MAGNETIC RESONANCE FINGERPRINTING USING A SPIN-ECHO PULSE SEQUENCE WITH AN ADDITIONAL 180 DEGREE PULSE

Abstract

The invention provides for a magnetic resonance system (100) for acquiring a magnetic resonance data from a subject (118) within a measurement zone (108) according to a magnetic resonance fingerprinting technique. The pulse sequence comprises a train of pulse sequence repetitions (302, 304). Each pulse sequence repetition has a repetition time chosen from a distribution of repetition times. Each pulse sequence repetition comprises a radio frequency pulse (306) chosen from a distribution of radio frequency pulses. The distribution of radio frequency pulses cause magnetic spins to rotate to a distribution of flip angles, and each pulse sequence repetition comprises a sampling event (310) at a sampling time chosen from a distribution of sampling times. Each pulse sequence repetition of the pulse sequence comprises a first 180 degree RF pulse (308) performed at a first temporal midpoint between the radio frequency pulse and the sampling event to refocus the magnetic resonance signal. Each pulse sequence repetition of the pulse sequence comprises a second 180 degree RF pulse (309) performed at a second temporal midpoint between the sampling event and the start of the next pulse repetition.

Claims

1. A magnetic resonance system for acquiring a magnetic resonance data from a subject within a measurement zone, wherein the magnetic resonance system comprises: a memory for storing machine executable instructions, and pulse sequence instructions, wherein the pulse sequence instructions cause the magnetic resonance system to acquire the magnetic resonance data according to a magnetic resonance fingerprinting technique, wherein the pulse sequence instructions comprises a train of pulse sequence repetitions, wherein each pulse sequence repetition has a repetition time chosen from a distribution of repetition times, wherein each pulse sequence repetition comprises a radio frequency pulse chosen from a distribution of radio frequency pulses, wherein the distribution of radio frequency pulses cause magnetic spins to rotate to a distribution of flip angles, and wherein each pulse sequence repetition comprises a sampling event where the magnetic resonance signal is sampled for a predetermined duration at a sampling time before the end of the pulse sequence repetition, wherein the sampling time is chosen from a distribution of sampling times, wherein the magnetic resonance data is acquired during the sampling event, wherein each pulse sequence repetition of the pulse sequence instructions comprises a first 180 degree RF pulse performed at a first temporal midpoint between the radio frequency pulse and the sampling event to refocus the magnetic resonance signal, and wherein each pulse sequence repetition of the pulse sequence instructions comprises a second 180 degree RF pulse performed at a second temporal midpoint between the sampling event and the start of the next pulse repetition in order to reduce the effect of inhomogeneities in the magnetic field used in the measurement zone; a processor for controlling the magnetic resonance system, wherein execution of the machine executable instructions causes the processor to: acquire the magnetic resonance data by controlling the magnetic resonance system with pulse sequence instructions; and calculate the abundance of each of a set of predetermined substances by comparing the magnetic resonance data with a magnetic resonance fingerprinting dictionary, wherein the magnetic resonance fingerprinting dictionary contains a listing of calculated magnetic resonance signals in response to execution of the pulse sequence instructions for a set of predetermined substances.

2. The magnetic resonance system of claim 1, wherein the magnetic resonance system further comprises, wherein the magnetic resonance system is a magnetic resonance imaging system, wherein the measurement zone is an imaging zone: a magnet for generating a main magnetic field within the measurement zone; a magnetic field gradient system for generating a gradient magnetic field within the measurement zone to spatially encode the magnetic resonance data; and wherein the pulse sequence instructions further comprises instructions to control the magnetic field gradient system to for performing spatial encoding of the magnetic resonance data during acquisition of the magnetic resonance data, wherein the spatial encoding divides the magnetic resonance data into discrete voxels.

3. The magnetic resonance system of claim 2, wherein execution of the machine executable instructions further causes the processor to calculate the magnetic resonance fingerprinting dictionary by modeling each of the predetermined substances as a single spin with the Bloch equations for each of the discrete voxels.

