Coronary Angiography with Magnetization-Prepared T. . Contrast Jean H. Brittain, Bob S. Hu, Graham A. Wright, Craig H. Meyer, Albert Macovski, Dwight G. Nishimura

A magnetization-prepared, T,-weighted sequence (T, Prep) is used to suppress muscle and venous structures. When com- bined with lipid suppression, this technique improves the vi- sualization of the coronary arteries. T, Prep was designed to be robust in the presence of flow as well as 8, and B, inho- mogeneities and may be combined with virtually any imaging technique. Here, it is implemented with both a single-slice spiral acquisition and a multi-slice spiral method that acquires up to 15 slices in a single breath-holding interval. Key words: coronary artery imaging; magnetization-prepared contrast; angiography; magnetic resonance imaging.

INTRODUCTION

Each year, as many as 1.5 million Americans suffer a myocardial infarction and approximately 500,000 die as a result (1). Early detection of coronary artery disease could identify patients for noninvasive treatments such as life-style changes and pharmaceuticals, which have been shown to reverse the progress of coronary artery stenoses (2).

A variety of magnetic resonance techniques have been developed to address this challenge (3-16). Many of these imaging methods use lipid suppression to generate contrast between the arteries of interest and epicardial fat (5-13, 15). Visualization of vessels that reside in a lipid environment, as is common for the proximal coronary arteries, is improved via these techniques. More distal segments of the arteries, however, are typically adjacent to the muscle of the myocardium and therefore are not well depicted. To address this issue, magnetization trans- fer contrast (MTC) (17-19) has been implemented in conjunction with several of the lipid-suppressed meth- ods (10-12). While MTC does enhance contrast between muscle and blood, it also necessitates a high specific absorption rate (SAR).

We propose a different approach, a T,-weighted mag- netization-prepared sequence (T, Prep), which sup- presses both muscle and venous structures. The latter is

MRM 33689-696 (1995) From the Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Division of Cardiovascular Medicine (B.S.H.). Stanford University, Stanford, California, and the Department of Medical Biophysics and Sunnybrook Health Science Centre (G.A.W.), University of Toronto, Toronto, Ontario, Canada. Address correspondence to: Jean H. Brittain, MSEE., 120 Durand Building, Department of Electrical Engineering, Stanford, CA 94305-4055. Received October 11, 1994; revised January 20, 1995; accepted January 20, 1995. This work was supported by NIH NS 29434, NIH HL 39297, NSF BCS 9058556, NIH HL 47448 and GE Medical Systems. J.H.B. was also sup- ported by a National Science Foundation Graduate Research Fellowship.

Copyright 0 1995 by Williams & Wilkins All rights of reproduction in any form reserved.

0740-31 94/95 $3.00

beneficial in coronary artery imaging because the pres- ence of coronary veins can result in confusion or view obstruction. Recognizing the technical challenges in- volved in cardiac imaging, T, Prep is designed to be robust in the presence of flow as well as B, and B, inhomogeneities, and it may be utilized with virtually any imaging sequence.

The T, weighting generated by this sequence sup- presses muscle because the T, of muscle is much less than that of oxygenated blood. Conveniently, the T, of oxygenated blood is also greater than that of venous blood. As the oxygen saturation (%O,) of blood decreases the concentration of deoxyhemoglobin increases. The paramagnetic nature of deoxyhemoglobin causes local variations in the field surrounding the intact red blood cells in which it is contained. Spins undergoing ex- change andlor bounded diffusion through these suscep- tibility-induced field gradients experience irreversible dephasing. Thus, as the %O, of blood decreases, its T2 decreases as well (20, 21).

