WO2008132686A1

WO2008132686A1 - Quantification for mr parameters such as t1 or t2 in a sub-region of a subject

Info

Publication number: WO2008132686A1
Application number: PCT/IB2008/051622
Authority: WO
Inventors: Christian Stehning; Hannes Dahnke
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-04-27
Filing date: 2008-04-28
Publication date: 2008-11-06

Abstract

The practical use of existing techniques for measuring characteristic magnetic resonance (MR) parameters such as spin-lattice relaxation time ( T1 ), spin-spin relaxation time ( T2 ), proton density ( p ), T2*, etc., is limited by the extended periods of time required for the measurement. It is thus desirable to have a method of measuring or quantifying characteristic MR parameters in a shorter period of time. Accordingly, a method of quantifying at least one characteristic MR parameter in a subject is hereby disclosed. The method comprises identifying (103) a region to be scanned in the subject, defining (105) a sub-region within the identified region, acquiring (107) MR data from the defined sub-region, and processing (109) the acquired MR data to quantify the at least one characteristic parameter in the defined sub-region.

Description

QANTIFICATION FOR MR PARAMETERS SUCH AS Tl OR T2 IN A SUB-REGION OF A SUBJECT

FIELD OF THE INVENTION

The invention relates to the field of magnetic resonance (MR), particularly to measurement of characteristic MR parameters.

BACKGROUND OF THE INVENTION

The publication entitled "Rapid T₁ mapping using multislice echo planar imaging", by Stuart Clare and Peter Jezzard in the journal Magnetic Resonance in Medicine (2001), volume 45, issue 4, pages 630-634, describes an approach to speeding up the measurement and mapping of T₁ values in the human brain. The technique involves using echo planar imaging (EPI) together with a conventional [invert] — TI — [excite] technique, and enables a sixty-slice brain- volume to be acquired in just three minutes.

SUMMARY OF THE INVENTION

The practical use of existing techniques for measuring characteristic MR parameters such as spin-lattice relaxation time (T₁), spin-spin relaxation time (T₂), proton density ( p ),the spin-spin relaxation time in the presence of local field inhomogeneities ( T₁ ^* ), and the spin-lattice relaxation time in the rotating frame of reference, (T_x ), etc., is limited by the extended periods of time required for the measurement. As a result, artifacts caused by motion could degrade the quality or accuracy of the measurement. Also, it is difficult to reliably follow dynamic changes in the parameters due to the poor temporal resolution.

It is thus desirable to have a method of measuring or quantifying characteristic MR parameters in a shorter period of time. It is an insight of the inventors that, for most MR applications, quantification of characteristic MR parameters in a small, user-defined region of interest (ROI) is sufficient. Quantification is usually not necessary in the rest of the anatomy that is being subject to examination.

Accordingly, a method of quantifying at least one characteristic MR parameter in a subject is hereby disclosed. The method comprises identifying a region to be scanned in the subject, defining a sub-region within the identified region, acquiring MR data from the defined sub-region, and processing the acquired MR data to quantify the at least one characteristic parameter in the defined sub-region.

By confining quantification to a predefined ROI, i.e., the defined sub-region, and excluding regions outside the predefined ROI, the time required to measure or quantify one or more characteristic MR parameters is reduced. The reduction in total measurement time is a result of both the reduction in time required for acquiring the MR data as well as for processing it.

Furthermore, a computer program for quantifying at least one characteristic MR parameter in a subject is also disclosed herein. The computer program comprises instructions for identifying a region to be scanned in the subject, defining a sub-region within the identified region, acquiring MR data from the defined sub-region, and processing the acquired MR data to quantify the at least one characteristic parameter in the defined sub- region, when the computer program is run on a computer.

