WO2016188974A1

WO2016188974A1 - Mri using sense with acquisition of undersampled reference data via an ultrashort echo time sequence

Info

Publication number: WO2016188974A1
Application number: PCT/EP2016/061623
Authority: WO
Inventors: Peter Boernert; Kay Nehrke; Johannes Petrus Groen; Jouke Smink
Original assignee: Koninklijke Philips N.V.
Priority date: 2015-05-27
Filing date: 2016-05-24
Publication date: 2016-12-01

Abstract

The invention provides for a magnetic resonance imaging system. The magnetic resonance imaging system comprises a magnetic resonance imaging magnet for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject located within an imaging volume, a magnetic field gradient coil for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume, a coil array comprising a plurality of antenna elements configured for acquiring magnetic resonance data, a memory for storing machine executable instructions, and a processor, wherein execution of the instructions causes the processor to control the magnetic resonance imaging system to: - acquire with the coil array for each antenna element of the coil array first coil element reference data using ultra-short echo time imaging, the first coil element reference data forming a first set of coil array reference data of the imaging volume, reconstruct a first coil sensitivity map for each antenna element of the coil array using the first set of coil array reference data, the first coil sensitivity maps forming a first set of coil sensitivity maps, acquire with the coil array for each antenna element of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data of the imaging volume, reconstruct a first coil image for each antenna element of the coil array using the first set of coil array image data, the first coil images forming a first set of coil images, reconstruct a first magnetic resonance image according to the SENSE protocol using the first set of coil images and the first set of coil sensitivity maps.

Description

MRI USING SENSE WITH ACQUISITION OF UNDERSAMPLED REFERENCE

DATA VIA AN ULTRASHORT ECHO TIME SEQUENCE

TECHNICAL FIELD OF THE INVENTION

The invention relates to magnetic resonance imaging, in particular to acquiring magnetic resonance images using the SENSE reconstruction technique. BACKGROUND OF THE INVENTION

In magnetic resonance imaging (MRI) parallel imaging techniques are known. These techniques use a set of independent, decoupled receive coils for parallel magnetic resonance acquisition, thereby increasing the signal-to-noise ratio (SNR) compared to a single coil. These single coils of a multi-coil setup in general have better filling factors, i.e. the fraction of the coil detection volume filled with sample is higher, but these coils have non-uniform receive sensitivities and different spatial locations. Thus, the magnetic resonance (MR) signals detectable by the coils are sensitivity encoded giving another and alternative mean in MRI to perform spatial encoding parallel to the usual Fourier signal encoding. Using a set of those coils one can under-sample the k-space, i.e. the MRI data space, to accelerate scanning, applying appropriate image reconstruction techniques or methods for reconstructing magnetic resonance images, which are free of under- sampling/unfolding artifacts, and to combine the individual coil images. One example of such an image reconstruction technique that furthermore performs the image combination of images generated with a plurality of coils is the sensitivity encoding or SENSE

reconstruction technique. SENSE can also be applied if no under-sampling is performed and yield the optimal image combination in terms of the signal-to-noise ratio.

Using the SENSE reconstruction technique, an accurate knowledge of the receiving coil sensitivities is required to combine magnetic resonance data acquired by a multi element coil array used as a set of receiving coils. These coil sensitivities may be estimated from low resolution reference scans, because coils sensitivities change very slowly in space according to Maxwell laws.

The SENSE reconstruction technique was introduced by the journal article Pruessmann et al., "SENSE: sensitivity encoding for fast MRI," Magnetic Resonance in Medicine, 42:952-962 (1999). The terminology to describe the SENSE reconstruction is well known and has been the subject of many review articles and is present in standard texts on Magnetic

Resonance Imaging. For example "Handbook of MRI Pulse Sequences" by Bernstein et. al., published by Elsevier Academic Press in 2004 contains a review of the SENSE

reconstruction technique on pages 527 to 531.

The ISMRM-2013 (p0769) abstract by A.P. Aitken et al. on 'Rapid acquisition of PET attenuation maps form high under sampled UTE images using sparse-sense reconsruction' ). This concerns reconstruction of a magnetic resonance image that is then used as a PET-attenuation map. The acquisition of the magnetic resonance signal is done using an undersampled dual echo UTE Sparse-SENSE approach.

SUMMARY OF THE INVENTION

The invention provides for a magnetic resonance imaging system, a computer program product and a method in the independent claims. Embodiments are given in the dependent claims.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method or computer program product.

Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit," "module" or "system." Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer executable code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A 'computer-readable storage medium' as used herein encompasses any tangible storage medium which may store instructions which are executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. The computer-readable storage medium may also be referred to as a tangible computer readable medium. In some embodiments, a computer-readable storage medium may also be able to store data which is able to be accessed by the processor of the computing device. Examples of computer- readable storage media include, but are not limited to: a floppy disk, a magnetic hard disk drive, a solid state hard disk, flash memory, a USB thumb drive, Random Access Memory (RAM), Read Only Memory (ROM), an optical disk, a magneto-optical disk, and the register file of the processor. Examples of optical disks include Compact Disks (CD) and Digital Versatile Disks (DVD), for example CD-ROM, CD-RW, CD-R, DVD-ROM, DVD-RW, or DVD-R disks. The term computer readable-storage medium also refers to various types of recording media capable of being accessed by the computer device via a network or communication link. For example a data may be retrieved over a modem, over the internet, or over a local area network. Computer executable code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wire line, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

A computer readable signal medium may include a propagated data signal with computer executable code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

'Computer memory' or 'memory' is an example of a computer-readable storage medium. Computer memory is any memory which is directly accessible to a processor. 'Computer storage' or 'storage' is a further example of a computer-readable storage medium. Computer storage is any non- volatile computer-readable storage medium. In some embodiments computer storage may also be computer memory or vice versa.

A 'processor' as used herein encompasses an electronic component which is able to execute a program or machine executable instruction or computer executable code. References to the computing device comprising "a processor" should be interpreted as possibly containing more than one processor or processing core. The processor may for instance be a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed amongst multiple computer systems. The term computing device should also be interpreted to possibly refer to a collection or network of computing devices each comprising a processor or processors. The computer executable code may be executed by multiple processors that may be within the same computing device or which may even be distributed across multiple computing devices.

Computer executable code may comprise machine executable instructions or a program which causes a processor to perform an aspect of the present invention. Computer executable code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the "C" programming language or similar programming languages and compiled into machine executable instructions. In some instances the computer executable code may be in the form of a high level language or in a pre-compiled form and be used in conjunction with an interpreter which generates the machine executable instructions on the fly.