Since their introduction (22), T,-weighted, magnetiza- tion-prepared techniques have been used in a variety of applications including T, measurement (23, 24) and con- trast development for lesion characterization (25, 26). However, B, and B, inhomogeneities have posed signif- icant challenges for these techniques which have tradi- tionally utilized a 9O0-~-18Oo-~-9O0 combination for prep- aration of T, contrast (22,25,26). To gain insensitivity to Bo and B, inhomogeneities as well as to flow, T, Prep employs ideas originally implemented in two angio- graphic sequences designed to maximize blood signal at late echo times (27, 28). These sequences, however, are limited to projection or single-slice acquisitions since they isolate the region of interest with a final spatially selective refocusing pulse. T, Prep combines the robust- ness of these methods with the flexibility of a magneti- zation-prepared sequence.

T, Prep may be combined with virtually any imaging technique and thus may be adapted for use in a variety of vascular regions. We will first describe the design of the T, Prep sequence and then demonstrate its effectiveness in suppressing muscle and venous structures in coronary images acquired with two rapid imaging sequences: sin- gle-slice spiral acquisition and multi-slice spiral acquisi- tion.

METHOD T, Prep Sequence

The T, Prep sequence shown in Fig. 1 is designed to store T,-weighted magnetization along M, in a manner that is robust in the presence of flow as well as B,, and B, inhomogeneities. A 90, excitation is followed by a train

689

Brittain et al. 690

Gz

Rf

Spoiler n MLEV-Weighted 180's

1 4 * 4 t180*

FIG. 1. T2 Prep sequence illustrated with four refocusing pulses.

of 180;pulses, which are equally separated by an interval t180. At the echo of the final refocusing pulse, the T,- weighted magnetization is returned to the longitudinal axis.

To minimize flow-sensitivity, the entire T, Prep se- quence is nonselective. Thus, there are no gradient mo- ments to be nulled. In addition, the regular refocusing achieved by the train of 180; pulses lessens the amount of dephasing due to flow through susceptibility gradi- ents. Assuming ideal RF homogeneity, phase accrued by spins moving at a constant velocity through local B, inhomogeneities will be refocused on each even echo if the B, inhomogeneity can be modeled as a linear gradient (29). The validity of such a model improves as t180 decreases because spins travel a shorter distance between pulses (27).

Ideally, the 180; pulses perfectly refocus the transverse magnetization generated by the initial 90; excitation and invert any longitudinal magnetization. However, errors in the refocusing train due to B, and B, inhomogeneity can excite spurious signals. Two compensation schemes are used to minimize these errors. First, the 180; pulses are weighted in an MLEV pattern (30, 31). This method modulates the sign of the refocusing pulses in a manner which compensates for pulse imperfections and yields superior refocusing following 2" pulses where n is an integer. The second method involves the use of compos- ite 180; refocusing pulses. These 90; 180; 90; pulses provide second-order correction to errors in B, ampli- tude and provide accurate inversion of any longitudinal magnetization (32). The use of these two compensation strategies yields a dramatic reduction in spurious signals. All RF pulses used in the T, Prep sequence are rectan- gular with minimum width and maximum amplitude (duration of 90' excitation = 406 ps for our hardware). Given these parameters, if only one of the two compen- sation methods is implemented in studies of static phan- toms, the MLEV modulation results in a slightly larger reduction of spurious signals than the composite 180' pulses. However, neither the MLEV nor the composite 180' pulse method alone is as effective as both tech- niques used together. Those spurious signals which re- main despite compensation are modulated at the Nyquist frequency by phase-cycling of both the initial excitation and the "tipup" pulse.

A [-go:] pulse is the intuitive choice to return the T,-weighted magnetization to the longitudinal axis. However, the performance of a T, Prep sequence using this pulse is sensitive to B, inhomogeneity. Instead, a

composite 270; [-360;] pulse (33) is utilized. Figure 2 compares the simulated results from a T, Prep sequence with four composite refocusing pulses (t180 = 24 ms) weighted in an MLEV pattern followed either by a [ -903 (Fig. 2a) or a 270; [-360:] combination (Fig. 2b). The resulting M, is plotted as a function of B, inhomogeneity and off-resonance. Relaxation was neglected in these simulations.

