WO2014072867A1

WO2014072867A1 - Non-reference phase-sensitive inversion recovery imaging

Info

Publication number: WO2014072867A1
Application number: PCT/IB2013/059656
Authority: WO
Inventors: Jinnan Wang; Peter Boernert
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-11-07
Filing date: 2013-10-25
Publication date: 2014-05-15

Abstract

A magnetic resonance imaging system that can acquire PSIR images with only half of the image time by using a new reconstruction technique. The system (1) includes a background phase unit (26), a phase correction unit (30), and an image correction unit (32). The background phase unit (26) constructs a background phase image from a received reconstructed phase image based on region growing of a selected homogeneous region by adding pixels within a phase difference criteria of the homogeneous region and adding inverted pixels outside the phase difference criteria. The phase correction unit (30) corrects the phase image with the background phase image to create a true phase image. The image correction unit (32) corrects a received reconstructed magnitude image based on the true phase image.

Description

NON-REFERENCE PHASE-SENSITIVE INVERSION RECOVERY IMAGING

The following relates generally to medical imaging. It finds particular application in conjunction with magnetic resonance imaging, image reconstruction, and imaging sequences, and will be described with particular reference thereto. However, it will be understood that it also finds application in other usage scenarios and is not necessarily limited to the aforementioned application.

Phase sensitive inversion recovery (PSIR) is a technique which identifies a polarization of a spin magnetization of a patient's tissues in a magnetic resonance imaging system. The spins measured in the patient's tissues are received as inverted phases, e.g. 180° phase difference. The phase difference can be used to identify properties of the tissues where the difference is measured. For example, in myocardium infarction detection, the separation of the spins using PSIR separates luminal angiography with a negative spin polarity, and hemorrhage with a positive spin polarity. The PSIR technique extends the dynamic range of MR images where the phase information doubles the amount of information collected, e.g. a |Mz/Mo| range of [0,1] extended to Mz/Mo [-1,1].

Current PSIR techniques use multiple acquisition sequences or pulses to obtain the phase information. For example, a Tl TFE based PSIR pulse sequence includes a series of pulses, a, to obtain image information and a second series of pulses, Θ, to obtain the phase information between each repetition time (TR). Other approaches vary the timing of the a pulses and/or Θ pulses with interlacing, alternating, and the like. The various techniques result in two acquisitions, one acquisition for a phase reference and a second acquisition for a regular image acquisition. The multiple acquisitions prolong the acquisition time which means longer scan times for the patient. In time-sensitive acquisitions, prolonged sequences can preclude applicability of the PSIR technique.

Current image reconstructions receive complex magnetic resonance signals which include a phase and a magnitude components. Usually, in most MR applications, the phase information is discarded after image reconstruction. However, as mentioned above, this comes along with a loss of dynamic range and contrast in inversion recovery type or other phase sensitive acquisitions which is not desirable. For a PSIR sequence, a first acquisition reconstructs both a magnitude image and a phase map, and a second acquisition reconstructs a second magnitude reference image and a second phase map. The second reference image is used to correct the phase polarity of the magnitude image of the first reconstruction. In phase sensitive MR images, the different polarity of the spins are revealed as flipped phases. For example, spins of different magnetization polarity will have 180° difference in phases which with variation of background phases superimposed make difficult the identification of the true phases.

The following discloses a new and improved non-reference phase-sensitive inversion recovery (nrPSIR) imaging which addresses the above referenced issues, and others.

In accordance with one aspect, a magnetic resonance image reconstruction system includes a background phase unit, a phase correction unit, and an image correction unit. The background phase unit constructs a background phase image from a received reconstructed phase image based on growing a selected homogeneous region by adding pixels within a phase difference criteria of the homogeneous region and adding inverted pixels outside the phase difference criteria. The phase correction unit corrects the phase image with the background phase image to create a true phase image. The image correction unit corrects a received reconstructed magnitude image based on the true phase image.