The computer executable code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It is understood that each block or a portion of the blocks of the flowchart, illustrations, and/or block diagrams, can be

implemented by computer program instructions in form of computer executable code when applicable. It is further under stood that, when not mutually exclusive, combinations of blocks in different flowcharts, illustrations, and/or block diagrams may be combined. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

A 'user interface' as used herein is an interface which allows a user or operator to interact with a computer or computer system. A 'user interface' may also be referred to as a 'human interface device.' A user interface may provide information or data to the operator and/or receive information or data from the operator. A user interface may enable input from an operator to be received by the computer and may provide output to the user from the computer. In other words, the user interface may allow an operator to control or manipulate a computer and the interface may allow the computer indicate the effects of the operator's control or manipulation. The display of data or information on a display or a graphical user interface is an example of providing information to an operator. The receiving of data through a keyboard, mouse, trackball, touchpad, pointing stick, graphics tablet, joystick, gamepad, webcam, headset, pedals, wired glove, remote control, and accelerometer are all examples of user interface components which enable the receiving of information or data from an operator.

A 'hardware interface' as used herein encompasses an interface which enables the processor of a computer system to interact with and/or control an external computing device and/or apparatus. A hardware interface may allow a processor to send control signals or instructions to an external computing device and/or apparatus. A hardware interface may also enable a processor to exchange data with an external computing device and/or apparatus. Examples of a hardware interface include, but are not limited to: a universal serial bus, IEEE 1394 port, parallel port, IEEE 1284 port, serial port, RS-232 port, IEEE-488 port, Bluetooth connection, Wireless local area network connection, TCP/IP connection, Ethernet connection, control voltage interface, MIDI interface, analog input interface, and digital input interface.

A 'display' or 'display device' as used herein encompasses an output device or a user interface adapted for displaying images or data. A display may output visual, audio, and or tactile data. Examples of a display include, but are not limited to: a computer monitor, a television screen, a touch screen, tactile electronic display, Braille screen, Cathode ray tube (CRT), Storage tube, Bi-stable display, Electronic paper, Vector display, Flat panel display, Vacuum fluorescent display (VF), Light-emitting diode (LED) displays, Electroluminescent display (ELD), Plasma display panels (PDP), Liquid crystal display (LCD), Organic light- emitting diode displays (OLED), a projector, and Head-mounted display. Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins using the antenna of a magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical image data. A Magnetic Resonance Imaging (MRI) image is defined herein as being the reconstructed two or three dimensional visualization of structural and in particular anatomic data contained within the magnetic resonance imaging data. This visualization can be performed using a computer.

Using an array of coil elements for generating a magnetic resonance image, in general no fixed geometry between the coils and a subject located within an imaging volume is given. Furthermore, the coupling between the conductive subject and the coil array may change from study to study. Coils located on or near by the surface of the subject receive MRI signals in an inherently non-uniform fashion. Therefore, in order to generate a magnetic resonance image with an array of coil elements, spatial information from coils at different spatial positions has to be recovered. The key information required is information on the spatial distribution of the receive sensitivities of the coils.

SENSE is an image based reconstruction method, wherein images from each antenna element of a coil array with reduced field of view (FOV) are reconstructed and merged using knowledge of individual coil sensitivities. It thus allows reconstructing a magnetic image from a plurality of coil images of independent, decoupled coils based on coil sensitivity maps reconstructed for each of these coils. Here, it is understood that the SENSE protocol in its most general form applies to the reconstruction of a magnetic resonance image using a set of coil images and a set of coil sensitivity maps. The SENSE reconstruction scheme may be applied to sets of aliased coil images as well as to sets of non-aliased coil images. Aliased coil images may result from coil element image data which is under-sampled in k-space.

As used herein a parallel imaging method encompasses imaging methods using a plurality of receiving coils for magnetic resonance imaging. Furthermore, parallel imaging allows utilizing spatial information related to the coils of a coil array for reducing the conventional Fourier encoding. Parallel imaging methods are able to accelerate and require less time for acquiring magnetic resonance imaging data which can be reconstructed into magnetic resonance images. Alternatively, keeping total scanning time fixed parallel imaging methods allow to increase the spatial resolution. Spatial information obtained from arrays of RF coils sampling data in parallel may be used to perform some portion of spatial encoding usually done by gradient fields, typically the phase encoding gradient. Thus, MRI acquisition times may be speeded up without a need for faster switching gradients or for additional RF power deposited.

In order to accelerate the imaging, SENSE may not only take into account the knowledge of coil sensitivities to combine a set of coil images, but also to remove Fourier unfolding artifacts resulting from aliasing coil images. In SENSE the conventional Fourier encoding may be reduced by utilizing spatial information about the individual antenna elements of a multi element coil array. This reduction in the Fourier encoding allows acquiring coil array image data necessary for a magnetic resonance image more rapidly.

First, low-resolution, fully Fourier-encoded coil element reference data for each coil antenna element may be used for sensitivity assessment. Based on these coil element reference data a sensitivity map of each coil antenna element may be derived.

Parallel imaging reconstruction may then be efficiently performed by creating an aliased image for each antenna element using discrete Fourier transformation, in case of k-space under-sampled data. From the set of aliased coil images a full FOV magnetic resonance image may be reconstructed using the spatial information provided by the set of coil sensitivity maps. Parallel reconstruction techniques can be used to improve the image quality with increased signal-to-noise ratio, spatial resolution, and reduce artifacts, helping further to increase the temporal resolution in dynamic MRI scans.

Parallel imaging techniques allow diminishing SNR in proportion to the numbers of reduction factors. R is the factor by which the number of k-space samples is reduced. In standard Fourier imaging reducing the sampling density results in the reduction of the FOV, causing aliasing. Thus, in order to use the SENSE reconstruction scheme for acceleration, an aliased image is generated for each antenna element of the coil array using discrete Fourier transformation.

The dependency between the magnetic resonance image and the coil images of the antenna elements of the coil array, in case of aliased images, is described by the sensitivity maps of the antenna elements, which describe the spatial dependency of the Bi field of the coil array. Thus, the sensitivity, e.g. provided by a set of coil sensitivity maps, may be used to remove the aliasing of the coil images.

In general it is assumed that coil sensitivity maps are smooth functions in space. Therefore, low resolution estimates might be sufficient for a large part of the maps. By rapidly acquiring such a low resolution estimate on the sensitivity of the receiving coils and use this information for reducing the Fourier encoding of coil data, the magnetic resonance image reconstruction may be speeded up by a total factor of two to three, even though additional data acquisition and processing is required to provide a sensitivity map for each antenna element.

In order to perform high quality SENSE reconstruction, whether for aliased or non-aliased coil images, detailed knowledge of the sensitivities for the individual antenna elements of the coil array is necessary. A more accurate estimation of the coil sensitivities may be obtained from high resolution data, however, this requires additional scan time, which is not desired in terms of scan efficiency and might increase the risk of motion artifacts.

Coil sensitivities may be compromised for various reasons, e.g. motion, main field non-uniformity, etc. Thus, in case of imprecise sensitivity maps errors may appear in the SENSE reconstruction results. The coil sensitivities are usually obtained via a SENSE reference scan which is based on conventional low resolution three dimensional (3D)

Cartesian pulse sequences. However, conventional Cartesian pulse sequences are using finite echo times which might be unable capturing transverse magnetization from fast decaying MR signals. Such fast decaying MR signals are present in case of fast T₂ relaxing components, like solid or trapped water protons in highly ordered structures. The decaying time of signals from those fast T₂ relaxing components may be too short, such that they do not contribute to the detectable signal at given echo times (TE) larger than the decaying time.