The improvement in off-resonance performance with the composite "tipup" pulse which is demonstrated in Fig. 2 may be understood if one considers the effective axis of rotation, Bcff, in the two cases:

[ l l

where B, is the strength of the magnetic field in the z direction, y is the gyromagnetic ratio, and B, and w are the strength and frequency of the applied RF pulse re- spectively. Let us ignore the refocusing train and con- sider only the initial 90; and the subsequent [ - 90y or

-300 -200 -100 0 100 200 300 a Off-Resonance (Hz)

a, 1.4

E

U 3 4-l .-

1.2

a 5 1

a, > - 0.8 CG a,

.- -

0.6

-300 -200 -100 0 100 200 300

b Off-Resonance (Hz)

FIG. 2. Mz following a T2 Prep sequence with four refocusing pulses (t180 = 24 ms) plotted as a function of off-resonance and relative B, amplitude. The magnetization is returned to the z-axis by (a) [-go,", (b) 270," [-360,"].

T2 Prep Coronary Angiography

Rf

691

' n -A-

rtl80* U U J

270; [-360;]. Note in Fig. 3 that while B, is reversed i? direction between the 90; and the [-go:], (BO - w / y ) k does not change. Thus the 90; and the [ -903 rotate about different effective axes and a 90; [-90:] combination does not return the magnetization precisely to the longi- tudinal axis. In contrast, the 90; and the 270; do rotate about the same axis and complete a full-revolution plus or minus any error in B, amplitude. While the [ - 3603 rotates about a different axis than the 90; 270; combina- tion, it provides some compensation for any B, ampli- tude error. Therefore, the composite "tipup" more accu- rately returns the magnetization to the longitudinal axis.

A large spoiling gradient (amplitude = 0.9 G/cm, du- ration = 6 ms) that dephases any transverse magnetiza- tion remaining after the "tipup" completes the T, Prep sequence, yielding a T,-weighted M, from the entire ex- cited volume. This magnetization may then be sampled by any imaging sequence which is appropriate for the given application.

Steady-State Signal: Single-Slice Spiral Acquisition

The implementation of the T, Prep sequence with a single-slice spiral acquisition (5) is illustrated in Fig. 4. Each TR, T, Prep is followed by a spatial-spectral exci- tation and the acquisition of one spiral interleaf. Because the spatial-spectral pulse selectively excites water while isolating the slice of interest, this sequence rejects lipids in addition to suppressing muscle and venous structures with T, Prep. Furthermore, the spiral acquisition has excellent flow properties which are well-suited to coro- nary imaging (5, 34).

Given the time intervals TE, ta, and tb as shown in Fig. 5, and assuming ideal pulses with zero transverse signal prior to the 8 imaging excitation and the initial goo exci- tation, the steady-state signal produced by stationary ma- terial will be:

where

E, = exp( -$), 141

z

I Y

FIG. 3. Effective axis of rotation for 90," and [-go,"] pulses.

Gx

-TE- I t TR b

FIG. 4. T2 Prep implemented with a single-slice spiral acquisition.

90 180 18oTipup 8 90 180 ... Rf

4-- TE

FIG. 5. Time intervals TE, ta, and tb used in simulation of steady- state signal.

Ez = exp( -:). [51

If 8 = goo, this reduces to

M,,(stationary) = Mo[l - E, + (1 - Eb)E,Ez]. 161

Note that with t, = 0 and tb = m this would reduce to pure T, contrast as expected.

Since the 0 imaging pulse is slice selective, we can assume that flowing material, such as blood, will not experience the previous 8 pulses. Thus, the steady-state signal of blood will be:

If e = goo,

Comparison of Eq. [Z] and Eq. [7] reveals that in-flow enhancement increases the steady-state signal of blood by a factor

1 - E,EbE, cose 191

As expected, this ratio is maximized when 0 = goo, the point of maximum static-material saturation.

The steady-state signal for arterial blood, muscle, and the venous blood found in the coronary veins is plotted as a function of TE in Fig. 6. Note the resultant contrast between arterial blood and both muscle and venous

1 - E0E$2 '

692

0.5

Brittain et al.