In accordance with another aspect, a method of magnetic resonance imaging receiving a magnitude and a phase image reconstructed from magnetic resonance signals of a single magnetic resonance imaging sequence. A background phase image is constructed from a selected at least one homogeneous growing region in the phase image and iteratively adding neighboring pixels within a phase difference criteria of the growing region or adding inverted pixels outside the phase difference criteria. The phase image is corrected with the background phase image to create a true phase image. The received magnitude image is corrected based on the true phase image.

In accordance with another aspect, a magnetic resonance imaging system includes one or more processors and a display device. The one or more processors are configured to construct a background phase image from a received reconstructed phase image based on growing a selected homogeneous region by adding pixels within a phase difference of the homogeneous region and adding inverted pixels outside the phase difference. The one or more processors are further configured to filter the pixels of the constructed background phase image with a low pass filter and subtract the constructed background phase image from the phase image to create a true phase image. The one or more processors are further configured to correct a received reconstructed magnitude image for phase based on the true phase image. The display device displays the corrected received reconstructed magnitude image.

One advantage is a single acquisition which provides phase-sensitive magnetic resonance images.

Another advantage resides in use of existing magnetic resonance hardware and software.

Another advantage resides in the reduction in scan time for phase-sensitive recovery imaging of patients.

Still further advantages will be appreciated to those of ordinary skill in the art upon reading and understanding the following detailed description.

The invention may take form in various components and arrangements of components, and in various steps and arrangement of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

FIGURE 1 schematically illustrates an embodiment of an nrPSIR system.

FIGURES 2A-2C illustrate exemplary images of a magnitude image, a PSIR image, and a nrPSIR image, respectively.

FIGURE 3A-3B schematically illustrate an embodiment of a general phase correction data flow and a background phase estimation data flow.

FIGURE 4 flowcharts one method of using an embodiment of non-reference phase-sensitive inversion recovery.

With reference to FIGURE 1, an embodiment of an nrPSIR system 1 is schematically illustrated. The system 1 includes a MR scanner 2 such as an open system or c- type scanner, a horizontal bore scanner, and the like shown in a cross section view. The scanner includes an opening or bore that defines an examination region in which a subject 4 is placed for a spectroscopic and/or imaging examination. The MR scanner 2 includes one or more main magnets 6 with a C-shape ferrous flux return path in an open system, one or more radio frequency (RF) coils 8, and one or more gradient coils 10. A C-type main magnet 6 generates a vertical static Bo field such as vertical static field. Alternatively, a bore magnet generates a horizontal static Bo field.

The system 1 includes a sequence controller 12 which controls the operation of the imaging sequence, a RF transmitter unit 14 controlling the operation of the RF coils 8, and a gradient controller 16 controlling the operation of the gradient coils 10. The communication between the controlling unit and the corresponding coils can be wireless or wired. The RF coils 8 generate radio frequency pulses which excite and manipulate resonance in tissue of the subject 4. The RF coils 8 can include a whole body coil and/or a local coil such as a torso coil, hand coil, shoulder coil, knee coil, etc. The one or more gradient coils 10 generate gradient magnetic fields across the static magnetic field to spatially encode the induced resonance, induced gradient echoes, and the like. The sequence controller 12 configures the RF coils and the gradient coils to excite and manipulate resonance in tissues of the subject.

The system 1 includes a RF receiver unit 18, which receives MR signals. As the resonance decays in the tissue of the subject, weak radio frequency signals or MR signals are received by a radio frequency antenna such as the RF coils 8 and/or local coils, and sent to the RF receiver unit 18. A reconstruction unit 20, such as a processor, receives RF data or MR signals from the RF receiver 18 and reconstructs one or more images from the received MR signals such as image slices, a volume, etc. from a single imaging acquisition sequence. The reconstructed images include a magnitude image (I_m) 22 and a phase map (I_p) 24 which can be stored in a memory such as server or processor memory, local disk storage, cloud storage, and/or a storage management system such a Picture Archiving and Communication System (PACS), a Radiology Information System (RIS), and the like.

The system includes a background phase unit 26, a filter unit 28, a phase correction unit 30, and an image correction unit 32. The background phase unit 26 constructs a background phase image (Ibck) 34 from the phase map 24 based on a region growing of selecting a homogeneous region of the phase map and adding pixels of the phase map to the growing region based on a region growing algorithm.