If those fast relaxing species are not sufficiently embedded in "long-lasting" MR signal sources, the Cartesian SENSE reference scan is blind to them and misestimates coil sensitivities at their location. By failing to visualize these fast T₂ relaxing components incomplete sensitivity estimates are provided, due to a lack of information. This effect may result in the inability to reconstruct short T₂-signals in SENSE accelerated parallel imaging applications using sequences capable to see the fast decaying signals. This is immediately obvious in case of non-accelerated short T2 imaging scans (R=l) where the SENSE algorithm is used to guide optimal image combination. If coil sensitivity information is missing or compromised at certain locations, coil combination is not correct.

Furthermore, strong local main field inhomogeneities may pose problems on conventional Cartesian gradient echo imaging. In presence of strong local main field variations, intra voxel dephasing takes place and becomes even more serious in case of large voxel sizes. The intra voxel dephasing may result in a complete signal cancelation. Such signal drop outs in the source data of a SENSE reference scan, which uses large voxel sizes, may also result into blind spots in the estimated coil sensitivities.

Typical SENSE reference scans measure rather large voxel sizes, e.g. 7 x 7 x 7 mm³, which may comprise areas of strong subject-induced susceptibility changes, like air- tissue interfaces, and poor magnet homogeneity, e.g. at the edges of the usable magnet field of view. In those areas of strong subject-induced susceptibility changes and/or poor magnet homogeneity, conventional coil sensitivity estimates and consequently SENSE reconstruction can be compromised.

To overcome these problems the present invention proposes the use of ultrashort echo time imaging for SENSE reference scanning. Based on ultra-short echo time imaging fast decaying components may be captured directly from the free induction decay (FID), avoiding the above mentioned signal losses. This applies to T₂- relaxation as well as intra voxel dephasing.

In one aspect the invention provides a magnetic resonance imaging system comprising: a magnetic resonance imaging magnet for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject located within an imaging volume, a magnetic field gradient coil for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume, a coil array comprising a plurality of antenna elements configured for acquiring magnetic resonance data, a memory for storing machine executable instructions, and a processor. Execution of the instructions causes the processor to control the magnetic resonance imaging system to:

acquire with the coil array for each antenna element of the coil array first coil element reference data using ultra-short echo time imaging.

A 'coil array' as used herein encompasses a magnetic resonance imaging coil which comprises multiple antenna elements, i.e. a multi-element magnetic resonance imaging coil. The coil array may function as a transmit and/or a receive coil for performing magnetic resonance imaging. Coil array image data as well as the coil array reference data as used herein is magnetic resonance imaging data acquired using the coil array. Each part of the coil array data is magnetic resonance imaging data from each individual coil array. The first set of coil array reference data comprises first coil element reference data acquired for each antenna element of the coil array. 'Coil element image data' and 'coil element reference data' as used herein encompasses magnetic resonance imaging data acquired by an antenna element.

Reference data may differ from image data only regarding the resolution of the underlying acquisition. In some examples they may further differ regarding the pulse sequence used to acquire the data and/or the degree of sampling of k-space.

The machine executable instructions may comprise pulse sequence data. Pulse sequence data as used herein encompasses data that may be used to control the magnetic resonance imaging system to acquire magnetic resonance data according to a particular magnetic resonance imaging protocol. The pulse sequence data may for instance be in the form of commands which may be executed or it may be in the form of a timing diagram or timing information which may be converted by a program into commands for controlling the magnetic resonance imaging system.

The pulse sequence data may in particular encompass data that may be used to control the magnetic resonance imaging system to acquire the magnetic resonance data according to an ultra-short echo time imaging protocol.

An ultra-short echo time image as used herein encompasses an image reconstructed from a free induction decay data where the free induction decay occurred on an extremely short timescale. Ultra-short echo time imaging as used herein refers to magnetic resonance imaging using pulse sequences with an echo time of 300 or less. The echo time as used herein refers to the time interval between the end of a radio frequency (RF) pulse of a pulse sequence and the beginning of the sampling window, i.e. of a readout gradient. The readout gradient is the magnetic field gradient applied during the period when the receiver components are on and the signal is being sampled. The FID refers to the electromagnetic resonance signal of the nuclei of the subject after their excitation.

Using ultra-short echo time sequences, i.e. with echo times of 300 or less, fast decaying components could be captured directly from the FID avoiding unnecessary delays which result into signal loss. Thus, ultra-short echo time imaging allows the detection of signal components with T₂ relaxation times on the order of only a few hundred

microseconds, as they occur in highly ordered tissues like bone, tendons, ligaments, menisciand periosteum. The achievable echo time for ultra-short echo time imaging methods is mainly determined by the transmit/receive switching times of the MR system components and the receive coils. These are typically on the order of 100 μβ, but may be reduced even down to the order of 10 and less.

Examples of tissues with short T₂ values are tendons with about 0.3-7 ms, cortical bone with 0.5 ms or dentine with 0.15-0.2 ms. Fast decaying T₂ signals from such tissues are invisible for conventional MRI using echo times of the order of milliseconds. These tissues thus lead to blind spots in sensitivity maps. Using ultra short echo time imaging with echo times of 300 μβ or less removes such blind spots from the sensitivity maps.

Furthermore, potential signal dropouts due to intra voxel dephasing caused by strong local magnetic main field variations may be prevented by echo times shorter than the timescale of dephasing. Based on coil sensitivity maps, which do not suffer from blind spots due to intra voxel dephasing and even take short T₂-signals into account, a high image quality of the final magnetic resonance image may be enabled.

Furthermore, a coil sensitivity map for each antenna element of the coil array is reconstructed using the first set of coil array reference data, the coil sensitivity maps forming a first set of coil sensitivity maps. Sensitivity maps describe for each antenna element of the coil array the spatial dependency of the Bi field, i.e. they quantify the relative weighting (in amplitude and phase) of signals from different points of origin within the FOV of each antenna element of the coil array. The sensitivity maps may be reconstructed by reconstructing a low-resolution coil reference image for each antenna element of the coil array using the first set of coil array reference data. Thus, a coil reference image may be acquired separately for each antenna element at full field-of-view. The resulting coil reference images may be normalized by dividing them by a low-resolution image of a FOV covering the FOV of all antenna elements for which a sensitivity map is to be reconstructed. Such an image for normalization may be acquired with a magnetic resonance imaging coil which images a large region, e.g. a body coil.

Execution of the instructions further causes the processor to control the magnetic resonance imaging system to: acquire with the coil array for each antenna element of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data of the imaging volume.

The first coil element image data may be under-sampled in k-space or fully

Fourier-encoded, i.e. fully sampled in k-space. Acquiring under-sampled coil element image data allows reducing the number of phase-encoding steps and thus may lead to a significant saving in the amount of time required to acquire magnetic resonance imaging data. When such under-sampled coil element image data is reconstructed into images, significant fold- over artifacts are present. This phenomenon, known as aliasing, occurs because an insufficient number of frequency components have been sampled to uniquely distinguish all spatial locations. The spatial information related to the first set of coil array reference data may be used to supplement the missing information in the coil element image data due to k- space under-sampling and undo the signal superposition underlying the fold-over effect, i.e. resolving the aliasing of the coil images.