Arterial Blood t ’L J

TE (ms)

FIG. 6. Simulated steady-state signal as a function of T f resulting from T2 Prep with single-slice spiral acquisition.

structures. This simulation models a cardiac-gated se- quence with a heart rate of 60 beatdmin (TR = 1 s), t, = 20 ms, and 8 = 90°. Assuming t180 = 24 ms, relaxation times were drawn from the literature as follows: T,(cardiac muscle) = 50 ms (35, 36); T,(cardiac mus- cle) = 900 ms (35, 36); T,(arterial blood) = 223 ms (28); T,(venous blood of coronary veins with %O, = 20%) = 35 ms (28, 37); T,(arterial and venous blood) = 1000 ms (38).

Steady-State Signal: Multi-Slice Spiral Acquisition

The spiral-based sequence can be extended to a multi- slice form as shown in Fig. 7 (5, 8). A single T, Prep is followed by a spatial-spectral 8 excitation and the acqui- sition of one spiral interleaf for each slice of interest. Note that for each subsequent slice, the interval between the T, Prep “tipup” and the 8 pulse for that slice in- creases and the time remaining before the next T, Prep 90° excitation decreases (to increments and t, decrements by Stim,). Therefore, the steady-state signal analysis must account for increasing TI contrast in later slices. Figure 8 illustrates the resulting steady-state signal as a function of slice assuming TE = 48 ms, Stime = 40 ms and 8 = 45O. Note that while the signal does recover at different rates, contrast is well maintained even in later slices. This

~~~ ~~~

4 TR w

FIG. 7. T2 Prep implemented with a multi-slice spiral acquisition.

1 2 3 4 5 6 7 8 9 10 Slice

FIG. 8. Simulated steady-state signal as a function of slice result- ing from T, Prep with multi-slice spiral acquisition. Differences in T, yield contrast loss in later slices.

simulation maintains the assumption that flowing mate- rials, such as blood, will not experience the previous 8 pulses. If slices are prescribed in opposition to flow direction, this assumption will be valid if the average flow velocity is greater than that necessary to supply fresh blood to the entire displayed vessel length in an interval TR - (N - l)Stime, where N is the number of slices. For example, if 10 slices of 5 mm each are ac- quired in a coronary sequence with TR = 1 s and Stime = 40 ms, this minimum average velocity is 7.8 cm/s if slices are prescribed perpendicular to the vessel. In compari- son, peak coronary velocities are reported as 30 cm/s for the left main coronary artery (LM) and 22 cm/s for the right coronary artery (RCA) (9, 39). While the simulation neglects any effect that individual imaging excitations have on adjacent slices, in reality there is some overlap if slices are contiguous. For this reason, a 8 less than goo is typically selected.

Acquisition Parameters

Both the single-slice and multi-slice spiral sequences using T, Prep were implemented on a 1.5 T GE Signa with shielded gradients where the maximum gradient amplitude is 1 G/cm and maximum gradient slew rate is 2 G/cm/ms. Twenty spiral interleaves were acquired in a single breath-holding interval, with each interleaf utiliz- ing 17.6 ms of data acquisition time gated to diastole by a plethysmograph trigger. Except where noted, the in- plane resolution was 1.07 X 1.07 mm. Volunteers were positioned supine with a 12.7-cm surface coil on their chest for signal reception.

RESULTS Parameter Selection

The contrast generated by T, Prep sequences can be manipulated by judicious choice of the two parameters shown in Fig. 1: TE and t180.

As demonstrated in Fig. 6, selection of TE is inherently a compromise between signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). Whereas CNR initially in-