Furthermore, an MR system configured to quantify at least one characteristic MR parameter in a subject, the MR system comprising an identification unit for identifying a region to be scanned in the subject, a control unit for defining a sub-region within the identified region, a data acquisition unit for acquiring MR data from the defined sub-region, and a data processing unit for processing the acquired MR data to quantify the at least one characteristic MR parameter in the defined sub-region.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will be described in detail hereinafter, by way of example, on the basis of the following embodiments or implementations, with reference to the accompanying drawings, wherein: FIGURE 1 shows a flow chart of some possible implementations of the method of quantifying characteristic MR parameters, as disclosed herein;

FIGURES 2a and 2b show the top view and side view of an MR phantom, respectively;

FIGURE 3 shows a possible implementation of the disclosed method; FIGURE 4 shows a second possible implementation of the disclosed method; and

FIGURE 5 shows an MR system configured to implement an exemplary aspect of the method of quantifying characteristic MR parameters, as disclosed herein. Corresponding reference numerals when used in the various figures represent corresponding elements in the figures.

DETAILED DESCRIPTION OF EMBODIMENTS FIGURE 1 shows an implementation of the method of quantifying characteristic MR parameters as disclosed herein. Input requirements are specified in an input step 101 (DAT). Based on the input requirements, a region of a subject's anatomy that is to be scanned is identified in an identifying step 103 (dotted box labelled ID). Within the region identified in the identifying step 103, a sub-region in which the one or more characteristic MR parameters are to be measured is defined in a definition step 105 (dotted box labelled DEF). Based on the input requirements specified in the input step 101 and the sub-region defined in the definition step 105, MR data are acquired in an acquisition step 107 (ACQN). The acquired MR data are processed in a processing step 109 (PROC) to yield the measured values of the specified characteristic MR parameter or parameters, in an output step 111. The input requirements specified in the input step 101 may include the particular characteristic MR parameter or parameters to be quantified, for example, T₁ , T₂ , p , T₁ ^* , etc. The input requirements may also or alternatively be a specific location in the subject's anatomy to be monitored, e.g., a small lesion in the liver or the brain or other specific region of the brain, etc. The input requirements may also or alternatively be a change in contrast in a specific region, as would be useful in, for example, bolus tracking studies. The identification step 103 may include a number of different sub-steps, for example acquisition of MR data 103 a (ACQ) followed by a reconstruction sub-step 103r (RECON) followed by a selection sub-step 103s (SEL). For example, MR data is acquired from the abdominal region of the subject in the acquisition sub-step 103 a. The data is reconstructed into multiple slices in the reconstruction sub-step 103r, which are manually scanned by a radiologist to detect a slice that contains a suspected lesion, for example a tumor, in the selection sub-step 103s. Alternatively, the selection of the slice containing the suspected lesion may be done automatically based on pattern recognition or pattern matching techniques, etc. In another exemplary implementation, the identification step 103 may include scanning the subject using a different imaging modality, for example computed tomography (CT), positron emission tomography (PET) or X-ray imaging, in the acquisition sub-step 103a. The data obtained from the scan is reconstructed to yield images of the subject in the reconstruction sub-step 103r. The obtained images are screened to detect the portion of the subject's anatomy containing a suspected lesion and the appropriate slice of the anatomy is selected in the selection sub-step 103s. Using appropriate coordinate transformation techniques (from the CT/PET/X-ray system to the MR system), the same portion or slice of the subject's anatomy that contains the suspected lesion may now be scanned on the MR scanner.

The identification step 103 may include collecting a low-resolution scout scan to identify which part of the subject's anatomy needs to be studied more closely. Alternatively, the identification step 103 may include collecting a diagnostic T₁ -weighted or T₂ -weighted image to qualitatively assess the T₁ and/or T₂ distributions in a section of the anatomy. The identification step 103 may be repeated if necessary in order to accurately identify the region to be scanned. The repetition may be multiple runs of the same scanning sequence or may be a sequence of different scanning sequences or it could be a combination of the two. For example, a second scout scan may be acquired based on the results of the first scout scan. Alternatively, one or more low-resolution scout scans may be followed by the acquisition of a T₁ -weighted or T₂ -weighted image.

It is to be noted that the sub-steps 103a, 103r, 103s, shown in FIGURE 1 for the identification step 103 are merely by way of example; other processes and fewer or more processes may also or alternatively be performed.