The background phase image represents noise characteristic of the MR signal receive chain such as the receive coils. The filter unit 28 filters the pixels of the constructed background phase image with a low pass filter. The phase correction unit 30 subtracts the background phase image 34 from the phase map 24 to create a true phase image 36. In an alternative embodiment, the phase correction unit 30 uses a multiplication of a complex conjugate of the filtered background phase image with the phase map 24. The image correction unit 32 corrects the magnitude image 22 with the true phase image 36 to create a corrected magnitude image (I_COTr) 38.

The system includes a workstation 40, which includes a user interface, and an electronic processor or electronic processing device 42, a display device 44 which displays the corrected magnitude image 38, menus, panels, and user controls, and at least one input device 46 which inputs a healthcare practitioner selections and/or commands. For example, the healthcare practitioner can select the nrPSIR image sequence from a menu displayed on the display device. The workstation 40 can be a desktop computer, a laptop, a tablet, a mobile computing device, a smartphone, and the like. The display device 44 can include a computer monitor, a touch screen, Cathode ray tube (CRT), Storage tube, Flat panel display, 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 the like. The input device 46 can be a keyboard, a mouse, a microphone, and the like.

The various units or modules 20, 26, 28, 30, and 32 are suitably embodied by an electronic data processing device, such as the electronic processor or electronic processing device 42 of the workstation 40, or by a network-based server computer operatively connected with the workstation 40 by a network 48, or so forth. The user interface is suitably embodied by the workstation 40. Moreover, the disclosed background phase estimation, reconstruction, subtraction, filter, and correction techniques are suitably implemented using a non-transitory storage medium storing instructions (e.g., software) readable by an electronic data processing device and executable by the electronic data processing device to perform the disclosed techniques.

With reference to FIGURE 2A an exemplary uncorrected magnitude image 22 is shown. The image is a reconstructed uncorrected magnitude image (I_m) of a coronal view of a subject's neck. In an uncorrected magnitude image, the dark areas represent an absence of MR signals and light areas the presence of MR signals. In FIGURE 2B, the example magnitude image is corrected with a separately generated reference image using the prior art technique discussed in the background. As described in the background, this technique uses the longer scan time, about twice as long to acquire each of a magnitude image and a reference image. FIGURE 2C shows an example corrected image (Icon) using an embodiment of the present application. The corrected image compares with the corrected image of FIGURE 2B, but was acquired using a single imaging sequence in approximately half the time. The corrected image properly displays the negative signal 50 polarity of luminal angiography as dark including smaller branch blood vessels, and positive signal 52 polarity as gray or a lighter contrast.

With reference to FIGURE 3A, an embodiment of a general phase correction data flow is shown. The magnitude image (I_m) 22 and the phase map (I_p) 24 are received from the reconstruction unit. The background phase image (Ibc_k) 34 is estimated 54 from the phase map (I_p) as described above by the background phase unit. The algorithm operates on the voxels or pixels of the phase map to create the voxels or pixels of the background phase image. The background phase image 34 is subtracted 56 from the phase map 24 by the phase correction unit to create the true phase image 36. The true phase image 36 is used by the image correction unit 32 to correct the magnitude image 22 to produce the corrected image 38. Each pixel or voxel of the true phase map corrects 58 the corresponding pixel or voxel of the magnitude image. The correction can include a rebalancing of the contrasts and/or additional filtering of all or combinations of the pixels or voxels.