Execution of the instructions also causes the processor to control the magnetic resonance imaging system to: reconstruct a coil image for each antenna element of the coil array using the first set of coil array image data, the coil images forming a set of coil images, reconstruct a magnetic resonance image according to the SENSE protocol using the first set of coil images and the first set of coil sensitivity maps. Thus, providing a high quality magnetic resonance image in terms of SNR.

According to an example, the magnetic resonance imaging system further comprises a body coil configured for acquiring magnetic resonance data and execution of the instructions further causes the processor to control the magnetic resonance imaging system to: acquire body coil data of the imaging volume with the body coil using ultra-short echo time imaging and additionally use the body coil data for the reconstruction of the set of coil sensitivity maps.

A 'body coil' as used herein encompasses a magnetic resonance imaging coil which images a large region. In an example, the body coil may be a separate coil on its own. In another example, the body coil may be formed by multiple antenna elements of a coil array used collectively. In this case the data from the multiple antenna elements may be combined to form a single virtual body coil.

The body coil may be used as reference to compute coil sensitivities, i.e. the coil sensitivities of the coil array are computed relative to the body coil, assuming that the sensitivity of the body coil is homogeneous over the field of view. Any other coil having homogeneous coil sensitivity over the desired field of view may be used instead, including a virtual coil as described above.

Using a body coil may have the advantage that the body coil data can be applied for reconstructing the coil sensitivity maps. When reconstructing the coil sensitivity map for each antenna element, the coil element data for the respective antenna element sampled in k-space may be Fourier transformed, resulting in a low resolution image.

Additionally the body coil data acquired in k-space may also be Fourier transformed, resulting in a low resolution picture comprising all parts of the field of view sampled by each antenna element of the coil array. Using the image reconstructed with the body coil data, a sensitivity map for each antenna element of the coil array may be reconstructed using the first set of coil array reference data, which has been Fourier transformed into a first set of reference images. The body coil data acquired using ultra-short echo time imaging may be under-sampled in k-space. This reduces the amount of time required to acquire the magnetic resonance imaging data.

The reconstruction of a first set of coil sensitivity maps using the first set of coil array reference data and the body coil data may generally be formed as a quotient of low resolution coil array and body coil images, including some regularization. According to an example, execution of the instructions further causes the processor to control the magnetic resonance imaging system to: reconstruct a body coil image, wherein the reconstruction of the body coil image and the reconstruction of the set of coil images are performed independently of each other using gridding. 'Gridding' refers to the re- sampling of non-rectilinear or non-uniformly sampled k-space data onto a uniform rectilinear grid to enable Fast Fourier Transformation (FFT) reconstruction. This is commonly done by resampling the data after convolving them with a smooth kernel function and de-apodization after FFT. The entire reconstruction process, e.g. resampling of the data and FFT, is frequently referred to as gridding.

According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with an echo time of 100 or less. This may have the advantage that the echo time is short enough for imaging even very short T₂-signals and to avoid blind spots due to dephasing effects.

According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a three dimensional radial ultra-short echo time acquisition sequence. This may have the advantage that not only the echo time is short enough for imaging short T₂-signals and to avoid blind spots due to dephasing effects, but also that 3D radial sampling is rather robust against motion due to k- space center averaging effects.

The SENSE reference scan may measure a three dimensional (3D) data set with a low tip angle and low contrast gradient echo sequence. For this purpose a 3D radial (kooshball) UTE sequence is used to sample the FID directly after excitation ramping up the readout gradient as fast as possible to capture short T₂. Radial and in particular 3D radial sampling is rather robust against motion, due to k-space center averaging effects.

Furthermore, radial under-sampling may accelerate the data acquisition of this 3D sequence, because the radial trajectory is rather benign to under-sampling.

According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a two dimensional (2D) ultra-short echo time acquisition sequence. To achieve ultrashort echo times, half-sine RF excitation pulses are used, limiting the echo time to only the hardware switching time. Two excitations, one with a positive and the other with a negative slice-selection gradient, are required to excite a 2D slice. Thus, for a proper slice definition, two subsequent excitations with slice-selection gradients of opposite sign are applied, and their MR signals are appropriately added to form the signal of the desired slice using half-sine excitation. Spiral or radial sampling schemes or combinations of both may be used.

The 2D ultra-short echo time acquisition sequence may be extended to a 3D acquisition, wherein a 2D plane is acquired along radial spokes, while the direction perpendicular to the plane is acquired with Cartesian sampling, resulting in a stack-of-stars trajectory or wherein the 2D plane is acquired along spirals, while the direction perpendicular to the plane is acquired with Cartesian sampling, resulting in a stack-of-spirals trajectory.

According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a zero echo time acquisition sequence. Zero echo time (ZTE) imaging provides a robust 3D radial technique which is particularly suitable for direct MRI of tissues with very rapid transverse relaxation. In ZTE imaging the readout gradient is set before excitation performed with a short, hard radio frequency pulse. Therefore, gradient encoding starts instantaneously upon signal excitation, resulting in an actual TE of zero. Since only minimal time is required until the next RF pulse can be applied, short repetition times are enabled.

Since the readout gradient is already set before excitation of the RF impulse, there is no need for switching off the gradient between TR intervals. No fast magnetic field changes occur, because in ZTE the gradient changes slightly its orientation but not its strength. Consequently, ZTE imaging can be performed virtually silently. A ZTE scan may defmably run also between individual diagnostic imaging scans of an exam, which are separated by a break. Consequently, whenever there is a break, instead of letting the MRI run idle, ZTE-base coil sensitivity scanning may be performed in order to acquire updates for the coil sensitivities.

According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a single point imaging acquisition sequence. In single point imaging the RF pulse is applied in the presence of a gradient, but instead of acquiring the whole FID, only a single, phase-encoded data point is acquired. Phase-encoding in single point imaging has the advantage of avoiding image blur from resonance offsets arising, for example, from chemical shift and magnetic susceptibility differences. Due to the fact that only a single point is phase encoded and sampled per FID, long scanning times are implied. When scanning, one of the gradients is stepped through k- space in discrete steps, while the other gradients are held constant. On the other hand unfolding artifacts may be totally avoided by applying a single point imaging acquisition sequence. According to an example, the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a sweep imaging with Fourier transform (SWIFT) acquisition sequence. The scheme employs a sequence of frequency-modulated pulses with short repetition time that exceeds the pulse length by at least the amount of time needed for setting a new value (or orientation) of a magnetic field gradient used to encode spatial information. Thus, the different spin frequencies in the bandwidth of interest are excited sequentially in time using a frequency sweep. This sequential excitation has the advantage of reducing the need for short and high peak RF power to achieve a given flip angle. The frequency sweep can be produced by modulating either the static field or the frequency of RF irradiation. With SWIFT MRI, the signal acquired with a given setting of the gradient is processed to yield a single projection of the subject. To generate a 3D image, the frequency sweeps are repeated, each time with a different gradient orientation similar to 3D UTE and ZTE.