T2 Prep Coronary Angiography 693

a b C

FIG. 9. Oblique images through the left ventricle illustrating improved visualization of LAD segment in muscle environment with T2 Prep: 5-mm slice, 24-cm original FOV (shown cropped), 45" imaging excitation, slice number 6 of 9. Images obtained with (a) no preparation (window = 0/285), (b) T, Prep (tl8O/T€ = 12/48 ms, window = O/l49), (c) T2 Prep (tl8O/T€ = 12/96, window = O/l25).

a b FIG. 10. Axial images at the level of the proximal coronaries illustrating T2 Prep suppression of great cardiac vein (arrow): 10-rnm slice, 20-cm original FOV (shown cropped), 90" imaging excitation, single slice. Images obtained with (a) no preparation (window = 0/743), (b) T, Prep (tl8O/T€ = 24/96 ms, window =

0/243).

creases with TE, peaks, and then decreases, SNR is a monotonic function of TE, with increasing TE yielding lower SNR. This compromise is illustrated in Fig. 9. These oblique images from three multi-slice sets through the left ventricle were obtained without T, Prep (Fig. 9a) and with T, Prep using a constant t180 of 1 2 ms but different TEs of 48 and 96 ms, respectively, for Fig. 9b and Fig. 9c. (TEItl80 refocusing pulses are used to gen-

erate each image.) Note the left anterior descending ar- tery (LAD) which is made visible by the greater muscle and venous suppression afforded by a longer TE. The cost of this increased contrast is some loss of SNR. (The minimum and maximum signal displayed in each case are indicated in the figure captions: window = mini- mum/maximum.)

The second parameter selection, that of t180, involves a compromise between venous suppression and insensi- tivity to field inhomogeneities. With shorter t180, as mentioned previously, signal from spins moving through susceptibility-induced field gradients is better main- tained. However, changes in t180 also affect the T,/%O, relationship of blood (20, 21, 28). Specifically, as t180 is increased, the T, corresponding to a given %O, de- creases. Most of this reduction in T, occurs as t180 is increased from 1 to 40 ms (27).

The venous suppression achieved with T, Prep is il- lustrated in the axial images at the level of the proximal coronary arteries which are shown in Fig. 10. The image on the left (Fig. 10a) was acquired without T, Prep. Both the left arterial system and the great cardiac vein are clearly visualized. The image on the right (Fig. lob) was acquired with T, Prep using a t180 of 24 ms and TE of 96 ms. Note the resulting suppression of the great cardiac vein as well as the suppression of the chest wall.

Figure 11 illustrates the effect of more rapid refocusing

C

FIG. 11. Axial images at the level of the proximal coronaries illustrating dephasing in left atria (arrow) with increased t l80 due to flow through susceptibility variations: 11 -mm slice, 22-cm original FOV (shown cropped), 45" imaging excitation, slice 1 of 3. Images obtained with (a) no preparation (window = 0/430), (b) T2 Prep (tl80/T€ = 6/48 ms, window = 0/220), (c) T2 Prep (tl80/T€ = 24/48, window = 0/193).

694 Brittain et al.

DISCUSSION

As illustrated, T, Prep improves the visualization of cor- onary arteries which are adjacent to muscle and sup- presses coronary veins which might confuse or obstruct image interpretation.

Since both T, Prep and MTC methods yield muscle suppression, direct comparisons of blood-to-muscle con-

FIG. 12. Two short-axis images illustrating improved blood-to- septum contrast with T2 Prep: 7-mm slice, 24-cm original FOV (shown cropped), 30" imaging excitation, slices 2-3 of 7, 1.27 rnm x 1.27 mm in-plane resolution. Images obtained with (a) no preparation (window = 0/227), (b) T2 Prep (tl80/T€ = 4/96 ms, window = 0/77).

