A semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T

Abstract

Purpose: To design, construct, and validate a semiflexible 64-channel receive-only phased array for pediatric body MRI at 3T.

Methods: A 64-channel receive-only phased array was developed and constructed. The designed flexible coil can easily conform to different patient sizes with nonoverlapping coil elements in the transverse plane. It can cover a field of view of up to 44 × 28 cm(2) and removes the need for coil repositioning for body MRI patients with multiple clinical concerns. The 64-channel coil was compared with a 32-channel standard coil for signal-to-noise ratio and parallel imaging performances on different phantoms. With IRB approval and informed consent/assent, the designed coil was validated on 21 consecutive pediatric patients.

Results: The pediatric coil provided higher signal-to-noise ratio than the standard coil on different phantoms, with the averaged signal-to-noise ratio gain at least 23% over a depth of 7 cm along the cross-section of phantoms. It also achieved better parallel imaging performance under moderate acceleration factors. Good image quality (average score 4.6 out of 5) was achieved using the developed pediatric coil in the clinical studies.

Conclusion: A 64-channel semiflexible receive-only phased array has been developed and validated to facilitate high quality pediatric body MRI at 3T. Magn Reson Med 76:1015-1021, 2016. © 2015 Wiley Periodicals, Inc.

Keywords: MR phased array; body coils; flexible coils; parallel imaging; pediatric MRI.

Figure 1

(a) Coil layout of the 32-channel anterior coil. (b) Circuit schematic for a single coil element. (c) The anterior coil without the mechanical package: the coil elements only overlap in the S/I direction, creating high flexibility in the transverse plane. The cables were routed through the center of the coil arrays to achieve minimum coupling of the coils and cables. Floating cable traps are marked by yellow circles. (d) Demonstration of coil-to-patient matching on a doll for the anterior coils of a standard flexible 32-channel cardiac coil array (left) and the developed 64-channel pediatric coil (right). The anterior coil can automatically conform to the patient. R1.3: Floating cable traps marked.

Figure 2

λ/4 Baluns and channel decoupling: (a) self-resonance frequency of the ground loop formed by the attached cables depends on the location of the ground connection. Setting the ground connection at λ/4 results in highest common mode impedance. That breaks the ground loop and essentially has the same effect as conventional Baluns. A phantom study demonstrates good channel decoupling achieved by the 32-channel anterior coil: (b) images from all coil elements and (c) the corresponding images from all individual coil elements. (d) The noise correlation matrix of the 32-channel anterior coil also demonstrates good channel decoupling.

Figure 3

SNR comparison between the anterior coils from the 64-channel flexible coil and the standard 32-channel cardiac coil on (a) an unloaded rectangular phantom, (b) a loaded rectangular phantom, and (c) a pediatric shape loaded phantom. The 1D SNR map at two selected locations (highlighted by black lines) are shown in (d), (e) and (f) respectively. The 64-channel flexible coil achieved better SNR in all cases compared to the standard 32-channel coil. The achieved SNR gain over the standard coil is depth and load dependent.

Figure 4

Comparison of g-factor of the 64-channel flexible coil and the standard 32-channel coil under different acceleration factors. The experiment setups are shown in (a), and the g-factor maps are shown in (b). The 64-channel flexible coil has achieved better parallel imaging performance than the 32-channel standard coil for all acceleration factors. Note that lower torso mode of the 64-channel flexible coil was used.

Figure 5

A three-year-old male patient with clinical concerns of tethered cord: (a) T1-weighted FSE of cervical and upper thoracic spine and (b) its axial reformat; (c) T2-weighted FSE images of the sacrum; (d) 3D FSE images with an acceleration factor of 6; (e) T2-weighted images and (f) axial reformat; (g) post-contrast T1-weighted images; (h) images after maximum intensity projection show good image quality of the kidney. (a), (b), (c), (e) and (f) were imaged using the posterior mode, and the other images were imaged using the middle torso mode. The array enabled detailed imaging of multiple body parts without repositioning the patient, in this case a full spine, abdomen, and pelvis.