4. The magnetic resonance system of claim 2, wherein the spatial encoding is one-dimensional, wherein the discrete voxels are a set of discrete slices, wherein the method further comprises the step of dividing the magnetic resonance data into the set of slices, wherein the abundance of each of a set of predetermined substances is calculated within each of the set of slices by comparing the magnetic resonance data for each of the set of slices with the magnetic resonance fingerprinting dictionary.

5. The magnetic resonance system of claim 4, wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a constant magnetic field gradient in a predetermined direction during the execution of the pulse sequence.

6. The magnetic resonance system of claim 4, wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a one dimensional readout gradient at least partially during the sampling event.

7. The magnetic resonance system of claim 2, wherein the spatial encoding is three dimensional, wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a three dimensional readout gradient least partially during the sampling event.

8. The magnetic resonance system of claim 2, wherein the spatial encoding is performed as multislice encoding, wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a slice selecting gradient during the radio frequency pulse, wherein the spatial encoding is further performed by controlling the magnetic field gradient system to produce a phase selection gradient or a slice selection gradient during the first 180 degree RF pulse, and wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a readout gradient during the sampling event.

9. The magnetic resonance system of claim 2, wherein the spatial encoding is performed as non-Cartesian spatial encoding, wherein the spatial encoding is performed by controlling the magnetic field gradient system to produce a readout gradient during the sampling event which samples k-space in a non-Cartesian order.

10. The magnetic resonance system of claim 1, wherein the magnetic resonance system is a NMR spectrometer, wherein execution of the machine executable instructions further causes the processor to calculate the magnetic resonance fingerprinting dictionary by modeling each of the predetermined substances as a single spin with the Bloch equations for each of the discrete voxels.

11. The magnetic resonance system of claim 1, wherein the calculation of the abundance of each of the predetermined tissue types within each of discrete voxels by comparing the magnetic resonance data for each of the discrete voxels with the pre-calculated magnetic resonance fingerprinting dictionary is performed by: expressing each magnetic resonance signal of the magnetic resonance data as a linear combination of the signal from each of the set of predetermined substances, and determining the abundance of each of the set of predetermined substances by solving the linear combination using a minimization technique.

12. The magnetic resonance system of claim 1, wherein execution of the instructions further causes the processor to repeat measurement of the magnetic resonance data of at least one calibration phantom, wherein the at least one calibration phantom comprises a known volume of at least one of the set of predetermined substances.

13. A computer program product storing machine executable instructions and pulse sequence instructions for execution by a processor for controlling a magnetic resonance system for acquiring magnetic resonance data from a subject within a measurement zone, wherein the pulse sequence instructions cause the magnetic resonance system to acquire the magnetic resonance data according to a magnetic resonance fingerprinting technique, wherein the pulse sequence instructions comprises a train of pulse sequence repetitions, wherein each pulse sequence repetition has a repetition time chosen from a distribution of repetition times, wherein each pulse sequence repetition comprises a radio frequency pulse chosen from a distribution of radio frequency pulses, wherein the distribution of radio frequency pulses cause magnetic spins to rotate to a distribution of flip angles, and wherein each pulse sequence repetition comprises a sampling event where the magnetic resonance signal is sampled for a predetermined duration at a sampling time before the end of the pulse sequence repetition, wherein the sampling time is chosen from a distribution of sampling times, wherein the magnetic resonance data is acquired during the sampling event, wherein each pulse sequence repetition of the pulse sequence instructions comprises a first 180 degree RF pulse performed at a first temporal midpoint between the radio frequency pulse and the sampling event to refocus the magnetic resonance signal, and wherein each pulse sequence repetition of the pulse sequence instructions comprises a second 180 degree RF pulse performed at a second temporal midpoint between the sampling event and the start of the next pulse repetition in order to reduce the effect of inhomogeneities in the magnetic field used in the measurement zone, wherein execution of the machine executable instructions causes the processor to: acquire the magnetic resonance data by controlling the magnetic resonance system with pulse sequence instructions, and calculate the abundance of each of a set of predetermined substances by comparing the magnetic resonance data with a magnetic resonance fingerprinting dictionary, wherein the magnetic resonance fingerprinting dictionary contains a listing of calculated magnetic resonance signals in response to execution of the pulse sequence instructions for a set of predetermined substances.