on susceptibility-induced dephasing. These axial images taken at the level of the proximal coronary arteries were obtained without T, Prep (Fig. l l a ) and with T, Prep using a constant echo time of 48 ms but differing t180 of 6 ms (Fig. 1lb) and 24 ms (Fig. l l c ) . Note that the blood within the left atria, which has flowed from the lungs and through regions of large susceptibility variation, is better maintained with faster refocusing.

Multi-Slice Images

Using the sequence illustrated in Fig. 7, up to 15 images may be acquired in a single breath-holding interval. Fig- ures 12a and 12b show the second and third images from two, short-axis, multi-slice sets acquired without and with T2 Prep (tl80ITE = 24/96 ms), respectively. Notice the improved contrast between septum and blood as well as the clear delineation of the trabeculations achieved with T2 Prep.

Figure 13 exhibits four images taken from two, 10- image, multi-slice, short-axis sets moving from midven- tricle to the base of the heart. Comparing the images acquired without T, Prep (Fig. 13a) to those acquired with T, prep using t180 of 12 ms and TE of 48 ms (FQ.

FIG. 13. Four short-axis images illustrating improved visualization with T, Prep of LAD and ClRC in muscle environment as they

cropped), 45" imaging excitation, slices 1-4 of 10. Images ob-

(tl8O/T€ = 12/48 ms, window = 0/1 62).

13b), we note the improved visualization with T, prep of the LAD and the circumflex ('IRC) arteries as they ap-

approach the LM: 5-mm slice, 24-cm original Fov (shown

tained with (a) no preparation (window = 0/308), (b) T2 prep proach the origin of the LM.

Tz Prep Coronary Angiography

trast achieved and SAR levels required by the two tech- niques are of interest. Anecdotal comparisons reveal that T, Prep yields equal or greater blood-to-muscle contrast than pulsed-MTC (19) sequences. The SAR associated with a single-slice T, Prep sequence utilizing four com- posite refocusing pulses and a composite “tipup” is 0.27 W/kg for a 70-kg patient. While this is well below regu- lated limits and below the power deposition of typical MTC techniques, SAR must be considered in selecting TE and t180. For a given TE, more rapid refocusing necessitates a greater number of 180’ pulses and thus higher SAR. However, the number of refocusing pulses possible with a given power deposition level is doubled by replacing composite 180’ pulses with their noncom- posite form. Thus, more rapid refocusing is achieved for the same SAR with some increase in sensitivity to B, inhomogeneity. It is possible that there are some MTC effects associated with the T2 Prep sequence. A more comprehensive study must be undertaken to evaluate these effects and to fully compare the relative benefits of MTC and T, Prep.

In addition to improved depiction of the coronary ar- teries, T2 contrast, such as that achieved by T2 Prep, has other potential applications in the M R evaluation of car- diac function. These include estimation of cardiac ejec- tion fraction, detection of ischemic regions (40), and identification of patients at risk of heart transplant rejec- tion (41). Furthermore, different choices of TE, t180, and imaging method could clearly adapt T, Prep to diverse applications such as abdominal imaging, evaluation of deep venous thrombosis, and T, estimation (24). We have demonstrated its use with single-slice and multi- slice spiral acquisitions; however, it has also been imple- mented with turbo-spiral (42), interleaved spiral-ring (42), and rapid 3-D (43) acquisitions. Other obvious choices include segmented k-space, echo-planar, and in- terleaved echo-planar approaches.