According to an example, the first coil element image data is acquired using ultra-short echo time imaging. Thus, regarding the detection of fast decaying T₂ signals, the resolution of the first coil element image data may be on the same level as the coil sensitivity maps. Therefore, allowing taking full advantage of the information provided by the reference scans. However, reference scans based on ultra-short echo time imaging may also be advantageous for first coil element image data not acquired using ultra-short echo time imaging. Coil element image data not acquired using ultra-short echo time imaging may be influenced by dephasing effects which may be efficiently taken into account by reference scans based on ultra-short echo time imaging.

According to an example, the first coil element reference data is sampled in k- space for values of k below a predetermined threshold for the acquisition of the first coil element reference data. This may have the advantageous to allow a fast scan with coarse resolution in order to provide an immediate estimate for possible changes of the coil sensitivities.

According to an example, the first coil element reference data is under- sampled in k- space for the acquisition of the first coil element reference data. In under- sampling selected frequency components are not sampled. The components which are not sampled may be based on uniform or non-uniform under-sampling patterns or distributions. This may have the advantage that due to under-sampling, data acquisition may be significantly accelerated. Since the ultra-short echo time imaging allows for coil sensitivity maps without blind spots, the aliasing of the coil images may still be effectively removed. Furthermore, successively acquired highly under-sampled second coil array reference data may be used to successively reconstruct subsequent second coil sensitivity maps. These additional coil sensitivity data or their respective maps may be merged with previously acquired ones, using appropriate image processing to correct for potential rigid body displacements in case of motion during sampling or during the exam.

In an example, with a body coil, the body coil data may be under-sampled in addition to the first set of coil array reference data. This may lead to a saving in the amount of time required to acquire magnetic resonance imaging data. It may further be possible to accurately image or acquire magnetic resonance imaging data which represents the imaging volume by using key elements or a smaller subset of k-space. The coil element reference data corresponding to each antenna element of the coil array may be under-sampled in k-space to the same degree or different degrees.

In another example the first coil element reference data and the body coil data are under-sampled to a different degree. This embodiment may be advantageous because it may be possible to reconstruct either the first coil element reference data or the body coil data using the data which is sampled more than the other. For instance if the body coil data is more under-sampled in k-space than the coil element reference data then the coil element reference data may be used to partially reconstruct the body coil data. This may be advantageous because this may further reduce the amount of time for generating a magnetic resonance image.

In another example the under-sampling of k-space of the body coil and/or array coil is non-uniformly distributed in k-space. For instance the k-space from the body coil may be more densely sampled for low values of k-space.

According to an example, the first coil element image data is under-sampled in k-space for the acquisition of the first set of coil array image data, thus allowing for a faster acquisition of data.

The under-sampling of the first and optionally of the second coil element reference data appears to lead only to a low level of folding-in artefacts because of the radial scanning of k-space that is used in the UTE acquisition. In an alternative approach an initial estimate of the coil element reference data may be made at full sampling of the centre region of k-space. The (second) coil element reference data may be acquired by under-sampling, which provide a time update of the initial coil element reference data. This update appears also to be relatively insensitive to folding-in artefacts. Notably, as in radial scanning of k- space, there is inherently a dense sampling of the centre region of k-space that allows under- sampling that invokes only a low folding-in artefact level.

The magnetic resonance imaging system of any previous claim, wherein execution of the instructions further causes the processor to control the magnetic resonance imaging system to: successively acquire second sets of coil array reference data of the imaging volume with the coil array using ultra-short echo time imaging, wherein each of the second sets of coil array reference data comprises second coil element reference data acquired for each antenna element of the coil array, for each of the second sets of coil array reference data performing the following steps:

- reconstructing a second coil sensitivity map for each antenna element of the coil array using the respective second set of coil array reference data, the respective second coil sensitivity maps forming a second set of coil sensitivity maps,

successively acquire second sets of coil array image data of the imaging volume with the coil array, wherein each of the second sets of coil array image data comprises second coil element image data acquired for each antenna element of the coil array, for each of the second sets of coil array image data reconstructing a second coil image for each antenna element of the coil array using the respective second set of coil array image data, the second coil images forming a second set of coil images, and

reconstructing a second magnetic resonance image according to the SENSE protocol using the respective second set of coil images and the respective second set of coil sensitivity maps.

This may have the advantage that changes of the coil sensitivities can be taken into account. In general, reconstructing a set of coil sensitivity maps once would be sufficient. As long as no changes of the position of the antenna element relative to each other and relative to the subject occur and as long as the subject itself remains unchanged, a set of coil sensitivity maps once reconstructed may be suitable for an arbitrary number of magnetic resonance image reconstructions performed afterwards. However, due to motions, the relative position of the antenna elements of the coil array to a subject located within the imaging volume may change with time. In order to take these changes into account, it may be beneficial to reconstruct updated sets of coil sensitivity maps in certain time intervals. Thus, it may be guaranteed that the current coil sensitivity maps used for image reconstruction are adequately precise, since it is avoided that the time passing between the state of the system for which the coil sensitivity maps are reconstructed and the state of the system for which the coil images are reconstructed become too large. In an example, each of the second sets of coil sensitivity maps may be compared with the first set of coil sensitivity maps and/or a preceding second set of coil sensitivity maps. In case the second set of coil sensitivity maps has not changed or the changes are negligible, the reconstruction of second magnetic resonance images may be performed using the first set of coil sensitivity maps and/or a preceding second set of coil sensitivity maps instead of the current second set of coil sensitivity maps. It may be beneficial for this comparison to acquire second sets of coil array reference data with a rather low precision and only in case the comparison indicates that considerable sensitivity changes have occurred to acquire a second set of coil array reference data with a suitable precision to reconstruct a precise second set of coil sensitivity maps in order to replace the previous ones.

Thus changes of the coil sensitivities over time may be adequately reproduced by a series of second sets of coil sensitivity maps. These second sets of coil sensitivity maps, which have been successively generated, may be used to track the development of the sensitivity over time and take this development over time into account for the reconstruction of the magnetic resonance image. During examination, highly under-sampled and therefore short acquisitions of second sets of coil array reference data may be used to check the actual MRI data acquisition.

Successively acquired highly under-sampled 3D data may be merged with previously acquired data, using appropriate image processing to correct for potential rigid body displacements in case of motion during sampling. Furthermore, they may be used to update and/or correct available coil sensitivity information.

According to an example, each of the second sets of coil array reference data is sampled on a distinct, non- identical k- space trajectory. This may have the advantage that potential sub-sampling artifacts may be reduced and/or avoided. Foosball interleaved scans may advantageously be sampled on distinct, non-identical k-space trajectories, in each repetitive step to reduce potential subsampling artefacts.