14. A method of operating a magnetic resonance system for acquiring magnetic resonance data from a subject within a measurement zone, wherein the magnetic resonance system comprises: a memory for storing pulse sequence instructions, wherein the pulse sequence instructions cause the magnetic resonance system to acquire the magnetic resonance data according to a magnetic resonance fingerprinting technique, wherein the pulse sequence instructions comprises a train of pulse sequence repetitions, wherein each pulse sequence repetition has a repetition time chosen from a distribution of repetition times, wherein each pulse sequence repetition comprises a radio frequency pulse chosen from a distribution of radio frequency pulses, wherein the distribution of radio frequency pulses cause magnetic spins to rotate to a distribution of flip angles, and wherein each pulse sequence repetition comprises a sampling event where the magnetic resonance signal is sampled for a predetermined duration at a sampling time before the end of the pulse sequence repetition, wherein the sampling time is chosen from a distribution of sampling times, wherein the magnetic resonance data is acquired during the sampling event, wherein each pulse sequence repetition of the pulse sequence instructions comprises a first 180 degree RF pulse performed at a first temporal midpoint between the radio frequency pulse and the sampling event to refocus the magnetic resonance signal, and wherein each pulse sequence repetition of the pulse sequence instructions comprises a second 180 degree RF pulse performed at a second temporal midpoint between the sampling event and the start of the next pulse repetition in order to reduce the effect of inhomogeneities in the magnetic field used in the measurement zone; wherein the method comprises the steps of: acquiring the magnetic resonance data by controlling the magnetic resonance system with pulse sequence instructions; and calculating the abundance of each of a set of predetermined substances by comparing the magnetic resonance data with a magnetic resonance fingerprinting dictionary, wherein the magnetic resonance fingerprinting dictionary contains a listing of calculated magnetic resonance signals in response to execution of the pulse sequence instructions for a set of predetermined substances.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In the following preferred embodiments of the invention will be described, by way of example only, and with reference to the drawings in which:

[0055] FIG. 1 illustrates an example of a magnetic resonance imaging system;

[0056] FIG. 2 illustrates a method of operating the magnetic resonance imaging system of FIG. 1;

[0057] FIG. 3 illustrates an example of a pulse sequence; and

[0058] FIG. 4 illustrates a further example of a pulse sequence.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0059] 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.

[0060] FIG. 1 shows an example of a magnetic resonance imaging system 100 with a magnet 104. The magnet 104 is a superconducting cylindrical type magnet 104 with a bore 106 through it. The use of different types of magnets is also possible; for instance it is also possible to use both a split cylindrical magnet and a so called open magnet. A split cylindrical magnet is similar to a standard cylindrical magnet, except that the cryostat has been split into two sections to allow access to the iso-plane of the magnet, such magnets may for instance be used in conjunction with charged particle beam therapy. An open magnet has two magnet sections, one above the other with a space in-between that is large enough to receive a subject: the arrangement of the two sections area similar to that of a Helmholtz coil. Open magnets are popular, because the subject is less confined. Inside the cryostat of the cylindrical magnet there is a collection of superconducting coils. Within the bore 106 of the cylindrical magnet 104 there is an imaging zone 108 where the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

[0061] Within the bore 106 of the magnet there is also a set of magnetic field gradient coils 110 which is used for acquisition of magnetic resonance data to spatially encode magnetic spins within the imaging zone 108 of the magnet 104. The magnetic field gradient coils 110 connected to a magnetic field gradient coil power supply 112. The magnetic field gradient coils 110 are intended to be representative. Typically magnetic field gradient coils 110 contain three separate sets of coils for spatially encoding in three orthogonal spatial directions. A magnetic field gradient power supply supplies current to the magnetic field gradient coils. The current supplied to the magnetic field gradient coils 110 is controlled as a function of time and may be ramped or pulsed.