With reference to FIGURE 3B, an embodiment of a background phase estimation 54 data flow is diagrammatically illustrated. The phase map (I_p) 24 is received from the reconstruction unit. Each pixel or voxel of the phase map includes a measure of phase corresponding the spatial coordinates. A homogeneous region 60 of the phase map 24 is selected as the seed of the growing algorithm. The homogeneous region(s) can be obtained by generating a gradient image of the phase map and selecting a region where the gradient is approximately zero. The selected region of the phase map with a phase 4)_reg becomes an initial portion of the background phase image 34. In an iterative process, neighboring pixels or voxels are selected 62 which have a phase φ_ηρ based on a nearness to the growing region. The phase of selected neighboring pixels or voxels are compared 64 to the phase of the growing region. In another embodiment, the comparison includes a confidence measure based on the strength of the received MR signal. The confidence measure can be used to further refine the comparison. If phase of the selected pixels or voxels are close to the phase of the growing region, for example within 90°, the pixels or voxels with the phase value are added 66 to the growing region, e.g. background phase image. If the phase of the selected pixels or voxels are not close to that of the growing region, the phase of the selected pixels or voxels are inverted and then the inverted pixels or voxels are added 68 to the growing region. The phase values of the pixels or voxels can be inverted by multiplying a phase value by o^~m. The voxels whose phase is not within the closeness criteria can be replaced with their complex conjugate and the complex conjugate added to the growing region. The iterative process continues until all the pixels or voxels of the phase image are selected 70 and either added, or inverted and then added to the background phase image.

The background phase image is filtered 72 with a low pass filter to remove noise. The filtering is selected based on the noisiness of the region being imaged and in increased for organs which are inherently high in noise and decreased for those with little noise. The filtering in another embodiment is performed during the region growing process. After filtering, the background phase image 34 is subtracted from the phase image as described in reference to FIGURE 3A. In other embodiments, different growing techniques can be performed such as selecting different seed points, averaging multiple grown regions, and the like. In other embodiments, different filtering techniques can be performed to filter the phase differences between pixels or voxels.

FIGURE 4 flowcharts one method of using an embodiment of non-reference phase-sensitive inversion recovery (nrPSIR). In a step 73, the complex MR signals are received and reconstructed into the magnitude image 22 and reconstructed into the phase map 24 or image. In a step 74, the magnitude image and phase map are received. The magnitude and phase map can be received directly from the reconstruction unit which reconstructs the magnitude image and phase map based on received MR signals of a single imaging sequence such as a Tl sequence.

The background phase image is constructed from a selected homogeneous growing region of the phase image and grown by iteratively adding neighboring pixels within a phase difference of the growing region or adding inverted pixels outside the phase difference in a step 76. For example, if a neighboring pixel with a value of 10° and the selected homogeneous region has a phase value of 115°, then the difference is more than 90°. The neighboring pixel would be inverted or flipped 180° to a phase value of 190°. The background phase image includes an estimate of the background phase for each spatial location.

The background phase image grown from the phase map is filtered in a step 78 with a low pass filter. The filtered background phase image is subtracted from the phase image in a step 80. The subtraction removes the background phase image from the phase image to produce a true phase image.

In a step 82, the true phase image is used by the image correction unit to correct the received magnitude image. The correction recovers the phase inversion of the magnitude image to extend the sensitivity of the pixels for a full range of difference phases which are combined in the uncorrected magnitude image. The corrected magnitude image is displayed on a display device 44 and/or stored in a step 84. The storing can include a storage management system such as a Picture Archiving and Communication System (PACS), a Radiology Information System (RIS), and the like.

It is to be appreciated that in connection with the particular illustrative embodiments presented herein certain structural and/or function features are described as being incorporated in defined elements and/or components. However, it is contemplated that these features may, to the same or similar benefit, also likewise be incorporated in other elements and/or components where appropriate. It is also to be appreciated that different aspects of the exemplary embodiments may be selectively employed as appropriate to achieve other alternate embodiments suited for desired applications, the other alternate embodiments thereby realizing the respective advantages of the aspects incorporated therein.

It is also to be appreciated that particular elements or components described herein may have their functionality suitably implemented via hardware, software, firmware or a combination thereof. Additionally, it is to be appreciated that certain elements described herein as incorporated together may under suitable circumstances be stand-alone elements or otherwise divided. Similarly, a plurality of particular functions described as being carried out by one particular element may be carried out by a plurality of distinct elements acting independently to carry out individual functions, or certain individual functions may be split- up and carried out by a plurality of distinct elements acting in concert. Alternately, some elements or components otherwise described and/or shown herein as distinct from one another may be physically or functionally combined where appropriate.