According to an example, the second coil element reference data of each second set of coil array reference data is more under-sampled in k-space than the first coil element reference data and the reconstruction of each of the second coil sensitivity maps is performed using additionally the first set of coil array reference data. This may have the advantage that the acquisition time for the second set of coil array reference data is short and that changes of the coil sensitivities are still taken into account. The first set of coil array reference data may be used as a bases and changes may taken into account by transforming the first set of coil array reference data with the second set of coil array reference data into a set of coil array reference data sufficiently representing the present state of coil sensitivities. In an example, the first set of coil array reference data is used for the reconstruction of a second coil sensitivity map by using a set of coil array reference data previously used, which has been calculated based on the first set of coil array reference data.

A computer program product may in particular be used for upgrade already install sets of executable instructions and to enable magnetic resonance imaging systems to be operated by a processor according to the method claimed here.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

Fig. 1 shows a block diagram which illustrates an example of a method,

Fig. 2 shows a block diagram which illustrates an example of a method,

Fig. 3 shows a block diagram which illustrates an example of a method,

Fig. 4 shows a schematic diagram of a UTE pulse sequence,

Fig. 5 shows a schematic diagram of a ZTE pulse sequence,

Fig. 6 shows a schematic diagram illustrating an example of a magnetic resonance imaging system. DETAILED DESCRIPTION OF THE EMBODIMENTS

Like numbered elements in these figures are either equivalent elements or perform the same function. Elements which have been discussed previously will not necessarily be discussed in later figures if the function is equivalent.

FIG. 1 shows a block diagram which illustrates an embodiment of a method according to the invention. This method may be implemented as a computer-implemented method, a computer program product, and also as instructions stored on a computer-readable storage medium. In step 100 a set of coil array reference data of an imaging volume is acquired with a coil array using ultra-short echo time imagining. In step 102 body coil data is acquired with a body coil using ultra-short echo time imagining. Either step 100 or 102 may be performed first. During steps 100 and 102 the body coil data and/or the coil array reference data are under-sampled. In step 104 a set of coil sensitivity maps is reconstructed using the set of coil array reference data and the body coil data. In step 106 coil array image data of the imaging volume is acquired. In step 108 a set of coil images is reconstructed using the set of coil array image data. In step 110 the magnetic resonance image is reconstructed according to the SENSE protocol using the set of coil images and the set of coil sensitivity maps.

FIG. 2 shows a block diagram which illustrates the reconstruction of a magnetic resonance image according to an example using the SENSE protocol. A first set of coil array reference data in k-space comprising coil element reference data 200, 201, 202, 203 is acquired using ultra-short echo time imaging. In one example the coil element reference data 200, 201, 202, 203 may be under-sampled in k-space, in another example it is not under-sampled. This set of coil array reference data is Fourier transformed into a set of four coil reference images 210, 211, 212, 213. Additionally, body coil data 220 is acquired using ultra-short echo time imaging and Fourier transformed into a body coil image 230. In one example the body coil data 220 may be under-sampled in k-space, in another example it is not under-sampled. With the body coil image 230, a coil sensitivity map 240, 241, 242, 243 for each antenna element of the coil array is reconstructed using the four coil reference images 210, 211, 212, 213. Furthermore, a set of coil array image data 250, 251, 252, 253 is acquired in k-space for each antenna element of the coil array. In one example the coil array image data 250, 251, 252, 253 may be under-sampled in k-space, in another example it is not under-sampled. In one example the coil array image data 250, 251, 252, 253 is acquired using ultra-short echo time imaging, in another example ultra-short echo time imaging is not used. The set of coil array data is Fourier transformed into a set of coil images 250, 251, 252, 253. The main difference between coil images 260, 261, 262, 263 and coil reference images 210, 211, 212, 213 is that the resolution of the images 260, 261, 262, 263 is significantly higher. The set of coil images 260, 261, 262, 263 and the set of coil sensitivity maps 240, 241, 242, 243 are used to reconstruct according to the SENSE protocol a magnetic resonance image 270 of the whole imaging volume. The SENSE protocol describes the relation between the set of coil images 260, 261, 262, 263, the set of coil sensitivity maps 240, 241, 242, 243 and the magnetic resonance image 270 of the whole imaging volume by a matrix equation, which is solved according to the SENSE protocol to calculate the magnetic resonance image 270.

FIG. 3 shows a block diagram which illustrates an example of a method which applies a series of second sets of coil sensitivity maps. Step 300 comprises steps 100-104 of Fig. 1 , i.e. the acquisition of the first coil element reference data, the body coil data and the reconstruction of the sensitivity maps. Step 302 comprises steps 106-110 of Fig. 1, i.e. the acquisition of the first coil element image data, the reconstruction of coil images as well as the reconstruction of a first magnetic resonance image. Step 302 may be repeated in order to reconstruct additional magnetic resonance images with the first set of coil sensitivity maps. In step 304 second coil element reference data is acquired with each antenna element of the coil array using ultra-short echo time imaging and a second set of coil sensitivity maps is reconstructed. Additional second body coil data may be acquired using ultra-short echo time imaging and be additionally used for the reconstruction of the respective second set of coil sensitivity maps. In step 306 second coil element image data is acquired and a second set of coil images is reconstructed. Finally, in step 308 a second magnetic image according to the SENSE protocol is reconstructed using the second set of coil images reconstructed in step 306 and the second set of coil sensitivity maps reconstructed in step 304. Steps 306 and 308 may be repeated in order to successively reconstruct a series of second magnetic images with a series of second sets of coil images and the second set of coil sensitivity maps reconstructed in step 304. Step 304 corresponds to step 300 reconstructing an additional second set of coil array reference data and an additional second set of coil sensitivity maps is reconstructed. Steps 306 and 308 correspond to step 302. Furthermore, step 304 may also be repeated in order to successively reconstruct further second sets of coil sensitivity maps each

corresponding to a current state of coil sensitivities and using these further second sets of coil sensitivity maps for reconstructing further second magnetic images, when repeating steps 306 and 308, in order to take into account changes of the coil sensitivities over time.

FIG. 4 shows a schematic diagram of a basic pulse sequence for 3D radial UTE imaging. A hard RF block pulse for excitation and ramp sampling for the readout gradient G_r is used in acquisition. K-space is sampled with isotropic angular spacing, thus cover a sphere in k-space. Here, the echo time TE is defined as the interval between the end of the RF pulse and the beginning of the sampling window.

Using low tip-angle excitations allows keeping the RF pulse significantly shorter than 100 μβ, i.e. the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of a RF excitation pulse at the Larmor frequency is low. TE results from the time needed for coil ring down as well as the time required for tuning of the coils. After the pulse, the energy stored in the transmit coil must ring down to allow safe tuning of the receive coils. While ring down typically takes only a few microseconds, tuning of the coils may be more time consuming. Fast switching coils may ensure a minimal TE below 100 and minimize signal decay prior to acquisition.

Here, an initial non-selective RF block pulse is used for excitation. Sampling is started simultaneously with the rising slope of the readout gradient G_r to sample free induction decay. The beginning of data acquisition coincides with the origin of k-space which is sampled radially starting from k=0. This results in a kooshball-like structure in k-space, since the endpoints of the radial profiles lie on the surface of a sphere.