[0062] Adjacent to the imaging zone 108 is a radio-frequency coil 114 for manipulating the orientations of magnetic spins within the imaging zone 108 and for receiving radio transmissions from spins also within the imaging zone 108. The radio frequency antenna may contain multiple coil elements. The radio frequency antenna may also be referred to as a channel or antenna. The radio-frequency coil 114 is connected to a radio frequency transceiver 116. The radio-frequency coil 114 and radio frequency transceiver 116 may be replaced by separate transmit and receive coils and a separate transmitter and receiver. It is understood that the radio-frequency coil 114 and the radio frequency transceiver 116 are representative. The radio-frequency coil 114 is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Likewise the transceiver 116 may also represent a separate transmitter and receivers. The radio-frequency coil 114 may also have multiple receive/transmit elements and the radio frequency transceiver 116 may have multiple receive/transmit channels.

[0063] The subject support 120 is attached to an optional actuator 122 that is able to move the subject support and the subject 118 through the imaging zone 108. In this way a larger portion of the subject 118 or the entire subject 118 can be imaged. The transceiver 116, the magnetic field gradient coil power supply 112 and the actuator 122 are all see as being connected to a hardware interface 128 of computer system 126. The computer storage 134 is shown as containing pulse sequence instructions 140 for performing a magnetic resonance fingerprinting technique.

[0064] The pulse sequence instructions comprise a train of pulse sequence repetitions. Each pulse sequence repetition has a repetition time chosen from a distribution of repetition times. Each pulse sequence repetition comprises a radio-frequency pulse chosen from a distribution of radio-frequency pulses. The distribution of radio-frequency pulses may be used to cause magnetic resonance spins to rotate to a distribution of different flip angles. The different radio-frequency pulses for instance may use a different amplitude, duration or shape to cause a particular magnetic spin to rotate to a particular or different flip angle. The different radio-frequency pulses may have a different effect on different types of magnetic spins and cause them to rotate to different distributions of flip angles. Each pulse sequence repetition further comprises a sampling event where the magnetic resonance signal is sampled for a predetermined duration at a sampling time before the end of the pulse sequence repetition. The sampling time is chosen from a distribution of sampling times. The magnetic resonance data is acquired during the sampling event. Each pulse sequence repetition of the pulse sequence instructions comprises a first 180° radio-frequency pulse performed at a first temporal midpoint between the radio-frequency pulse and the sampling event to refocus the magnetic resonance signal. Each pulse sequence repetition of the pulse sequence instructions comprises a second 180° radio-frequency pulse performed at a second temporal midpoint between the sampling event and the start of the next pulse repetition. The computer storage 134 is further shown as containing magnetic resonance data 142 that was acquired using the pulse sequence instructions 140 to control the magnetic resonance imaging system 100. The computer storage 134 is further shown as containing a magnetic resonance fingerprinting dictionary 144. The computer storage is further shown as containing a magnetic resonance image 146 that was reconstructed using the magnetic resonance data 142 and the magnetic resonance fingerprinting dictionary 144.

[0065] The computer memory 136 contains a control module 150 which contains such code as operating system or other instructions which enables the processor 130 to control the operation and function of the magnetic resonance imaging system 100.

[0066] The computer memory 136 is further shown as containing a magnetic resonance fingerprint dictionary generating module 152. The fingerprint generating module 152 may model one or more spins using the Bloch equation for each voxel to construct the magnetic resonance fingerprinting dictionary 144. The computer memory 136 is further shown as containing an image reconstruction module that uses the magnetic resonance data 142 and the magnetic resonance fingerprinting dictionary 144 to reconstruct the magnetic resonance image 146. For example the magnetic resonance image 146 may be a rendering of the spatial distribution of one or more of the predetermined substances within the subject 118.

[0067] The example of FIG. 1 could be modified so that the magnetic resonance imaging system or apparatus 100 is equivalent to a Nuclear Magnetic Resonance (NMR) spectometer. Without gradient coils 110 and the gradient coil power supply 112 the apparatus 100 would perform a 0-dimensional measuremetn in the imaging zone 108. FIG. 2 shows a flowchart which illustrates a method of operating the magnetic resonance imaging system 100 of FIG. 1. First in step 200 the magnetic resonance data 142 is acquired by controlling the magnetic resonance imaging system with the pulse sequence instructions 140. Next in step 202 the abundance of each of the set of the predetermined substances is calculated by comparing the magnetic resonance data 142 with the magnetic resonance fingerprinting dictionary 144. The abundance for instance may be plotted or displayed in the magnetic resonance image 146.

