Short-axis images of the heart for a patient with inferior and inferolateral MI and a smaller anterolateral MI are shown in comparing (a) the “standard” breath-held, segmented IR-turboFLASH, (b) a single free-breathing, single-shot IR-trueFISP image, (c) average of the first 8 repetitions free-breathing with motion correction, (d) average of the best 8 of 29 repetitions free-breathing with motion correction, and (e) average of all 29 repetitions free-breathing with motion correction. The acquisition time was the same (16 heartbeats) for cases in and c) In this example, the spatial resolution was 1.4 × 2.7 × 6mm3 with FOV 350 × 350 mm2 and TI = 300 msec. The smaller anterolateral MI, which is difficult to detect without averaging (), is readily discernible after averaging with motion correction (). Short-axis images for a second patient with anterolateral MI are shown in as another example.
FIG. 3 Short-axis images of the heart for a patient with inferior and inferolateral MI and a smaller anterolateral MI comparing (a) the “standard” breath-held, segmented IR-turbo-FLASH, (b) a single free-breathing, single shot IR-true-FISP image, (more ...)
FIG. 4 Short-axis images of the heart for a second patient with anteroseptal MI comparing (a) the “standard” breath-held, segmented IR-turbo-FLASH, (b) a single free-breathing, single shot IR-true-FISP image, (c) average of the first 8 repetitions (more ...)
The motion parameters and mean square error (arbitrary units) of the registration are plotted in for the first patient shown in , where each time image frame is spaced by 2 heartbeats at approximately 73 bpm. The oscillatory characteristic is due to sampling at close to two images (4 heartbeats) per respiratory cycle. The error and fit parameters are plotted in after sorting based on mean square error. The average in is composed of the first 8 frames after sorting, i.e., 8 with minimum error. The mean in-plane translation for the 6 patients was 3.1 ± 1.6 pixels, and the maximum translation ranged from 3.5 to 15 pixels. The maximum in-plane rotation ranged from 1.1 to 11.5°.
Motion parameters and mean square registration error (arbitrary units) (a) versus acquired frame number and (b) sorted based on mean square error.
Motion-corrected averaging was applied to long-axis images acquired during free breathing. In the case of long axis imaging there is substantial through-plane motion and the slice with MI is not in every frame due to respiratory motion. shows and average of the best 8 frames (i.e., with lowest registration error) using frame 2, which contains the MI as the target for registration; the sorted error for this case is plotted in . shows and average of the best 8 frames using frame 13 (a slice without MI) as the target for registration; the sorted error for this case is plotted in . The respiratory phase corresponding to is close to end-expiration; thus, there are a greater number of frames that fit closely as seen by plot of error in . The desired slice with MI was not optimally acquired at end-expiration and, therefore, the averaging was limited.
FIG. 6 Motion-corrected averages of long-axis images acquired during free breathing illustrating significant through-plane motion. The images shown represent averages of images selected from two different respiratory phases: (a) a slice which contains the MI (more ...)
The magnetization during inversion recovery for the two sequences was simulated to compare the CNR between MI and normal myocardium. The difference in magnetization ΔM
is approximately 1.4:1 greater for the true-FISP readout as seen in ; however, the SNR loss due to greater bandwidth used by true-FISP was 2.6:1. Note that the plot in has a staircase appearance due to the segmented acquisition with interleaved phase encode order. The CNR for the segmented turbo-FLASH method is predicted to be approximately 2.7 times that of the true-FISP with SENSE for a single frame without averaging using the specific imaging parameters used after accounting for acceleration loss and SENSE g
-factor. The maximum value of the SENSE g
-factor for rate 2 acceleration using the custom eight-element linear array is typically 1.05 in the heart region. The CNR for the image with 8 averages is improved by √8 ≈ 2.8, yielding approximately the same CNR as the breath-held turbo-FLASH acquired in the same time. The calculations for the CNR comparison are described by Eq .
The measured CNR for free-breathing motion corrected IR-true FISP with 8 averages was 0.96 ± 0.13 (mean ± SD, n
= 6) times that for breath-held IR-turbo-FLASH, which is in close agreement with prediction. Averaging was extended for 30 images acquired in 60 heartbeats to further increase CNR (approximately double that of the IR-turbo-FLASH case) with minimal motion blurring or loss of detail after registration (see ).
Magnetization during inversion recovery for MI and normal myocardium and specified imaging parameters for (a) segmented turbo-FLASH and (b) single-shot true-FISP with R = 2 SENSE acceleration.
The comparison of infarct size measurements is shown in , which plots the size for the single-shot true-FISP sequence versus the breath-held, segmented turbo-FLASH. The bold-line corresponds to the average of first 8 frames after motion correction, and the lighter line is for the first frame without averaging. The infarct size for averaged images is in close agreement with the turbo-FLASH method (R2 = 0.98) over a wide range of MI sizes, while the single frame data are more scattered (R2 = 0.87), as expected.
FIG. 8 Comparison of infarct size measured in the breath-held, segmented turbo-FLASH images with motion corrected, free-breathing, single-shot, true-FISP images. The bold line and circles show results from averaging eight true-FISP images; the thin line and (more ...)