SPIN ECHO MR IMAGING

Abstract

MR imaging comprising the steps of: subjecting an object (10) to an imaging sequence of RF pulses and switched magnetic field gradients (GS, GP, GM), which imaging sequence is a steady state sequence comprising a plurality of repeatedly applied acquisition blocks (21), wherein each acquisition block (21) comprises two units (22, 23) in immediate succession, namely: i) a first unit (22) starting with an excitation RF pulse radiated toward the object (10), with the duration of the first unit being an integer multiple of a given time interval T, and ii) a second unit (23) starting with a refocusing RF pulse radiated toward the object (10) and comprising a readout magnetic field gradient (GM) and a phase encoding magnetic field gradient (GP), with the duration of the second unit (23) being an integer multiple of the time interval T, acquiring one or more phase-encoded spin echo signals (31, 32) in a sequence of acquisition blocks (21), and reconstructing one or more MR images from the acquired spin echo signals (31, 32). Moreover, the invention relates to a MR device (1) and to a computer program for a MR device (1).

Claims

1. A method of magnetic resonance (MR) imaging of an object in the examination volume of a MR device, the method comprising the steps of: subjecting the object to an imaging sequence of RF pulses and switched magnetic field gradients (GS, GP, GM), which imaging sequence is a steady state sequence comprising a plurality of repeatedly applied acquisition blocks, wherein each acquisition block comprises two units in immediate succession, namely: i) a first unit starting with an excitation RF pulse radiated toward the object (10), with the duration of the first unit being an integer multiple of a given time interval T, and ii) a second unit starting with a refocusing RF pulse radiated toward the object and comprising a readout magnetic field gradient (GM) and a phase encoding magnetic field gradient (GP), with the duration of the second unit being an integer multiple of the time interval T, acquiring one or more phase-encoded spin echo signals in a sequence of the acquisition blocks being repeatedly applied, such that at least one spin echo signal is acquired in any of the first or second unit at an integer number of the time interval T after the refocusing pulse and an integer number of the time interval T before either the end of said first or second unit or next spin echo signal acquired in said first or second unit and reconstructing one or more MR images from the acquired spin echo signals.

2. The method of claim 1, wherein the first unit also includes a readout magnetic filed gradient (GM) and-temporal integrals of the magnetic field gradient (GM) applied during the first and second units correspond to each other so as to produce the spin echo signals.

3. The method of claim 1, wherein the temporal integral of the magnetic field gradient in at least one of the gradient directions is the same over each time interval T of the acquisition block.

4. The method of claim 3, wherein the temporal integral of the magnetic field gradient is the same over each time interval T of the acquisition block independently for each gradient direction, except the phase-encoding direction.

5. The method of claim 1, wherein the excitation and refocusing RF pulses each have a flip angle of 20°-90°, preferably 50°.

6. The method of claim 1, wherein the phase difference between the excitation RF pulse and the refocusing RF pulse is at least 30°.

7. The method of claim 1, wherein at least two spin echo signals are acquired during the second unit.

8. The method of claim 1, wherein the first unit further comprises a switched readout magnetic field gradient, with a gradient echo signal being acquired during the first unit.

9. The method of claim 1, wherein the flip angles and/or phases of the RF excitation and refocusing pulses are varied in the sequence of acquisition blocks.

10. The method of claim 1, wherein the repetition time of the imaging sequence, is less than 100 ms.

11. A magnetic resonance (MR) device comprising at least one main magnet coil for generating a uniform, steady magnetic field within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from an object positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit for reconstructing MR images from the received MR signals, wherein the MR device is arranged to perform the following steps: subjecting the object to an imaging sequence of RF pulses and switched magnetic field gradients (GS, GP, GM), which imaging sequence is a steady state sequence comprising a plurality of repeatedly applied acquisition blocks, wherein each acquisition block comprises two units in immediate succession, namely: i) a first unit starting with an excitation RF pulse radiated toward the object, with the duration of the first unit being an integer multiple of a given time interval T, and ii) a second unit starting with a refocusing RF pulse radiated toward the object and comprising a readout magnetic field gradient (GM) and a phase encoding magnetic field gradient (GP), with the duration of the second unit being an integer multiple of the time interval T, acquiring one or more phase-encoded spin echo signals in a sequence of the acquistion blocks being repeatedly applied, such that at least one spin echo signal is acquired in any of teh first or second unit at an integer number of the time interval T after the refocusiing pulse and an integer number of the time interval T I before either the end of said first or second unit or next spin echo signal acquired in said first or second unit and reconstructing one or more MR images from the acquired spin echo signals.

12. A computer program to be run on a magnetic resonance (MR) device, which computer program comprises instructions for: generating an imaging sequence of RF pulses and switched magnetic field gradients, which imaging sequence is a steady state sequence comprising a plurality of repeatedly applied acquisition blocks, wherein each acquisition block comprises two units in immediate succession, namely: i) a first unit starting with an excitation RF pulse radiated toward the object, with the duration of the first unit being an integer multiple of a given time interval T, and ii) a second unit starting with a refocusing RF pulse radiated toward the object and comprising a readout magnetic field gradient (GM) and a phase encoding magnetic field gradient (GP), with the duration of the second unit being an integer multiple of the time interval T, acquiring one ore more phase-encoded spin echo signals in a sequence of the acquisition blocks being repeated applied, such that at least one spin echo signal is acquired in any of teh first or second unit at an integer number of the time interval T after the refocusiing pulse and an integer number of the time interval T before either the end of said first or second unit or next spin echo signal acquired in said first or second unit and reconstructing one or more MR image from the acquired spin echo signals.