[0068] Magnetic Resonance (MR) fingerprinting is a new and very promising technique for the determination of tissue types by comparison of an MR measurement to a number of pre-calculated dictionary entries.

[0069] This invention builds upon the idea of MR fingerprinting in combination with an MR of scanner of reduced complexity and dedicated sequences and reconstruction algorithms to open up new opportunities for very efficient cancer screening or quantitative large-volume measurements.

[0070] Magnetic resonance fingerprinting has a high potential for accurate tissue characterization. Still, the current technique is based on a voxel-wise analysis of MR images and therefore is both time-consuming and expensive.

[0071] Some examples may provide for a way to efficiently detect and quantify the existence of specific tissue types while:

1. Reducing hardware cost and energy consumption
2. Increasing patient throughput

[0072] This may enable new applications for early cancer detection or for body fat quantification.

[0073] Examples may possibly have one or more of the following features:

1. An MRI system with reduced hardware requirements: Low-performance x- and y-coils are possible; these coils may even be left out completely (a z-gradient coil can be designed to be very efficient).
2. A dedicated image acquisition sequence for B0-independent magnetic resonance fingerprinting
3. A dedicated reconstruction algorithm which determines relative and absolute volumes of different tissue types
4. A display device to visualize the findings

[0074] Instead of producing and analyzing medical images based on voxels, some example methods described here yields a tissue component analysis of a whole z-slice. A single dedicated fingerprint measurement (duration of a few seconds) is performed without employing in-plane (x, y) gradients. The tissue composition of the whole slice and the relative abundance of the tissue components are determined automatically from the resulting signal.

[0075] The MR sequence to be used preferably fulfills two requirements: First, it is sensitive to tissue-specific parameters (e.g. T1 and T2 values, others are conceivable, too) to encode the tissues of interest and allow quantitative tissue characterization by matching the measured signal against a dictionary (MR fingerprinting). Second, the signal is independent of non-tissue specific parameter variations (e.g. B.sub.0 variations), so that matching the tissue components is possible over the whole slice.

[0076] FIG. 4 illustrates one example of such a sequence, which is sensitive to T.sub.1 and T.sub.2 but independent of B.sub.0 variations. The sequence is based on a random or otherwise freely chosen list of flip angles α.sub.i and delay times t.sub.i. After the first RF pulse with flip angle α1, an echo is produced after a delay of 2t.sub.1 and the signal is recorded (ADC1). Another echo step with length 2t.sub.1b ensures that the dephasing is again eliminated before the next part of the fingerprint sequence begins with flip angle α.sub.2 and delay t.sub.2.

[0077] The additional echoes after the measurement points ADC.sub.i can be kept as short as possible with t.sub.1b=t.sub.2b= . . . A slice-selection gradient is switched on for each RF pulse using the z gradient coil.

[0078] FIG. 3 shows a portion an exemplary pulse sequence 300. The pulse sequence 300 may be used for generating or calculating the pulse sequence instructions 140. In this timing diagram a first pulse sequence repetition 302 is shown and a second pulse sequence repetition 304 is shown. Each pulse repetition begins with a radio-frequency pulse 306. The duration of the pulse repetition varies from pulse repetition to pulse repetition. There is a duration 310 where the radio-frequency signal is measured. The time between the radio-frequency pulse 306 and the measurement duration 310 is also varied as is the amplitude and/or shape of the particular radio-frequency pulses 306. This pulse sequence 300 also shows two 180° refocusing pulses 308, 309 per repetition 302, 304. The first refocusing pulse 308 is located at the temporal midpoint between the radio-frequency pulse 306 and the measurement duration 310. The second radio-frequency pulse 309 is located between the midpoint of the measurement duration 310 and the start of the next pulse 306. The first refocusing pulse 308 causes the radio-frequency signal to be refocused when the measurement 310 is made. The second refocusing pulse 309 causes the signal to be refocused when the next pulse 304 starts.

