Quantitative contrast enhanced black-blood imaging using quadruple-inversion recovery

• US 20060184002A1
• Filed: 03/21/2006
• Published: 08/17/2006
• Est. Priority Date: 12/19/2002
• Status: Active Grant

• Current Assignee: Washington University In St Louis
• Original Assignee: University of Washington
• Inventors: Yuan, Chun, Yarnykh, Vasily L.
• Granted Patent: US 7,627,359 B2
• Time in Patent Office: Days
• US Class Current: 600/410
• CPC Class Codes: G01R 33/5601 involving use of a contrast agent for contrast manipulation, e.g. a paramagnetic, super-paramagnetic, ferromagnetic or hyperpolarised contrast agent
• CPC Class Codes: G01R 33/5607 by reducing the NMR signal of a particular spin species, e.g. of a chemical species for fat suppression, or of a moving spin species for black-blood imaging

Abstract

A reduced field-of-view (FOV) imaging technique combines suppression of signals from outer volume and inflowing blood. Both outer volume and blood suppression are achieved using an SFQIR (Small-FOV Quadruple-inversion-Recovery) preparative pulse sequence including two double-inversion pulse pairs separated by appropriate delays. Within each pair, inversion pulses are successively applied to the imaged slice and the slab orthogonal to the imaging plane, with the thickness equal to the FOV size in the phase-encoding direction. Each double-inversion results in a reinversion of the magnetization in a central part of the FOV, while outer areas of the FOV and inflowing blood remain inverted. The SFQIR module was implemented for single-slice and multislice acquisition with a fast spin-echo readout sequence. Timing parameters of the sequence corresponding to the maximal suppression efficiency can be found by niiziig variation of the normalized signal over the entire range of T₁ occurring in tissues.

Claims

1. A method for suppressing components of a magnetic resonance signal used for imaging a site in a subject, said components corresponding to undesired portions of the site, comprising the steps of:

(a) generating a sequence of radiofrequency (RF) pulses, a total number of the RF pulses being an even number at least equal to four;

(b) applying said RF pulses to the subject so that the RF pulses are grouped into a plurality of double-inversion procedures, each double-inversion procedure including a first selective RF pulse executed synchronously with a magnetic field gradient pulse targeted to select a first volume and a second selective RF pulse executed synchronously with a magnetic field gradient pulse targeted to select a second volume that crosses and is generally transverse to the first volume, wherein the first and second selective RF pulses in each double-inversion procedure are immediately adjacent to each other in time;

(c) applying a first predefined time delay between one of the first and second selective RF pulses in one procedure and a corresponding one of the first and second selective RF pulses in a successive double-inversion procedure; and

(d) for a double-inversion procedure selected to be a last double-inversion procedure before the signal used to produce an image is acquired, applying a second predefined time delay following the corresponding one of the first and the second selective RF pulses, before acquiring the signal that will be processed to produce the image of the site, the components of the signal from the undesired portions of the site being substantially suppressed when producing the image of the site as a result of applying the sequence of RF pulses that includes the first predefined time delay and the second predefined time delay.

2. The method of claim 1, wherein durations of the first predefined time delay and the second predefined time delay are optimized to provide a maximum suppression of one of:

(a) a contribution due to flowing blood at the site; and

(b) a contribution due to tissue that is outside a desired region to be imaged at the site.

3. The method of claim 2, wherein the other of the contribution to the signal due to flowing blood at the site, and the contribution to the signal due to tissue that is outside a desired region to be imaged at the site, is also substantially reduced.

4. The method of claim 1, wherein the first selective RF pulses are employed to select a slab at the site, the slab comprising the first volume.

5. The method of claim 4, wherein the second selective RF pulses are employed to select a slice at the site, the slice comprising the second volume.

6. The method of claim 4, wherein the second selective RF pulses are employed to select a plurality of slices included in another slab at the site, said other slab comprising the second volume.

7. The method of claim 1, wherein the first volume and the second volume are generally orthogonal to each other.

8. The method of claim 1, further comprising the step of determining durations of the first and the second predefined time delays by applying an optimization algorithm over a range of relaxation times T₁ for tissues at the site being imaged.

9. The method of claim 8, wherein the step of determining the durations comprises the step of employing a Gauss-Newton minimization technique in a reiterative process.

10. The method of claim 1, wherein producing the image of the site, using the signal from which undesired components of the signal have been removed, also derives at least one of the following benefits:

(a) suppresses undesired artifacts in the image;

(b) increases a resolution of the image; and

(c) decreases a time required to scan the site and produce the image for display.