In a definition step 105, a sub-region (within the region identified in the identification step 103) is defined. This sub-region forms the region of interest (ROI) in which the one or more characteristic MR parameters are to be measured. The definition of the sub-region may be done either manually or automatically. For example, the region identified in the identification step 103 is displayed on a touch-screen display, as indicated by sub-step 105dis (DIS). A radiologist or other operator delineates the desired sub-region manually from inside the identified region displayed on the screen, as indicated by sub-step 105 dm (DELN- M). The defined sub-region may be a lesion such as a tumor or an aneurysm, a region of myocardial infarct, a plaque structure, a particular section of a blood vessel such as the aorta, a specific region of the brain, etc. Alternatively, the delineation of a desired sub-region from the region identified in the identification step 103 may be done automatically as indicated by sub-step 105da (DELN-A). For example, a sub-processing unit (not shown) may use pattern matching or pattern recognition techniques to automatically delineate a sub-region from the identified region. A simple technique may be to threshold on intensity values in the identified region; of course, other, more sophisticated techniques may also be used. In an acquisition step 107, MR pulse sequences consisting of a sequence of radio-frequency (RF) pulses and gradient pulses are applied to acquire MR data that may later be processed in the processing step 109 to extract information about the characteristic MR parameters specified in the input step 101. Examples of pulse sequences that may be used in the acquisition step 107, as well as processing techniques that may be used in the processing step 109, are explained with reference to FIGURES 3 and 4 below.

FIGURE 2a shows a cylindrical MR phantom 200 containing five circular regions 202a, 202b, 202c, 202d and 202e. The five circular regions represent five cylindrical containers, each containing a liquid with a different characteristic MR parameter value, e.g., different values for T₁ and T₂ relaxation times, proton density, etc. Only one of the cylindrical containers 202a is shown in FIGURE 2b in an orthogonal view, for the sake of clarity. The dotted line 204 in both the figures indicates the region from which MR data are to be collected in the acquisition step 107.

The spatial delineation depicted by the dotted line 204 may be achieved by an appropriate pulse sequence, i.e., an appropriate order of RF pulses and selection gradient pulses, which acting together, generate an MR signal from a small, user-defined ROI while suppressing the MR signal from outside the user-defined ROI. Furthermore, the pulse sequence may also be designed to allow for measurement of the characteristic MR parameters like T₁ and T₂. Two examples of such MR pulse sequences are shown, merely by way of example, in FIGURES 3 and 4. It is furthermore possible to measure other parameters as well, either sequentially or simultaneously using an interleaved order of MR sequences. FIGURE 3 shows an exemplary implementation of the disclosed method. A two-dimensional (2D) "pencil beam" excitation pulse is applied to the phantom 200 of FIGURE 2a by applying an RF pulse 303 together with an appropriate spiral gradient waveform, applied in the physical x-y-plane. For illustration, the sinusoidal x-portion 301 of the gradient waveform, normally designated Gx in MR pulse sequence diagrams, is shown. This combination of RF and gradient pulses results in a cylindrical portion of the phantom being selectively excited, as shown by the bright circular region 304 and the bright, elongate shape 306, which are the top and side views of the region denoted by the dotted line 204 in FIGURES 2a and 2b, respectively. Since the excited region is a narrow, cylindrical region, this particular combination of RF and gradient pulses 303, 301 is called a "pencil beam" excitation.

It may be noted that other spatially selective pulse combinations such as a spectral-spatial pulse, BIR-4 pulses, or a sequence of slice-selective pulses applied along different physical axes etc., may also be used. The axes marked 'G' in FIGURE 3 represent the amplitude of the particular gradient pulses applied while the axes marked 'RF' represent the amplitude of the RF pulses applied. All the axes marked 't' indicate time and the axis marked T indicates the intensity of the MR signal received. It may be noted that the spatially selective excitation pulse may be preceded by magnetization preparation pulses, such as inversion pulses, or saturation pulses in order to facilitate simultaneous measurement of multiple characteristic MR parameters, such as T₁ ,

T₂ , T₁ ^* , and spin density p .