[0079] As in conventional MRF sequences, each sampling point ADCi may actually consist of a very fast series of multiple samplings of k-space. This may be Cartesian, spiral, or any other kind of k-space sampling.

[0080] The idea behind this sequence is the following: The refocusing 180-degree pulses 308, 309 ensure that at the time of the α.sub.i, pulses and at the time of the samplings ADCi, all spins are refocused. The dephasing caused by B.sub.o variations is therefore eliminated at the points in time of the α.sub.i pulses and the ADCi samplings, rendering the measured signal independent of B.sub.0. Additionally, a pre-calculation of the signal is simple when no dephasing effects need to be considered. In this case, the behavior of a single spin can be modelled, and for each time step t.sub.1, t.sub.1b, t.sub.2, t.sub.2b, etc., the evolution of the spin can be described by simple functions of the time constants T.sub.1 and T.sub.2.

[0081] The effect of using the two refocusing pulses 308 and 309 is that the effect of any inhomogeneities in the magnetic field is reduced or minimized. This may reduce the signal-to-noise in the end magnetic resonance fingerprinting chart and it also makes it easier to make the pre-calculated magnetic resonance fingerprinting dictionary. Without this compensation it may be necessary to include effects of the inhomogeneities in the calculations used to make the pre-calculated magnetic resonance fingerprinting dictionary.

[0082] With magnetic field gradients, the pulse sequence 300 illustrated in FIG. 3 may for instance be useful for a 0-dimensional measurement where the entire measurement zone or imaging zone has the data all acquired at once. The 0-dimensional measurement may for example be useful for a NMR spectrometer instead of a magnetic resonance imaging system. A more complicated pulse sequence may be constructed which includes magnetic field gradients for performing spatial encoding.

[0083] FIG. 4 shows a further example of a pulse sequence 400. In this example there are three different timelines shown. The first timeline 402 labels the radio-frequency pulse timeline. The timeline 404 shows when magnetic field gradients are applied. The third timeline labeled 406 shows when the measurements 310 are made. On the gradient timeline 404 there are three types of boxes labeled. The boxes labeled A 408, the boxes labeled B 410, and the boxes labeled C 412. The boxes labeled a 408 overlap with the radio-frequency pulses 306. The boxes labeled B overlap with the 180° radio-frequency pulses 308, 309. The boxes labeled C 412 overlap with the measurements 310. Each of the boxes represents a time period during which magnetic field gradients are set or varied according to the description of the different embodiments. In principle the radio-frequency timeline 402 can be used with most magnetic resonance techniques and k-space sampling schemes so as to enable a magnetic resonance fingerprinting technique to be applied using various magnetic resonance modalities or techniques.

[0084] For example if a constant magnetic field gradient were applied during the gradient timeline 404, there would be spatial encoding in slabs along the direction that the magnetic field gradient is applied. In another example a readout gradient may be applied only during the box C 412. For instance a one-dimensional or a three-dimensional readout gradient could be applied to obtain a one-dimensional or three-dimensional magnetic resonance fingerprint. In another example multi-slice encoding could be used. A slice-selecting gradient could be applied during the period a 408 during the radio-frequency pulse 306. The spatial encoding could further be performed by controlling the magnetic field gradient system to produce a phase or slice selection during the first 180° radio-frequency pulse 180. A readout gradient could then be applied during the time period C 412. Using the example shown in FIG. 4 it can be seen by the skilled individual how the base radio-frequency pulses illustrated in the timeline 402 could be applied in a general means to most magnetic resonance imaging sampling techniques.

[0085] The measured MR signal (a list of all the ADC.sub.i values) may be compared with the pre-calculated dictionary for all combinations of T.sub.1 and T.sub.2 to be expected in the volume. The dictionary is created by solving the Bloch equations for the fingerprinting sequence described above for different combinations of T.sub.1 and T.sub.2.

