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Steady-State Free Precession MR Imaging: Improved Myocardial Tag Persistence and Signal-to-Noise Ratio for Analysis of Myocardial Motion¹

¹ From the Department of Radiology, Stanford University, Lucas MRI/S Center, 1201 Welch Rd, Rm P 284, Palo Alto, CA 94304. From the 2002 RSNA scientific assembly. Received February 4, 2003; revision requested April 22; revision received May 13; accepted June 26. Supported by National Institutes of Health grants R01 HL46347 and P41 RR09784, a DFG Fellowship, and a Kyle Mann grant. M.M. supported by an RSNA 2002 Research Trainee Prize. Address correspondence to M.M. (e-mail: markl@s-word.stanford.edu).

View larger version (27K): [in a new window]   Figure 1. Top: Timing for k-space-segmented cine MR imaging. Bottom: Tagging preparation sequence. First electrocardiographic (ECG) cycle was used for steady-state preparation (SSprep) as follows: repetitive execution of the pulse sequence with data acquisition switched off. Tagging stripe or grid patterns were generated with typical SPAMM radiofrequency pulses and gradients (SPAMM-Tag) placed between spoiler gradients (S) and /2 pulses. R = R wave.

View larger version (46K): [in a new window]   Figure 2. Temporal evolution of the SNR difference, SNR(t), between tagged and nontagged parts for static phantoms with T1 = 1,560 msec (top) and T1 = 770 msec (bottom). Loss of tagging contrast as a function of time frame and associated tag fading is clearly visible in all cases. SSFP tagging demonstrates improved tagging contrast for most time frames. For flip angle = 20°, tag fading is reduced compared with that for higher flip angles; initial tag contrast is lower but still superior to that with SPGR tagging.

View larger version (36K): [in a new window]   Figure 3. Cumulative results of phantom experiments for different flip angles and relaxation times. In all graphs, results from conventional tagging measurements are shown for comparison and are represented by the data points on the right. Top left: Averaged SNR in nontagged parts of the phantom as a function of imaging flip angle. Substantially improved SNR for SSFP tagging is clearly visible for all flip angles. Top right: Tag-fading-time constant (Tf) obtained by exponential curve fitting to tag contrast curves. Values are comparable between both methods: similar if similar flip angles are used and reduced when larger flip angles are used. Bottom right: Tag persistence, calculated according to Equation (2) with SNRthresh = 6. SSFP tagging provided better tag persistence for most flip angles because of considerably increased SNR and tag contrast. Arrows indicate optimal trade-off between SNR gain and faster tag fading, as evidenced by maxima in tag persistence graph.

View larger version (145K): [in a new window]   Figure 4a. (a) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular basal section. The corresponding times after the R wave appear below each column. (a-c) Images in rows 2-4 are extracted from stripe-tagged SSFP cine series, each with different flip angles. For reference, images from an SSFP cine series acquired without tagging preparation but otherwise identical parameters are shown in top row. Conventional tagged gradient-echo images are depicted in bottom row. Respective flip angles are specified to right of each row. Variation of SNR, blood-tissue contrast, and tag persistence as a function of flip angle is evident. SSFP-tagged images with 10° and 30° flip angles demonstrate clearly visible stripe pattern even late in cardiac cycle; tag lines are mostly faded for larger flip angles and with conventional tagged SPGR imaging. Parameters were 3.8/1.9; bandwidth, ±125 kHz; field of view, 360 mm; section thickness, 8 mm; matrix, 256 x 160; views, eight per segment; temporal resolution, 30.4 msec; total acquisition time, 21 heartbeats. (b) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular middle section. (c) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular apical section.

View larger version (148K): [in a new window]   Figure 4b. (a) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular basal section. The corresponding times after the R wave appear below each column. (a-c) Images in rows 2-4 are extracted from stripe-tagged SSFP cine series, each with different flip angles. For reference, images from an SSFP cine series acquired without tagging preparation but otherwise identical parameters are shown in top row. Conventional tagged gradient-echo images are depicted in bottom row. Respective flip angles are specified to right of each row. Variation of SNR, blood-tissue contrast, and tag persistence as a function of flip angle is evident. SSFP-tagged images with 10° and 30° flip angles demonstrate clearly visible stripe pattern even late in cardiac cycle; tag lines are mostly faded for larger flip angles and with conventional tagged SPGR imaging. Parameters were 3.8/1.9; bandwidth, ±125 kHz; field of view, 360 mm; section thickness, 8 mm; matrix, 256 x 160; views, eight per segment; temporal resolution, 30.4 msec; total acquisition time, 21 heartbeats. (b) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular middle section. (c) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular apical section.

View larger version (145K): [in a new window]   Figure 4c. (a) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular basal section. The corresponding times after the R wave appear below each column. (a-c) Images in rows 2-4 are extracted from stripe-tagged SSFP cine series, each with different flip angles. For reference, images from an SSFP cine series acquired without tagging preparation but otherwise identical parameters are shown in top row. Conventional tagged gradient-echo images are depicted in bottom row. Respective flip angles are specified to right of each row. Variation of SNR, blood-tissue contrast, and tag persistence as a function of flip angle is evident. SSFP-tagged images with 10° and 30° flip angles demonstrate clearly visible stripe pattern even late in cardiac cycle; tag lines are mostly faded for larger flip angles and with conventional tagged SPGR imaging. Parameters were 3.8/1.9; bandwidth, ±125 kHz; field of view, 360 mm; section thickness, 8 mm; matrix, 256 x 160; views, eight per segment; temporal resolution, 30.4 msec; total acquisition time, 21 heartbeats. (b) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular middle section. (c) Short-axis images for cardiac phases during early and late systole at location corresponding to left ventricular apical section.

View larger version (52K): [in a new window]   Figure 5. Quantitative analysis of SNR for blood pool in left ventricle and nontagged (Myo) and tagged (tMyo) myocardium for basal, midventricular, and apical sections. Single data points (no tag) represent SNR achievable with typical SSFP and gradient-echo acquisitions without tagging. Compared with conventional tagging, SSFP tagging demonstrates enhanced SNR for almost all flip angles. SNR difference between nontagged and tagged myocardium (tag contrast) is increased for smaller SSFP flip angles and higher than that for conventional tagging; blood-myocardium SNR difference is still comparable to or higher than that of conventional tagging. Parameters were 3.8/1.9; bandwidth, ±125 kHz; field of view, 360 mm; section thickness, 8 mm; matrix, 256 x 160; views, eight per segment; temporal resolution, 30.4 msec; total acquisition time, 21 heartbeats. Solid gray line = blood, black line = nontagged myocardium, dotted gray line = tagged myocardium.

View larger version (100K): [in a new window]   Figure 6. Transverse sections for measurements including the /2 steady-storage scheme only (top) and with added grid-tagging preparation (bottom) for different cardiac phases in a healthy volunteer. Off-resonance ghosting artifacts (arrows) in the first time frame of the cine series are visible on both tagged and nontagged images. Tags have already faded in the image obtained at late diastole (bottom right). Parameters were 3.8/1.9; bandwidth, ±125 kHz; field of view, 360 mm; section thickness, 8 mm; matrix, 256 x 160; views, eight per segment; temporal resolution, 30.4 msec; total acquisition time, 21 heartbeats.