Following the pencil beam excitation, a selective refocusing pulse combination is employed, which consists of a combination of an RF pulse 307, preferably of 180° flip angle, and a trapezoidal gradient pulse 305 applied simultaneously. Other selective refocusing pulse combinations may also be used, e.g., multiple 90° pulses together with appropriate gradient pulses, etc. The selective refocusing pulse combination is applied orthogonal to the pencil beam excitation. For example, if the gradients 301 are applied along the physical x- and y-axes of a bore-type magnet system, then the selective refocusing pulse combination 305, 307 is applied along either the physical z-axis of the magnet system. By convention, in a bore-type (or closed) MR system, the horizontal axis that is parallel to the bore of the magnet is designated as the z-axis; the x-axis extends left-to-right of the system while the y-axis extends from top to bottom. In other words, if a human subject is positioned head-first and supine into the MR system, the SI (superior-inferior) axis would be the physical z-axis, the LR (left-right) axis would be the x-axis and the AP (anterior-posterior) axis would be the y-axis. These are, of course, only conventional representations, and adopting other coordinate systems would not deviate from the scope of the methods disclosed herein. In an open-type or gap-type MR system, the axis connecting the two magnet poles is designated as the physical z-axis. If a patient is position supine in the gap between the magnets, the left-to-right axis with respect to the patient is designated the physical y-axis while the head-to-toe axis is designated the x-axis.

In this particular implementation, the cylindrical region 306 that is excited by the pencil beam excitation pulse 301, 303 forms the identified region from within which a sub-region is to be defined. Application of the selective refocusing pulse combination 305, 307 orthogonal to the pencil beam excitation results in the excitation of a coin-shaped region represented by the circular region 308. This coin-shaped region forms the sub-region in which the characteristic MR parameters are to be measured. The dotted circle 204 is reproduced here for direct comparison with FIGURES 2a and 2b. Alternatively, other shapes may also be excited by an appropriate choice of pulse combinations. For example, a cube- shaped region could be excited by applying a slice selective excitation pulse (preferably of 90° flip angle) followed by a first orthogonal slice selective refocussing pulse (preferably of 180° flip angle), further followed by a second slice selective refocusing pulse (preferably of 180° flip angle) applied orthogonal to both the previous pulses.

Finally, a multi-echo readout comprising multiple selective refocusing pulse combinations 311, 319; 313, 323; 315, 327, interspersed with readout sequences 321, 325, 329 as shown in FIGURE 3, yields the images 312, 314, 316, respectively. By fitting an exponential curve 333 to the signal intensities in the acquired images 312, 314, 316 on a pixel-by-pixel basis, a family of curves representative of the transversal decay constant T₂ may be obtained. Alternatively, the signal intensities could be averaged in the circular region in each of the images 312, 314, 316 and fitted to an exponential curve to extract an average value for T₂ in the defined sub-region.

It may be noted that the coin-shaped sub-region defined in this particular embodiment is only an example. Other shapes are possible by modification of the RF and gradient pulse waveforms.

Furthermore, a non-selective variant of the excitation pulse may be applied, while spatially selective refocusing pulses may facilitate the localized measurement in a specific region as a further variant of the present method. Furthermore, a spatial localization of the measurement may be achieved by an array of appropriate signal reception coils with localized sensitivities, such as microcoils attached to a catheter tip, or a specific coil or coil combination out of a large coil array.

FIGURE 4 shows another exemplary implementation of the disclosed method, in which an MR sequence is used to measure the longitudinal relaxation time T₁ in a cylindrical ROI in the MR phantom shown in FIGURES 2a and 2b. In this specific implementation, a non-selective preparatory RF pulse 403 is applied. No gradient pulse is applied in this particular case, as shown by item 401. The preparatory pulse may be, for example, an inversion pulse (180° flip angle) or a saturation pulse. The preparatory pulse may alternatively be a series of inversion or saturation pulses. Alternative to non-selective preparatory pulses, it is possible to use selective preparatory pulses as well, in which case the RF pulse 403 would be applied together with a gradient pulse.