[0086] In order to determine the tissue composition of the whole slice, the signal is expressed as a (complex) linear combination of the N dictionary entries,

s=Σ.sub.k=0.sup.Na.sub.kd.sub.k

where s is the signal vector and d.sub.k are the dictionary entries. The coefficients a.sub.k≧0 are determined by the reconstruction algorithm. This is accomplished by solving the least squares problem
minimize

∥Da−s∥.sub.2

for

a.sub.k≧0

where D is the dictionary matrix with dictionary entries d.sub.k as columns and a is the vector of coefficients describing the contribution of the individual potential tissues components/tissue types to the detected signal.

[0087] Each dictionary entry is assigned to a certain tissue type. Thus, the coefficients a.sub.k yield an estimate for the relative abundance of the different tissue components in terms of the “number of spins” involved for each component.

[0088] In a further step, these relative “spin numbers” can be converted estimates of relative volumes or relative masses of the tissue components if the spin density of the different tissue types is known.

[0089] In some examples, the system does not produce spatially resolved images. The only spatial resolution is achieved in the z-direction (or other single direction) by applying the RF pulses shown in FIG. 4 in a slice selective manner. However, for each slice, the composition of tissue types is determined and can be visualized as numbers, bar graphs, etc. In the case of a multi-slice scan, the abundance of the different components can be displayed as a function of the z position.

[0090] In other examples, the system may be programmed in such a way that it alerts the operator if certain types of tissue are found (e.g., suspicious masses, potential tumors). It can also be programmed in such a way that it displays the total volume/relative abundance of specified tissues, e.g. metastases of a certain kind or fat fraction.

[0091] In one example, the MRI system contains no x or y gradient coils. Only a z gradient coil is provided.

[0092] In one example, the MRI system contains no gradient coil at all. A static z gradient is provided by a dedicated MR magnet with asymmetric windings.

[0093] In one example, a slightly higher spatial resolution, preferable in-plane, could be achieved by using spatially sensitive local reception coils, which are placed closed to the body surface.

[0094] In one example, a number of measurements are performed, while the patient table is moved stepwise automatically. In this way, a large part of the body or the whole body can be scanned.

[0095] In another example, using moving table technology, the patient is moved through a sensitive receive array (“car-wash approach”) to improve spatial resolution and SNR and to reduce costs of too many receivers.

[0096] In one example, a gauge measurement using a known volume of a known substance is performed once to determine the factor of proportionality linking the volume/mass of the substance to the value of the relative volume/mass determined by measurement. In this way, all subsequently measured relative volumes/masses can be converted to absolute tissue volumes/masses.

[0097] 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.

[0098] 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

[0099] 100 magnetic resonance system [0100] 104 magnet [0101] 106 bore of magnet [0102] 108 measurement zone or imaging zone [0103] 110 magnetic field gradient coils [0104] 112 magnetic field gradient coil power supply [0105] 114 radio-frequency coil [0106] 116 transceiver [0107] 118 subject [0108] 120 subject support [0109] 122 actuator [0110] 124 predetermined direction [0111] 125 slices [0112] 126 computer system [0113] 128 hardware interface [0114] 130 processor [0115] 132 user interface [0116] 134 computer storage [0117] 136 computer memory [0118] 140 pulse sequence instructions [0119] 142 magnetic resonance data [0120] 144 magnetic resonance fingerprinting dictionary [0121] 146 magnetic resonance image [0122] 150 control module [0123] 152 magnetic resonance fingerprint dictionary generating module [0124] 154 image reconstruction module [0125] 300 pulse sequence instructions [0126] 302 first pulse sequence repetition [0127] 304 second pulse sequence repetition [0128] 306 RF pulse [0129] 308 first 180 degree refocusing pulse [0130] 309 second 180 degree refocusing pulse [0131] 310 measurement or radio frequency signal [0132] 400 pulse sequence [0133] 402 RF pulse timeline [0134] 402 magnetic field gradient timeline [0135] 404 readout timeline [0136] 408 time period A [0137] 410 time period B [0138] 412 time period C