CONCLUSIONS

T, Prep suppresses both muscle and venous structures and thus improves visualization of the coronary arteries. It is robust in the presence of flow as well as B,, and B, inhomogeneities and can be implemented with virtually any imaging technique.

REFERENCES

1.

2.

3.

4.

5.

“Heart and Stroke Facts.” American Heart Association, Dal- las, Texas, 1993. D. Blankenhorn, S. Azen, D. Kramsch, W. Mack, L. Cashin- Hemphill, H. Hodis, L. DeBoer, P. Mahrer, M. Masteller, L. Vailas, P. Alaupovic, L. Hirsch, Coronary angiographic changes with lovastatin therapy. The monitored atheroscle- rosis regression study (MARS). The MARS Research Group. Ann. Int. Med. 119, 969-976 (1993). S. Wang, B. Hu, A. Macovski, D. Nishimura, Coronary an- giography using fast selective inversion recovery. Magn. Reson. Med. 18, 417-423 (1991). M. Doyle, M. Scheidegger, R. DeGraaf, J. Vermeulen, G. Pohost, Coronary artery imaging in multiple 1-sec breath holds. Magn. Reson. Imaging 11, 3-6 (1993). C. Meyer, B. Hu, D. Nishimura, A. Macovski, Fast spiral

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

695

coronary artery imaging. Mugn. Reson. Med. 28, 202-213 (1992). W. Manning, W. Li, N. Boyle, R. Edelman, Fat-suppressed breath-hold magnetic resonance coronary angiography. Cir- culation 87, 94-104 (1993). D. Pennell, J. Keegan, D. Firmin, P. Gatehouse, S. Under- wood, D. Longmore, Magnetic resonance imaging of coro- nary arteries: technique and preliminary results. Br. Heart J.

B. Hu, C. Meyer, A. Macovski, D. Nishimura, Multi-slice magnetic resonance coronary angiography, in “American College of Cardiology, 43rd Annual Scientific Session, 1994,” p. 89A. C. Paschal, E. Haacke, L. Adler, Three-dimensional MR im- aging of the coronary arteries: preliminary clinical experi- ence. J. Magn. Reson. Imaging 3,491-500 (1993). B. Hu, C. Meyer, A. Macovski, D. Nishimura, Clinical con- siderations in fast spiral magnetic resonance coronary artery imaging, in “Proc., SMRM, 12th Annual Meeting, 1993,” p. 551. Y. Wang, T. Grist, P. Christy, F. Korosec, K. Wedding, J. Polzin, M. Alley, C. Mistretta, Attempts to optimize 2D coronary artery MRA, in “Proc., SMRh4,lzth Annual Meet- ing, 1993,” p. 222. D. Li, C. Paschal, E. Haacke, L. Adler, Coronary arteries: three-dimensional MR imaging with fat saturation and mag- netization transfer contrast. Radiology 187,401-406 (1993). A. Duerinckx, M. Urman, D. Atkinson, 0. Simonetti, U. Sinha, B. Lewis, Limitations of MR coronary angiography, in “Proc., SMR, 1st Annual Meeting, 1994,” p. 81. Y. Wang, P. Christy, F. Korosec, M. Alley, T. Grist, J. Polzin, C. Mistretta, Respiratory motion artifacts in coronary imag- ing: breath-hold 2D, pseudo-gated 3D and multiple breath- hold 3D acquisitions, in “Proc., SMR, 2nd Annual Meeting, 1994,” p. 366. M. Scheidegger, G. Vassalli, 0. Hess, P. Boesiger, Validation of coronary artery MR angiography: comparison of mea- sured vessel diameters with quantitative contrast angiogra- phy, in “Proc., SMR, 2nd Annual Meeting, 1994,” p. 497. J. Debatin, G. McKinnon, D. Wetter, M. Unterweger, G. von Schulthess, Multishot echo planar imaging of the heart, in “Proc., SMR, 2nd Annual Meeting, 1994,” p. 1498. S. Wolff, R. Balaban, Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo. Magn. Reson. Med. 10, 135-144 (1989). R. Balaban, S. Chesnick, K. Hedges, F. Samaha, R. Heine- man, Magnetization transfer contrast in MR imaging of the heart. Radiology 180, 671-675 (1991). B. Hu, S. Conolly, G. Wright, D. Nishimura, A. Macovski, Pulsed saturation transfer contrast. Magn. Reson. Med. 26,