13. The method of claim 6 wherein the phase difference between the excitation RF pulse and the refocusing RF pulse is at least 50°.

14. The method of claim 10, wherein the repetition time of the imaging sequence is less than 20 ms.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

[0025] FIG. 1 schematically shows a MR device for carrying out the methods of the invention;

[0026] FIG. 2 shows a diagram illustrating the imaging sequence according to the invention;

[0027] FIG. 3 schematically shows an embodiment of the invention in which two spin echoes are generated during one acquisition block;

[0028] FIG. 4 schematically shows an embodiment of the invention in which three spin echoes are generated during one acquisition block;

[0029] FIG. 5 schematically shows an embodiment of the invention in which one gradient echo and two spin echoes are generated during one acquisition block.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] With reference to FIG. 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field B.sub.0 is created along a z-axis through an examination volume. The device further comprises a set of (1.sup.st, 2.sup.nd, and—where applicable—3.sup.rd order) shimming coils 2′, wherein the current flow through the individual shimming coils of the set 2′ is controllable for the purpose of minimizing B.sub.0 deviations within the examination volume.

[0031] A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

[0032] Most specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the examination volume. A digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a body RF coil 9 to transmit RF pulses into the examination volume. A typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the body RF coil 9.

[0033] For generation of MR images of limited regions of the body 10 by means of parallel imaging, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions. In parallel transmit applications, the array RF coils 11, 12, 13 may also be used for RF transmission, for example for the purpose of RF shimming.

[0034] The resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

[0035] A host computer 15 controls the current flow through the shimming coils 2′ as well as the gradient pulse amplifier 3 and the transmitter 7 to generate the imaging sequence of the invention. The receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

[0036] Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms, such like SENSE or SMASH. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

[0037] FIG. 2 shows a diagram illustrating an embodiment of the imaging sequence according to the invention. The magnetic field gradients GS (slice selection), GP (phase encoding), and GM (frequency encoding) are shown as a function of time t. The imaging sequence is a steady state sequence in which a basic acquisition block 21 is rapidly repeated without temporal delay between the repetitions. The acquisition block 21 comprises two units 22, 23 in immediate succession, namely a first unit 22 starting with an excitation RF pulse (not shown) radiated at point a. The duration of the first unit 22 equals the echo time TE of the depicted sequence. The second unit 23 starts with a refocusing RF pulse (not shown) at point b and comprises a readout magnetic field gradient 24 as well as a phase encoding magnetic field gradient 25. The phase encoding gradient 25 is balanced by a corresponding negative gradient pulse 26 at the end of the second unit 23. The duration of the second unit 23 is twice a given time interval T (which equals the echo time TE) in the depicted embodiment. The repetition time of the imaging sequence is the combined duration of the first and second units 22, 23, which is three times the time interval T. The phase-encoding is varied from repetition to repetition of the acquisition block 21 and correspondingly phase-encoded spin echo signals are acquired in the sequence of acquisition blocks 21. The interval during which MR signal acquisition takes place is designated by 27 in FIG. 2. Additional gradients 28, 28′, 29, 29′ are applied to suppress FID signals. Further, a gradient 30 is applied during the first unit 22. The temporal integral of the magnetic field gradient GM over the first half of the second unit 23 equals the temporal integral of the magnetic field gradient GM over the first unit 22. Likewise, the temporal integral of the magnetic field gradient GM over the second half of the second unit 23 equals the temporal integral of the magnetic field gradient GM over the first unit 22. The temporal integral of the magnetic field gradient GM is the same for each interval T, which is essential for the steady state imaging sequence of the invention because it ensures that spin echoes are formed at all integer multiples of T. This condition should be fulfilled for all gradient channels GS, GP, GM independently, wherein the variable phase-encoding magnetic field gradients are not taken into account. The phase-encoding is fully “rewound” (balanced). The temporal gradient integrals applied during the first and second units 22, 23 respectively correspond to each other such that refocusing of the spin echo signal is achieved in the middle of the second unit 23 in the depicted embodiment. Finally, a MR image is reconstructed from the spin echo signals acquired during the repetitions of the acquisition block 21.

[0038] In the variant shown in FIG. 3, two spin echoes 31, 32 are generated during the second unit 23. The inclined solid lines in the diagram schematically indicate the phase evolution of the transverse magnetization under the applied magnetic field gradients. The ratio of the durations of the first and second units 22, 23 is 1:3 in this embodiment.

[0039] In the embodiment shown in FIG. 4, the ratio of the durations of the first and second units 22, 23 is 2:3 such that an additional spin echo 41 is generated in the middle of the first unit 22.

[0040] In the embodiment shown in FIG. 5, the ratio of the durations of the first and second units 22, 23 is again 1:2 like in the embodiment of FIG 2. However, the magnetic field gradient GM is inverted during the first unit 22 such that a gradient echo 51 is generated.

[0041] Other combinations of durations of first and second units 22, 23 are conceivable, as long as the durations of the first unit 22 and the second unit 23 are integer multiples of the time interval T respectively. A further condition is that the temporal integral of the magnetic field gradient causing dephasing and rephasing of the transverse magnetization is the same over each interval T.