Following a delay AT , a 2D selective excitation beam consisting of an RF pulse 407 together with a sinusoidal gradient pulse 405 (or pencil beam excitation) as described with reference to FIGURE 3 is applied. The pencil beam excitation 405, 407 excites a cylindrical region as explained earlier. This cylindrical region forms the identified region from within which a sub-region is to be defined.

Following the pencil beam excitation, a selective refocusing pulse combination is employed, which consists of an RF pulse 411, preferably of 180° flip angle, and a trapezoidal gradient pulse 409 applied simultaneously, in a direction orthogonal to the direction of the pencil beam excitation, which results in the excitation of a coin-shaped region as explained with reference to FIGURE 3. This coin- shaped region forms the sub- region in which the characteristic MR parameters are to be measured. The final data acquisition is indicated by the acquisition block 413 (AQ); the data so acquired may be processed to generate images of the coin-shaped sub-region. By varying the delay AT and acquiring data multiple times, a series of datasets containing information about the distribution of T₁ -values in the coin-shaped sub-region is obtained. This is represented in the various images 410, 412, 414, 416, 418 and 420 as varying intensity values in the circular region from image-to-image. As an inversion pulse has been applied as the preparatory pulse, the intensity values of the pixels in the images starts from a negative maximum, crosses zero and reaches a positive maximum. If saturation pulses are used as preparatory pulses instead, the intensity values would start from zero and increase to a positive maximum. By fitting a mathematical model, for example an increasing exponential curve

425, to the signal intensities in the acquired images 410, 412, 414, 416, 418, 420, etc., on a pixel-by-pixel basis, a family of curves representative of the longitudinal decay constant T₁ may be obtained. Alternatively, the signal intensities may be averaged in the circular region in each of the images 410, 412, 414, 416, 418, 420, etc., and fitted to an exponentially increasing curve to extract an average value for T₁ in the defined sub-region.

In the various implementations mentioned above, only one characteristic MR parameter is being measured at one time. However, it is to be noted that multiple characteristic MR parameters may also be measured at the same time. For example, an RSLQ sequence [Andersen et ah, Magnetic Resonance in Medicine (1994), Volume 12, Pages 775- 784] may be used for simultaneous measurement of T₁ , T₂ and p . Other pulse sequences capable of measuring individual or multiple characteristic MR parameters may also be used.

The proposed methods may be performed as part of a regular patient examination. An example of a complete MR procedure may be as follows: After a patient is position properly in the MR scanner, a "scout scan" is performed to gain an overview of the anatomy. Contrast agents or targeted drugs or labelled cells are administered to the patient, if necessary. A T₁ -weighted or T₂ -weighted scan is performed to qualitatively assess the distribution of T₁ and T₂ in the volume being examined. For example, from the acquired images, areas of diagnostic or therapeutic interest may be found, as the signal intensity in those regions would be different (normally increased) due to a shortening of the longitudinal or transverse relaxation times, as a result of accumulation of the contrast agent or labelled cells, etc. These areas of increased signal intensity form the defined sub-regions in which the characteristic MR parameters are to be measured. By measuring the T₁ and T₂ values in these regions, it is possible to quantify the amount of contrast agent in the sub-region. Alternatively, a knowledge of the T₁ and T₂ values could allow for the characterization of native tissue (necrotic vs. viable) or plaque structures, for measuring oxygenation levels in regions of the brain during a functional MR imaging (fMRI) examination or in the lower extremities in diabetes patients, for quantification of contrast agent concentrations during a contrast-enhanced MR examination, etc. Furthermore, the characteristic MR parameters are independent of the employed MR sequence, which allows for a comparison with follow-up studies performed at a later point in time. Alternatively, a series of measurements could be performed sequentially to track changes dynamically during a single examination. In summary, confining measurement of characteristic MR parameters to a small, user-defined ROI reduces the total measurement time significantly, since the scan or acquisition time is approximately proportional to the size of the imaged volume and the data processing time is dependent on the amount of data. The reduced measurement time improves the temporal resolution of the measurement, which in turn facilitates better tracking of dynamic processes. Additionally, patient comfort is enhanced, often resulting in increased clinical throughput.