In short, the present specification has been set forth with reference to preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the present specification. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. That is to say, it will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications, and also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are similarly intended to be encompassed by the following claims.

Claims

CLAIMS: What is claimed is:

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

a background phase unit (26) which constructs a background phase image from a received reconstructed phase image based on growing a selected homogeneous region by adding pixels within a phase difference criteria of the homogeneous region and adding inverted pixels outside the phase difference criteria;

a phase correction unit (30) which corrects the phase image with the background phase image to create a true phase image; and

an image correction unit (32) which corrects a received reconstructed magnitude image based on the true phase image.

2. The system (1) according to claim 1, further including:

a reconstruction unit (20) which reconstructs the received magnitude image and the received phase image from magnetic resonance signals of a single magnetic resonance imaging acquisition sequence.

3. The system (1) according to either one of claims 1 and 2, wherein the phase difference criteria is 90°.

4. The system (1) according to any one of claims 1-3, further including:

a filter unit (28) which filters the constructed background phase image with a low pass filter; and

a display device (44) which displays the corrected received reconstructed magnitude image.

5. The system (1) according to any one of claims 1-4, wherein inverting the pixel of the phase image includes at least one of: changing the phase by 180°, determining a complex conjugate, or multiplying a phase value of the pixel by e^'m.

6. The system (1) according to any one of claims 1-5, wherein the selected homogeneous region is selected based on a gradient image of the phase image.

7. The system (1) according to any one of claims 1-6, wherein the correction of the background phase image includes a multiplication of a complex conjugate of the background phase image with the phase image.

8. The system (1) according to any one of claims 1-7, wherein pixels are selected for addition to the growing region based on a nearness to the growing region, and the growing include an iterative selection and addition of neighboring pixels until selection of all pixels of the phase image are selected.

9. The system (1) according to any one of claims 2-8, wherein the imaging sequence includes an inversion recovery (IR) imaging sequence.

10. The system (1) according to any one of claims 1-9, wherein the corrected received reconstructed image includes a phase-sensitive magnetic resonance image.

11. A method of magnetic resonance imaging, comprising:

receiving (74) a magnitude and a phase image reconstructed from magnetic resonance signals of a single magnetic resonance imaging sequence;

constructing (76) a background phase image from a selected at least one homogeneous growing region in the phase image and iteratively adding neighboring pixels within a phase difference criteria of the growing region or adding inverted pixels outside the phase difference criteria;

correcting (80) the phase image with the background phase image to create a true phase image; and

correcting (82) the received magnitude image based on the true phase image.

12. The method according to claim 11, wherein the phase difference criteria is 90°.

13. The method according to either one of claims 11 and 12, further including:

filtering (78) the constructed background phase image using a low pass filter; and

displaying (84) the corrected magnitude image on a display device.

14. The method according to any one of claims 1-13, wherein inverting of the pixel includes at least one of: changing the phase by 180°, determining a complex conjugate, or multiplying a phase value of the pixel by e^'m.

15. The method according to any one of claims 1-14, wherein the selected homogeneous region is selected based on a gradient image of the phase image.

16. The method according to any one of claims 11-15, wherein correcting the phase image includes a multiplication of a complex conjugate of the background phase image with the phase image.

17. The method according to any one of claims 11-16, wherein the imaging sequence includes an inversion recovery (IR) imaging sequence.

18. A non-transitory computer-readable storage medium carrying software which controls one or more electronic data processing devices to perform the method according to any one of claims 11-17.

19. An electronic data processing device configured to perform the method according to any one of claims 11-17.

20. A magnetic resonance imaging system (1), comprising:

one or more processors (42) configured to:

construct (76) a background phase image from a received reconstructed phase image based on growing a selected homogeneous region by adding pixels within a phase difference of the homogeneous region and adding inverted pixels outside the phase difference;

filter (78) the pixels of the constructed background phase image with a low pass filter;

subtract (80) the constructed background phase image from the phase image to create a true phase image;

correct (82) a received reconstructed magnitude image for phase based on the true phase image; and a display device (44) which displays (84) the corrected received reconstructed magnitude image.