11. A memory medium storing machine instructions for carrying out the steps of claim 1.

12. A method for magnetic resonance imaging a site in a body of a subject so that a contribution to an image of the site from tissue that is outside a desired region to be imaged at the site, is substantially reduced, comprising the steps of:

(a) generating at least two pairs of radio frequency (RF) pulses synchronously with magnetic field gradient pulses, each pair of RF pulses causing a double-inversion of the magnetization at the desired region;

(b) applying the at least two pairs of RF pulses to the subject as a preparative sequence of RF pulses executed prior to acquiring a signal usable for imaging the desired region, one RF pulse of each pair of RF pulses being targeted to select a first volume and the other RF pulse of each pair being targeted to select a second volume, wherein the first and second volumes extend generally transversely to each other, in a crossing relationship;

(c) applying a first time delay that was previously determined, between the RF pulses targeted at one of the first and second volumes occurring in successive pairs of the RF pulses; and

(d) applying a second time delay following the RF pulse targeted at the one of the first and second volumes in a last pair of RF pulses during the preparative sequence, before acquiring the signal for processing to produce an image of the site, the contribution to the image of the site from tissue that is outside a desired region being substantially suppressed when the signal is used to produce the image of the site, as a result of thus executing the preparative sequence.

13. The method of claim 12, further comprising the step of employing an even number of pairs of the RF pulses that occur within a repetition time being used for imaging the site.

14. The method of claim 12, wherein the first volume comprises a slab of tissue.

15. The method of claim 14, wherein the second volume comprises a slice of tissue that is substantially thinner in cross-section than the slab of tissue.

16. The method of claim 14, wherein the second volume comprises a slab of tissue that itself comprises a plurality of slices of tissue, any slice of which can be selectively imaged using the signal that was acquired.

17. The method of claim 16, further comprising the step of using the signal that was acquired to produce one of a plurality of images of selected slices from within the slab that comprises the plurality of slices, the signal that is processed to produce each image of the plurality of images being acquired after one RF pulse in the last of a pair of the RF pulses in each of a plurality of subsets of the pairs of RF pulses, where each subset of the pairs of RF pulses comprises a preparative sequence for each of the plurality of images.

18. The method of claim 12, wherein durations of the first and the second time delays are predetermined by minimizing a function that is defined for a range of relaxation times T₁ that is physiologically consistent with a variety of tissues at the site being imaged.

19. The method of claim 12, wherein a contribution to an image of the site from blood flowing through the site is also substantially reduced.

20. The method of claim 12, further comprising the step of predetermining durations of the first and the second time delays so as to substantially suppress a contribution to an image of the site, due to blood flowing through the site, while also substantially reducing the contribution to the image, due to the tissue that is outside the desired region.

21. The method of claim 12, wherein the first volume is generally orthogonal to the second volume, in forming the crossing relationship.

22. The method of claim 12, wherein durations of the first and second time delays are predetermined by employing a Gauss-Newton minimization technique in a reiterative process.

23. A preparative sequence of radio frequency (RF) pulses used in suppressing components of a magnetic resonance signal applied to image a site in a subject, said components corresponding to undesired portions of the site, comprising:

(a) at least two pairs of RF pulses, the RF pulses in each pair being executed in rapid succession, a first RF pulse in each pair being applied synchronously with a magnetic field gradient pulse targeted to select a first volume at the site, and a second RF pulse in each pair being applied synchronously with a magnetic field gradient pulse targeted to select a second volume at the site, the first volume extending generally transversely across the second volume;

(b) applying a first predefined time delay between one of the first and second RF pulses of one pair of the RF pulses and a corresponding one of the first and second RF pulses in a successive pair of RF pulses; and

(c) applying a second predefined time delay after the one of the first and second RF pulses of a pair of RF pulses that occurs just before executing an acquisition pulse sequence used to acquire the signal to image the site, the first and second predefined time delays being previously determined to optimally suppress an undesired contribution to the signal by a specific type of tissue at the site.

24. The preparative sequence of RF pulses of claim 23, wherein the first RF pulse in each pair causes at least a partial inversion of magnetization in the first volume, and the second RF pulse in each pair causes at least a partial inversion of magnetization in the second volume.

25. The preparative sequence of RF pulses of claim 23, wherein the specific type of tissue optimally suppressed in the image of the desired region is one of:

(a) any tissue that is outside a desired region to be imaged at the site; and

(b) blood flowing through the site.

26. The preparative sequence of RF pulses of claim 25, wherein a contribution to the image of a type of tissue that is not optimally suppressed and which is undesired in the image is at least substantially reduced by the preparative sequence.

27. The preparative sequence of RF pulses of claim 23, wherein the first and second predefined time delays are predetermined to achieve optimal signal suppression for a range of relaxation times T₁ corresponding to a physiological range for tissues comprising a living organism.

28. The preparative sequence of RF pulses of claim 23, wherein the acquisition pulse sequence used to acquire the signal is a fast spin-echo sequence comprising an excitation pulse, and a train of refocusing pulses.