Due to radial sampling, the center of the spherical k-space volume is oversampled and k-space is sampled with isotropic angular density. Thus, 3D radial sampling acquires k-space data within a sphere of radius k_max.

FIG. 5 shows a schematic diagram of a basic ZTE pulse sequence. RF excitation is performed with a short hard pulse, directly followed by data acquisition. Since in ZTE imaging the readout gradient G_r is set before excitation, the gradient encoding starts instantaneously upon signal excitation resulting in an actual TE of zero. With only minimal time required until the next RF pulse can be applied, very short repetition times TR are enabled. Since the readout gradient G_r is already set before excitation of the RF impulse, there is no need for switching off the gradient between TR intervals, allowing performing ZTE imaging virtually silently.

The data obtained with the ZTE scheme are slightly incomplete in the k-space center, since the finite duration of the RF pulse, transmit-receive switching, and the digital filter result in a total initial dead time Δ. However, this central k-space gap may be resolved by radial acquisition oversampling and algebraic reconstruction, involving finite support extrapolation. Other solutions to the missing data problem in the center of k-space are conceivable and known. ZTE provides high robustness against off-resonance as apparent at the tissue-air interfaces, allowing the depiction of considerable detail.

Fig. 6 shows an example of a magnetic resonance imaging system 600 according to an embodiment of the invention. The magnetic resonance imaging system 600 comprises a magnet 602. Within the magnet 602 there is an imaging volume 604. The imaging volume 604 is a zone where the magnetic field of the magnet 602 is uniform enough to perform magnetic resonance imaging. The subject 606 can be seen reposing on a subject support 608 with a portion of the subject 606 located within the imaging volume 604. The subject support 608 is attached to an optional actuator 621 that is able to move the subject support 608 and the subject 606 through the imaging volume 604. Also within a bore of the magnet 602 is a magnetic field gradient coil 610. The magnetic field gradient coil 610 typically comprises three separate gradient coil systems for the x, y, and z-directions.

Typically the z-direction is aligned with the magnetic field lines within the imaging volume 604. A gradient coil power supply 612 is shown as being connected to the magnetic field gradient coil 610. Above the imaging volume 604 is a coil array 614. The coil array 614 is shown as being comprised of four antennal elements 616. The actual number of antennal elements 616 and their arrangement space depends upon the geometry being imaged by the coil array 614. Above the coil array 614 is shown a body coil 618. Both the body coil 618 and the antenna elements 616 of the coil array 614 are shown as being connected to a radio frequency transceiver 620. The radio frequency transceiver 620 may be replaced in some embodiments by separate transmitters and receivers. Both the gradient coil power supply 612 and the radio frequency transceiver 620 are shown as being connected to a hardware interface 624 of a computer 622.

Within the computer 622 a processor 626 is able to send and receive instructions from the hardware interface 624. By means of the hardware interface 624 the processor 626 is able to control the operation and function of the magnetic resonance imaging system 600. The processor 626 is also connected to a user interface 628 which may be adapted for displaying data or renderings of magnetic resonance imaging to a user. The user interface 628 may also be adapted for receiving commands or instructions from a user for operating the magnetic resonance imaging system 600. The processor 626 is also connected to computer storage 630 and computer memory 632. Although a single computer 622 and a single processor 626 are shown it is understood that the terms a computer and a processor may refer to a plurality of computers and/or processors.

In the computer storage 630 is stored a pulse sequence 634. A pulse sequence

634 as used herein encompasses a set of instructions for operating a magnetic resonance imaging system 600 for acquiring a set of coil array reference data 638, body coil data 640 and a set of coil array image data 644. Herein the pulse sequence 634 comprises a set of instructions for acquisition of the set of coil array reference data 638 and body coil data 640 by way of the ultra-short echo time imaging. The storage 630 further contains a set of coil array reference date 638 that was acquired with the magnetic resonance imaging system 600. The computer storage 630 further contains body coil data 640 that was acquired by the magnetic resonance imaging system 600. The computer storage 630 further contains a set of coil sensitivity maps 642 that were calculated or reconstructed using the set of coil array reference data 638 and the body coil data 640. The computer storage 644 further contains a set of coil array image data 644 acquired by the magnetic resonance imaging system 600. The computer storage 630 further contains a set of coil images 646 that were calculated or reconstructed using the set of coil array image data 644. Finally the computer storage 630 also contains a magnetic resonance image 648 which is reconstructed using the magnetic resonance imaging data 646 and the set of coil sensitivity map 642.

The computer memory 632 contains several modules belonging to a computer program product for running and operating the magnetic resonance imaging system 600. The computer memory 632 contains a system control module 650. The system control module 650 controls the operation and functioning of the magnetic resonance imaging system 600. The computer memory 632 further contains a sensitivity map reconstruction module 652. The sensitivity map reconstruction module 652 contains instructions for use by the processor 626 to calculate coil sensitivity maps 642 using the body coil data 640 and the set of coil array reference data 638.

The memory 632 also contains a coil image reconstruction module 654. The coil image reconstruction module 654 contains instructions for the processor 626 to reconstruct a set of coil images 646 using the set of coil array image data 644. The memory 632 further contains an image reconstruction module 656. The image reconstruction module 656 contains instructions for the processor 626 to reconstruct a magnetic resonance image 648 according to the SENSE protocol 636 using the set of coil images 646 and the set of coil sensitivity maps 642.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope. LIST OF REFERENCE NUMERALS

200 coil element reference data

201 coil element reference data

202 coil element reference data

203 coil element reference data

210 coil reference image

21 1 coil reference image

212 coil reference image

213 coil reference image

220 body coil data

230 body coil image

240 coil sensitivity map

241 coil sensitivity map

242 coil sensitivity map

243 coil sensitivity map

250 first coil element image data

251 first coil element image data

252 first coil element image data

253 first coil element image data

260 coil image

261 coil image

262 coil image

263 coil image

270 magnetic resonance image

600 magnetic resonance imaging system

602 magnet

604 imaging volume

606 subject

608 subject support

610 magnetic field gradient coil

612 gradient coil power supply

614 coil array

616 antenna element 618 body coil

620 radio frequency transceiver

621 actuator

622 computer

624 hardware interface

626 processor

628 user interface

630 computer storage

632 computer memory

634 pulse sequence

636 SENSE protocol

638 set of coil array reference data

640 body coil data

642 set of coil sensitivity maps

644 set of coil array image data

646 set of coil images

648 magnetic resonance image

650 system control module

652 sensitivity map reconstruction module

654 coil image reconstruction module

656 image reconstruction module

Claims

CLAIMS:

1. A magnetic resonance imaging system (600) comprising:

a magnetic resonance imaging magnet (602) for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject (606) located within an imaging volume (604),

a magnetic field gradient coil (610) for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume (604),

a coil array (614) comprising a plurality of antenna elements (616) configured for acquiring magnetic resonance data,

a memory (630, 632) for storing machine executable instructions (650, 652,

654, 656), and

a processor (626), wherein execution of the instructions (650, 652, 654, 656) causes the processor (626) to control the magnetic resonance imaging system (600) to:

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element reference data using ultra-short echo time imaging, the first coil element reference data forming a first set of coil array reference data (638) of the imaging volume (604),

reconstruct a first coil sensitivity map for each antenna element (616) of the coil array using the first set of coil array reference data (638), the first coil sensitivity maps forming a first set of coil sensitivity maps (642),

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data (644) of the imaging volume (604), wherein the first coil element reference data is under-sampled in k- space for the acquisition of the first set of coil array reference data (638) and

reconstruct a first coil image for each antenna element (616) of the coil array using the first set of coil array image data (644), the first coil images forming a first set of coil images (646), reconstruct a first magnetic resonance image (648) according to the SENSE protocol (636) using the first set of coil images (646) and the first set of coil sensitivity maps (642) .