FIGURE 5 shows a possible embodiment of an MR system capable of implementing the methods disclosed herein. The MR system includes a data acquisition unit that comprises a set of main coils 501, multiple gradient coils 502 connected to a gradient driver unit 506, and RF coils 503 connected to an RF coil driver unit 507. The function of the RF coils 503, which may be integrated into the magnet in the form of a body coil, or may be separate surface coils, is further controlled by a transmit/receive (T/R) switch 513. The multiple gradient coils 502 and the RF coils are powered by a power supply unit 512. A transport system 504, for example a patient table, is used to position a subject 505, for example a patient, within the MR imaging system. A control unit 508 controls the RF coils 503 and the gradient coils 502. The control unit 508, though shown as a single unit, may be implemented as multiple units as well. The control unit 508 further controls the operation of a reconstruction unit 509. The control unit 508 also controls a display unit 510, for example a monitor screen or a projector, a data storage unit 515, and a user input interface unit 511, for example, a keyboard, a mouse, a trackball, etc.

The main coils 501 generate a steady and uniform static magnetic field, for example, of field strength IT, 1.5T or 3T. The disclosed methods may be employed at other field strengths as well. The main coils 501 are arranged in such a way that they typically enclose a tunnel-shaped examination space, into which the subject 505 may be introduced. Another common configuration comprises opposing pole faces with an air gap in between them into which the subject 505 may be introduced by using the transport system 504. To enable MR imaging, temporally variable magnetic field gradients superimposed on the static magnetic field are generated by the multiple gradient coils 502 in response to currents supplied by the gradient driver unit 506. The power supply unit 512, fitted with electronic gradient amplification circuits, supplies currents to the multiple gradient coils 502, as a result of which gradient pulses (also called gradient pulse waveforms) are generated. The control unit 508 controls the characteristics of the currents, notably their strengths, durations and directions, flowing through the gradient coils to create the appropriate gradient waveforms. The RF coils 503 generate RF excitation pulses in the subject 505 and receive MR signals generated by the subject 505 in response to the RF excitation pulses. The RF coil driver unit 507 supplies current to the RF coil 503 to transmit the RF excitation pulse, and amplifies the MR signals received by the RF coil 503. The transmitting and receiving functions of the RF coil 503 or set of RF coils are controlled by the control unit 508 via the T/R switch 513. The T/R switch 513 is provided with electronic circuitry that switches the RF coil 503 between transmit and receive modes, and protects the RF coil 503 and other associated electronic circuitry against breakthrough or other overloads, etc. The characteristics of the transmitted RF excitation pulses, notably their strength and duration, are controlled by the control unit 508.

It is to be noted that though the transmitting and receiving coil are shown as one unit in this embodiment, it is also possible to have separate coils for transmission and reception, respectively. It is further possible to have multiple RF coils 503, such as an array of RF coils, for transmitting or receiving or both. The RF coils 503 may be integrated into the magnet in the form of a body coil, or may be separate surface coils. They may have different geometries, for example, a birdcage configuration or a simple loop configuration, etc. The control unit 508 is preferably in the form of a computer that includes a processor, for example a microprocessor. The control unit 508 controls, via the T/R switch 513, the application of RF pulse excitations and the reception of MR signals comprising echoes, free induction decays, etc. User input interface devices 511 like a keyboard, mouse, touch- sensitive screen, trackball, etc., enable an operator to interact with the MR system.

The MR signal received with the RF coils 503 contains the actual information concerning the local spin densities in a region of interest of the subject 505 being imaged. Depending on the pulse sequences employed, the MR signal may also contain information about the T₁ and T₂ relaxation times in the defined sub-region. The received signals may be reconstructed by the reconstruction unit 509, and displayed on the display unit 510 as an MR image or an MR spectrum. It is alternatively possible to store the signal from the reconstruction unit 509 in a storage unit 515, while awaiting further processing. The reconstruction unit 509 is constructed advantageously as a digital image-processing unit that is programmed to derive the MR signals received from the RF coils 503.

