First-pass, contrast-enhanced imaging using saturation preparation for T1
-weighted contrast can be used to image multiple slices per heartbeat. Various imaging parameters have been proposed (1
) that trade spatial coverage for image quality. illustrates several acquisition strategies. shows the acquisition of a single slice per saturation preparation with relatively short TI. shows a single slice per saturation preparation, with a longer TI for increased T1
contrast and flatter response. The proposed method () uses a shot-to-shot, slice-interleaved acquisition with two slices acquired per saturation preparation, which thus reduces the net preparation time. Method 3 uses accelerated imaging to reduce the imaging time for each pair of slices; therefore, the methods all have the same imaging window. Method 3 provides twice the spatial coverage of method 2, using the same TI.
FIG. 1 Timing for various acquisition methods: (a) method 1 using single slice per saturation preparation with short TI, (b) method 2 using single slice per saturation preparation with longer TI for flatter response and increased T1 contrast, and (c) method (more ...)
Imaging time can be reduced by undersampled acquisition with full-FOV reconstruction employing either unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD) (4
) or parallel imaging methods, such as sensitivity encoding (SENSE) (6
). The temporal SENSE (TSENSE) (7
) method can be used with interleaved phase-encode acquisition order to adaptively derive or update B1
-map estimates, as well as for additional alias artifact suppression.
The acquisition order of the slices can also be interleaved spatially (i.e., for four slices, the slices can be acquired with shot-to-shot interleaving 131313 and 242424). Furthermore, the effective TR is increased by a factor of 2, which allows the use of an increased readout flip angle. The √R SNR loss from accelerated imaging is largely compensated for by this increased flip angle (8
). A longer preparation time (TI) can be used to further improve the image contrast and the point spread function (PSF). Slice interleaving combined with accelerated imaging maintains the same overall image acquisition window.
In this study, imaging was performed on a GE 1.5T CV/i
scanner, using a multishot echo-planar imaging fast gradient-recalled echo (EPI-FGRE) sequence with the following parameters: echo-train length = 4, TR = 6.9 ms, bandwidth = ±125 k, and 10% trigger window. The acquisition matrix was 128 × 80 with typically a 40 × 25 cm2
FOV, producing a nominal resolution of 3.1 × 3.1 mm2
and 8-mm slice thickness. Three methods were compared: method 1 (as described in Ref. 1
); method 2 (method 1 modified for improved image quality); and method 3, with slice interleaving and R = 2 TSENSE acceleration. The variable parameters are listed in . TI is defined at the center of the k
-space acquisition rather than at the first readout, as in Refs. 1
. A modified center-out k
-space acquisition order (1
) that acquires the central lines with the first echo was used for all methods. The TE for the first echo was approximately 1.6 ms for all methods, with interecho spacing of 0.75 ms. The effective TR for method 3 with slice interleaving was 13.8 ms, which permitted an increased flip angle. The number of short-axis slices acquired per heartbeat was a function of heart rate. The maximum number of slices that could be acquired for each method, assuming a 10% trigger window, is shown in .
Variable Parameters for Three Methods
FIG. 2 Spatial coverage (number of slices) vs. heart rate for (a) method 1 using TI = 80 ms, (b) method 2 using TI = 120 ms, and (c) method 3 using TI = 120 ms and TSENSE acceleration combined with slice interleaving (assuming a 10% trigger window in each case). (more ...)
The initial image for each slice was used as a reference for B1-map estimation, and did not have any saturation preparation. A fixed 10° readout flip angle was used for the initial reference image for all of the methods, which could also be used to normalize surface coil intensity variation. A standard GE four-element cardiac surface coil array was used. The subjects in this study (N = 6) were normal, healthy volunteers, who provided informed consent in accordance with an NIH-approved protocol. Images were acquired for 40 heartbeats beginning approximately 5 s prior to the administration of a single-dose bolus (0.1 mmol/kg) of contrast agent (Gadopentetate Dimeglumine; Berlex Magnevist) at 5 ml/s, followed by a saline flush (20 ml at 5 ml/s). The contrast agent was administered intravenously in the left antecubital vein.
Data were acquired using all three methods for each volunteer. Care was taken to use the same coil positioning and slice orientation for the three exams conducted on different days. The data were acquired over a 5-week period, and the order of the three methods varied. Reconstructions were performed using both TSENSE and UNFOLD. UNFOLD temporal filtering used 80% of the available bandwidth (1 dB) using a low-latency temporal filter design (10
). The signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) measurements were made in six equiangular segments of the left ventricle (LV) (short-axis slice). The measured post-enhancement SNR was calculated using an average of four image frames starting at the time of the peak contrast enhancement for myocardium. The precontrast SNR was calculated using an average of two frames preceding the bolus injection.