2. The magnetic resonance imaging system (600) of claim 1, wherein the magnetic resonance imaging system (600) further comprises a body coil (618) configured for acquiring magnetic resonance data and wherein execution of the instructions (650, 652, 654, 656) further causes the processor (626) to control the magnetic resonance imaging system (600) to:

- acquire body coil data (640) of the imaging volume (604) with the body coil

(618) using ultra-short echo time imaging and

additionally use the body coil data (640) for the reconstruction of the first set of coil sensitivity maps (642).

3. The magnetic resonance imaging system (600) of claim 1 or 2, wherein the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with an echo time of 100 or less.

4. The magnetic resonance imaging system (600) of any previous claim, wherein the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a three dimensional radial ultra-short echo time acquisition sequence.

5. The magnetic resonance imaging system (600) of any of claims 1 to 3, wherein the acquisition of the first coil element reference data by way of the ultra-short echo time imaging is performed with a zero echo time acquisition sequence.

6. The magnetic resonance imaging system (600) of any previous claim, wherein the first coil element reference data is sampled in k-space for values of k below a

predetermined threshold for the acquisition of the first set of coil array reference data (638).

7. The magnetic resonance imaging system (600) of any previous claim, wherein the first coil element image data is acquired using ultra-short echo time imaging.

8. The magnetic resonance imaging system (600) of any previous claim, wherein the first coil element image data is under-sampled in k-space for the acquisition of the first set of coil array image data (644).

9. The magnetic resonance imaging system (600) of any previous claim, wherein execution of the instructions (650, 652, 654, 656) further causes the processor (626) to control the magnetic resonance imaging (600) system to:

successively acquire second sets of coil array reference data of the imaging volume (604) with the coil array (614) using ultra-short echo time imaging, wherein each of the second sets of coil array reference data comprises second coil element reference data acquired for each antenna element (616) of the coil array,

for each of the second sets of coil array reference data performing the following steps:

reconstructing a second coil sensitivity map for each antenna element (616) of the coil array using the respective second set of coil array reference data, the respective second coil sensitivity maps forming a second set of coil sensitivity maps,

successively acquire second sets of coil array image data of the imaging volume (604) with the coil array (614), wherein each of the second sets of coil array image data comprises second coil element image data acquired for each antenna element (616) of the coil array,

for each of the second sets of coil array image data reconstructing a second coil image for each antenna element (616) of the coil array using the respective second set of coil array image data, the second coil images forming a second set of coil images, and

- reconstructing a second magnetic resonance image according to the

SENSE protocol (636) using the respective second set of coil images and the respective second set of coil sensitivity maps.

10. A magnetic resonance imaging system (600) comprising:

- a magnetic resonance imaging magnet (602) for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject (606) located within an imaging volume (604), a magnetic field gradient coil (610) for generating a gradient magnetic field for spatial encoding of a magnetic resonance signal of spins of nuclei within the imaging volume (604),

a memory (630, 632) for storing machine executable instructions (650, 652,

654, 656), and

- acquire with the coil array (614) for each antenna element (616) of the coil array first coil element reference data using ultra-short echo time imaging, the first coil element reference data forming a first set of coil array reference data (638) of the imaging volume (604),

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data (644) of the imaging volume (604), and

- reconstruct a first coil image for each antenna element (616) of the coil array using the first set of coil array image data (644), the first coil images forming a first set of coil images (646),

reconstruct a first magnetic resonance image (648) according to the SENSE protocol (636) using the first set of coil images (646) and the first set of coil sensitivity maps (642), wherein execution of the instructions (650, 652, 654, 656) further causes the processor (626) to control the magnetic resonance imaging (600) system to:

successively acquire second sets of coil array reference data of the imaging volume (604) with the coil array (614) using ultra-short echo time imaging, wherein each of the second sets of coil array reference data comprises second coil element reference data acquired for each antenna element (616) of the coil array, and wherein the second coil element reference data is under-sampled in k- space for the acquisition of the second set of coil array reference data (638)

for each of the second sets of coil array reference data performing the following steps: reconstructing a second coil sensitivity map for each antenna element (616) of the coil array using the respective second set of coil array reference data, the respective second coil sensitivity maps forming a second set of coil sensitivity maps,

reconstructing a second magnetic resonance image according to the SENSE protocol (636) using the respective second set of coil images and the respective second set of coil sensitivity maps.

11. The magnetic resonance imaging system (600) of claim 9, wherein each of the second sets of coil array reference data is sampled on a distinct, non- identical k-space trajectory.

12. The magnetic resonance imaging system (600) of claims 9 or 11, wherein the second coil element reference data of each second set of coil array reference data is more under-sampled in k-space than the first coil element reference data and the reconstruction of each of the second coil sensitivity maps is performed using additionally the first set of coil array reference data (638).

13. A computer program product comprising a computer readable storage medium having machine executable instructions (650, 652, 654, 656) and pulse sequence commands (634) for acquiring magnetic resonance data by way of a ultra-short echo time acquisition sequence embodied therewith, the executable instructions (650, 652, 654, 656) being executable by a processor (626) to cause the processor (626) to control a magnetic resonance imaging system (600) to:

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data (644) of the imaging volume (604), wherein the first coil element reference data is under-sampled in k-space for the acquisition of the first set of coil array reference data (638)

reconstruct a first coil image for each antenna element (616) of the coil array using the first set of coil array image data (644), the first coil images forming a first set of coil images (646),

reconstruct a first magnetic resonance image (648) according to the SENSE protocol (636) using the first set of coil images (646) and the first set of coil sensitivity maps (642).

14. A method of operating a magnetic resonance imaging system (600) comprising:

- a magnetic resonance imaging magnet (602) for generating a main magnetic field for orientating the magnetic spins of nuclei of a subject (606) located within an imaging volume (604),

a memory (630, 632) for storing machine executable instructions (650, 652,

654, 656), and

- a processor (626), wherein execution of the instructions (650, 652, 654, 656) causes the processor (626) to control the magnetic resonance imaging system (600) to:

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element reference data using ultra-short echo time imaging, the first coil element reference data forming a first set of coil array reference data (638) of the imaging volume (604), wherein the first coil element reference data is under-sampled in k- space for the acquisition of the first set of coil array reference data (638),

acquire with the coil array (614) for each antenna element (616) of the coil array first coil element image data, the first coil element image data forming a first set of coil array image data (644) of the imaging volume (604),