An identification unit, which may be part of the control unit 508, processes the appropriate MR data received from the RF coils in order to identify the region of the subject 505 that contains the sub-region in which the characteristic MR parameters are to be measured. A data processing unit, which may also be part of the control unit 508, processes the appropriate MR data acquired from the RF coils 503 to quantify the characteristic MR parameter or parameters of interest, in the defined sub-region. Alternatively, the identification and data processing units may be separate units (not shown) from the control unit. Furthermore, in embodiments providing an array of RF coils, a coil-selection unit is provided to enable the selection of one or more RF coils from the array of RF coils. The coil-selection unit may also be part of the control unit 508 or may be a separate unit (not shown).

The control unit 508 is capable of loading and running a computer program comprising instructions that, when executed on the computer, enables the computer to execute the various aspects of the methods disclosed herein. The computer program disclosed herein may reside on a computer readable medium, for example a CD-ROM, a DVD, a floppy disk, a memory stick, a magnetic tape, or any other tangible medium that is readable by the computer. The computer program may also be a downloadable program that is downloaded, or otherwise transferred to the computer, for example via the Internet. The transfer means may be an optical drive, a magnetic tape drive, a floppy drive, a USB or other computer port, an Ethernet port, etc.

The order in the described embodiments of the disclosed methods is not mandatory. A person skilled in the art may change the order of steps or perform steps concurrently using threading models, multi-processor systems or multiple processes without departing from the disclosed concepts.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The disclosed method can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the system claims enumerating several means, several of these means can be embodied by one and the same item of computer readable software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method of quantifying at least one characteristic magnetic resonance parameter in a subject, comprising: identifying a region to be scanned in the subject (103); defining a sub-region within the identified region (105); acquiring magnetic resonance data from the defined sub-region (107); and processing the acquired magnetic resonance data (109) to quantify the at least one characteristic parameter in the defined sub-region.

2. A method as claimed in claim 1, wherein more than one characteristic magnetic resonance parameter is quantified at the same time.

3. A method as claimed in claim 1, wherein the at least one characteristic magnetic resonance parameter is selected from a group consisting of spin-lattice relaxation time ( T₁ ), spin-spin relaxation time ( T₂ ), proton density ( p ), the spin-spin relaxation time in the presence of local field inhomogeneities ( T₁ ^* ), and the spin-lattice relaxation time in the rotating frame of reference, (T₁ ^').

4. A method as claimed in claim 1, wherein identifying the region to be scanned includes: acquiring magnetic resonance data (103a) from the subject; reconstructing one or more magnetic resonance images from the acquired data (103r); and selecting a target image containing a region of interest from the reconstructed magnetic resonance images (103s).

5. A method as claimed in claim 4, wherein defining the sub-region includes: displaying the target image (105dis); and manually delineating the region of interest in the target image (105dm).

6. A method as claimed in Claim 4, wherein defining the sub-region includes automatically delineating the region of interest in the target image (105da).

7. A computer program for quantifying at least one characteristic magnetic resonance parameter in a subject, the computer program comprising instructions for: identifying a region to be scanned in the subject; defining a sub-region within the identified region; acquiring magnetic resonance data from the defined sub-region; and processing the acquired magnetic resonance data to quantify the at least one characteristic parameter in the defined sub-region, when the computer program is run on a computer.

8. A magnetic resonance system configured to quantify at least one characteristic magnetic resonance parameter in a subject, the magnetic resonance system comprising: - an identification unit (508) for identifying a region to be scanned in the subject; a control unit (508) for defining a sub-region within the identified region; a data acquisition unit for acquiring magnetic resonance data from the defined sub-region; and - a data processing unit (508) for processing the acquired magnetic resonance data to quantify the at least one characteristic MR parameter in the defined sub-region.

9. A magnetic resonance system as claimed in claim 8, including: an array of radio -frequency receiver coils configured to receive magnetic resonance signals from the subject; and a coil- selection unit configured to select one or more radio-frequency receiver coils from the array of radio-frequency receiver coils, when the magnetic resonance system is in operation.