K. Thulborn, J. Waterton, P. Matthews, G. Radda, Oxygen- ation dependence of the transverse relaxation time of water protons in whole blood at high field. Biochim. Biophy. Acta

J. Gomori, R. Grossman, C. Yu-Ip, T. Asakura, NMR relax- ation times of blood: dependence on field strength, oxida- tion state, and cell integrity. I. Comput. Assist. Tomogr. 11,

A. Haase, Snapshot FLASH MRI. Applications to TI, Tz, and chemical-shift imaging. Magn. Reson. Med. 13, 77-89 (1990). T. Gneiting, S. Nekolla, J. Syha, M. Kienlin, A. Haase, Fast Tz imaging with Cam-Purcell snapshot-flash, in “Proc., SMRM, 11th Annual Meeting, 1992,” p. 1346. J. Brittain, G. Wright, J. Pauly, D. Nishimura, Flow-insensi- tive magnetization-prepared Tz measurement, in “Proc., SMRM, 12th Annual Meeting, 1993,” p. 1216.

70, 315-326 (1993).

231-240 (1992).

714, 265-270 (1982).

684-690 (1987).

696 Brittain et al.

25. J. Mugler, T. Spraggins, J. Brookeman, T,-weighted three- dimensional MP-RAGE MR imaging. J. Magn. Reson. Imag- ing 1, 731-737 (1991).

26. R. Edelman, B. Wallner, A. Singer, D. Atkinson, S. Saini, Segmented TurboFLASH: method for breath-hold imaging of the liver with flexible contrast. Radiology 177, 515-521 (1990).

27. G. Wright, D. Nishimura, A. Macovski, Flow-independent magnetic resonance projection angiography. Magn. Reson. Med. 17, 126-140 (1991).

28. G. Wright, B. Hu, A. Macovski, Estimating oxygen saturation of blood in vivo with MR imaging at 1.5T. J. Magn. Reson. Imaging 1, 275-283 (1991).

29. D. Nishimura, A. Macovski, J. Pauly, Magnetic resonance angiography. IEEE Trans. Med. Imaging MI-5, 140-151 (1986).

30. M. Levitt, R. Freeman, T. Frenkiel, Broadband beteronuclear decoupling. J. Magn. Reson. 47, 328-330 (1982).

31. A. Shaka, S. Rucker, A. Pines, Iterative Carr-Purcell trains. 1. Magn. Reson. 77, 606-611 (1988).

32. M. Levitt, R. Freeman, Compensation for pulse imperfec- tions in NMR spin-echo experiments. J. Magn. Reson. 43,

33. A. Shaka, R. Freeman, Composite pulses with dual compen- sation. 1. Magn. Reson. 55, 487-493 (1983).

34. D. Nishimura, P. Irarrazabal, C. Meyer, A velocity k-space analysis of flow effects in echo-planar and spiral imaging. M a p . Reson. Med. 33, 549-556 (1995).

35. R. Harrison, M. Bronskill, R. Henkelman, Magnetization

65-80 (1981).

transfer and T, relaxation components in tissue. Magn. Re- son. Med. 33, 490-496 (1995).

36. P. Bottomley, C. Hardy, R. Argersinger, G. Allen-Moore, A review of ‘H nuclear magnetic resonance relaxation in pa- thology: are TI and ?’, diagnostic? Med. Phys. 14, 1-37 (1987).

37. K. Gould, “Coronary Artery Stenosis.” Elsevier Science Pub- lishing Co., Inc., New York, 1991.

38. G. Wright, “Magnetic Resonance Relaxation Behavior of Blood: Study and Applications,” PhD thesis, Stanford Uni- versity, 1991.

39. P. Spiller, M. Friedrich, B. Politz, M. Block, U. Fermor, W. Hackbarth, J. Jehle, R. Korfer, H. Pannek, Measurement of systolic and diastolic flow rates in the coronary artery sys- tem by x-ray densitometry. Circulation 68, 337-347 (1983).

40. M. Ahmad, R. Johnson, H. Fawcett, M. Schreiber, Magnetic resonance imaging in patients with unstable angina: com- parison with acute myocardial infarction and normals. Magn. Reson. Imaging 6, 527-534 (1988).

41. G. Lund, R. Morin, M. Olivari, W. Ring, Serial myocardial T, relaxation time measurements in normal subjects and heart transplant recipients. J. Heart Transplant 7, 274-279 (1988).

42. A. Kerr, J. Pauly, C. Meyer, D. Nishimura, Turbo and seg- mented k-space imaging with spirals, in “Proc., S M R , 2nd Annual Meeting, 1994,” p. 461.

43. P. Irarrazabal, D. Nishimura, Efficient k-space trajectories for fast three-dimensional imaging, in “Proc., SMR, 2nd Annual Meeting, 1994,” p. 30.