29. The preparative sequence of RF pulses of claim 28, wherein suppression efficiency of the preparative sequence is defined by an integral:

$\mathrm{\Psi <br><br>}={\int <br><br>}_{T_1^{\min }}^{T_1^{\max }}<br><br>{\left(M_{\mathrm{zo}}/M_z^{\mathrm{FSE}}\right)}^2<br><br>\text{\hspace{1em}<br><br>}<br><br>d<br><br>T_1,\mathrm{where}<br><br>\text{;<br><br>}$

(a) T₁^min and T₁^max define a range of physiologically possible relaxation times T₁;

(b) M_z^FSE is the longitudinal magnetization produced by the fast spin echo sequence alone and M_z^FSE=1−

F exp(−

(TR−

2Nτ

)T₁);

(c) TR is the repetition time;

(d) N is an echo train length;

(e) τ

is equal to one half of the echo spacing;

(f) F is defined by;

F=[(cos β

)^NE^2N(1−

E)(1+E cos β

)+E(1−

cos β

)]/[1−

E² cos β

];

(g) β

is a flip angle for refocusing pulses;

(h) E is defined by;

E=exp(−

τ

/T₁);

(i) M_zo is the longitudinal magnetization of the outer volume of tissue produced by the preparative sequence of RF pulses applied before the fast spin echo sequence and is defined by;

$\begin{array}{c}{M}_{\mathrm{zo}}=<br><br>1-F<br><br>\text{\hspace{1em}<br><br>}<br><br>\exp <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(-\left(\mathrm{TR}-2<br><br>N<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\tau <br><br>}\right)/T_1\right)-\\ <br><br>2<br><br>\text{\hspace{1em}<br><br>}<br><br>\exp <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(-{\mathrm{TI}}_{2<br><br>o}/T_1\right)<br><br>\left[1-\exp <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(-{\mathrm{TI}}_{1<br><br>o}/T_1\right)\right]\times <br><br>\\ <br><br>\left[1-\exp <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(-\mathrm{TR}/T_1\right)\right]/\left[1-\exp <br><br>\text{\hspace{1em}<br><br>}<br><br>\left(-\mathrm{TR}/\left(N_s<br><br>T_1\right)\right)\right];<br><br>\end{array}$

(j) N_s is the number of slices being imaged;

(k) TI_1o is a duration of the first predefined time delay; and

(I) TI_2o is a duration of the second predefined time delay.

30. The preparative sequence of RF pulses of claim 29, wherein optimal durations of the first and the second predefined time delays being used in the preparative sequence were previously determined by a numerical minimization of the integral Ψ

.

31. The preparative sequence of RF pulses of claim 30, wherein components of the signal due to tissue outside of a desired region at the site are optimally suppressed in the signal by using the optimal durations of the first and the second predefined time delays.

32. The preparative sequence of RF pulses of claim 23, wherein the first volume at the site comprises a slab of tissue, and the second volume at the site comprises one of:

(a) a slice that is substantially thinner than the slab; and

(b) a plurality of slices that comprise another slab of tissue.

33. The preparative sequence of RF pulses of claim 32, wherein the first volume is generally orthogonal to the second volume.

34. A system for imaging a site in a subject, wherein specific components of a magnetic resonance signal acquired to image the site are substantially suppressed, said components corresponding to undesired portions of the site, comprising:

(a) apparatus adapted for producing an image of the site; and

(b) a computer coupled to the apparatus to control it, said computer including;

(i) a memory in which machine instructions are stored; and

(ii) a processor coupled to the memory, said processor executing the machine instructions to control the apparatus to carry out a sequence of operations, including;

(1) generating and applying pairs of radiofrequency (RF) inversion pulses synchronously with magnetic field gradient pulses, one RF inversion pulse in each pair being selective of a first volume of tissue at the site, and the other RF inversion pulse of each pair being selective of a second volume of tissue that transversely crosses the first volume;

(2) waiting for a first predefined time delay between one of the RF inversion pulses in one pair of RF inversion pulses and a corresponding RF inversion pulse in a subsequent pair of the RF inversion pulses;

(3) waiting for a second predefined time delay after the one RF inversion pulse in a pair of the RF inversion pulses occurring just before applying a sequence of RF pulses and magnetic field gradients used to acquire the signal; and

(4) after the second predefined time delay has elapsed, applying a sequence of RF pulses and magnetic field gradients to acquire the signal for use in producing an image of the site with the apparatus.

35. The system of claim 34, wherein the machine instructions cause the processor to repeat the sequence of operations a plurality of times for each of a plurality of selected slices comprising the second volume, enabling signals usable to produce images of the selected slices to be acquired, wherein the specific components of the signals acquired to image each slice are substantially reduced as a result of executing the sequence of operations.

36. The system of claim 34, wherein the first volume comprises a slab of tissue, and wherein the second volume comprises one of:

(a) another slice of tissue that is substantially thinner than the slab; and

(b) a plurality of slices comprising another slab, each of the plurality of slices being substantially thinner that the slab.

37. The system of claim 34, wherein durations of the first and second predefined time delays are determined to achieve optimal signal suppression for a range of relaxation times T₁ corresponding to a physiological range for tissues comprising a living organism.

38. The system of claim 34, wherein durations for the first and second predefined time delays are empirically determined.

39. The system of claim 34, wherein the specific components include a contribution to the signal from at least one of:

(a) any tissue that is disposed outside a desired region at the site that is to be included in the image; and

(b) blood flowing through the site.