Highly Accelerated Single Breath-Hold Coronary MRA with Whole Heart Coverage Using a Cardiac Optimized 32- Element Coil Array


T. Niendorf, C. J. Hardy, H. Cline, R. O. Giaquinto, A. K. Grant, N. M. Rofsky, D. K. Sodickson Applied Science Laboratory, GE Healthcare Technologies, Boston, MA, United States, Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, United States, GE Global Research Center, Niskayuna, NY, United States Purpose Clinical 3D coronary MRA (CMRA) is generally confined to multiple targeted thin slabs encompassing a particular segment of the coronary artery tree only due to the competing constraints of acquisition time, signal-to-noise ratio (SNR) and spatial resolution. Alternatively, the use of large imaging volumes, first made possible with free breathing techniques using mean acquisition times of approximately 13 min (1), supports the visualization of tortuous segments of the coronary arteries. Whole heart coverage breath-hold strategies have been elusive hitherto given practical constraints on the breath-hold duration, anatomic coverage and acquisition window length. For all of these reasons, a strategy employing many element RF coil arrays in conjunction with parallel imaging is conceptually appealing for the pursuit of comprehensive cardiac volumes in acceptable breath-hold times as recently demonstrated using mild accelerations and thick axial slabs (2). However, high baseline SNR is required to support the high accelerations required for such whole-heart breath-hold studies. To approach these challenges, the first aim of this study is to evaluate the performance of a new cardiac-optimised 32-element RF coil array for CMRA. Next we use this array in combination with highly parallel imaging to extend the achievable acceleration and volumetric coverage so as to achieve true whole-heart CMRA without exceeding clinically acceptable breath-hold times. Lastly the merits and limitations of the simplified whole heart coverage paradigm are discussed and its implications for clinical CMRA are considered. Methods An asymmetric cardiac-optimized 32-element two-dimensional array consisting of two clamshell formers, equipped with 21 anterior (∅=75 mm) and 11 posterior (∅=107 mm) elements, was designed. The posterior former was placed directly underneath the subject’s torso while the anterior former was positioned on the subjects left chest (Fig. 1). A 32-channel acquisition system including multiple sets of system electronics (GE Healthcare Technologies, Waukesha, WI, USA) was employed for signal transmission and reception (3). A fat saturated, ECG gated 3D SSFP pulse sequence was customized to synchronize the prospectively ECG gated data acquisition for all 32 channels. 3D SSFP was performed using: FOV=41 cm, data matrix=256x256, TE=1.9 ms, TR=3.7 ms. Data acquisition was completed in a single heartbeat for each acquired slice partition. Unaccelerated phantom experiments were performed to compare baseline SNR between the cardiac optimized 32element array and an 8-element cardiac coil (anterior/posterior 2 x 2 grid, element size 14 x 14 cm). The center of each coil was aligned with the S-I center position of the phantom (Fig 1). Accelerated whole-heart CMRA was then conducted on healthy adult volunteers, with simultaneous accelerations applied along both phase encoding directions. Net acceleration factors ranged from 8 (4x2, slice thickness=2 mm) to 16 (4x4, slice thickness=1mm). Large 3D axial slabs consisting of up to 120 slice partitions covering an S-I volume of 12 cm (interpolated voxel size of (0.8x0.8x1.0) mm) were acquired in a single breath-hold of 30 cardiac cycles. Images were reconstructed using the generalized encoding matrix (GEM) approach (4). For comparison the traditional targeted slab approach was applied in separate unaccelerated breath-hold scans to generate selective views of the right or the left coronary artery. Results For phantom studies using the 32-element cardiac array the approximate SNR obtained for a peripheral ROI placed 3cm below the anterior surface of the phantoms center (Fig 1) showed an SNR gain of approximately 350% as compared to the 8-element Fig.1: Coil positioning used (top) in phantom and (bottom) volunteer studies.

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@inproceedings{Niendorf2005HighlyAS, title={Highly Accelerated Single Breath-Hold Coronary MRA with Whole Heart Coverage Using a Cardiac Optimized 32- Element Coil Array}, author={Thoralf Niendorf and Christopher F.J Hardy and Harvey Cline and Randy O. Giaquinto and Angela K. Grant and Neil Rofsky and Daniel K. Sodickson}, year={2005} }