Insertable head gradient coils offer significant advantages such as high gradient strength and fast gradient switching speed owing to shorter distances to the target region of interest than whole-body cylindrical coils. To produce superior gradient performance, the local head coil is typically designed with an asymmetric configuration to accommodate both the shoulders and head of a patient, leading to tough dimensional constraints and practical limits to the coil implementation. In this paper, we propose a new cone-shaped model to improve the performance of the asymmetric head coils and to mitigate patient claustrophobia. The primary coils are designed with a larger diameter at the patient end for access and a smaller diameter at the service end to bring wires closer to the human head, while the secondary coils are arranged on a cylindrical former to improve coil efficiency. Two cases are studied in this paper. Case I: inner bore size at the patient end (diameter 42 cm) is fixed as the design reference. In this case, inner diameters at any other position vary with the conical tilting angles. Compared with a set of conical gradient coils designed with tilting angles ranging from 0 to 14°, it is found that the optimal coil performance is achieved at the tilting angle of 14°. The key performance parameters have been improved by 100%–200% for the transverse coils, and about 50% for the longitudinal coils compared with the cylindrical counterpart with the reference bore size (that is, the same diameter of 42 cm). The conical coils also produce less heat in the gradient structure and lower acoustic noise in the field of view. Case II: inner bore size at the iso-centre (diameter 34 cm) is set as the design reference. It is also found that, compared with 34 cm diameter cylindrical coils, the conical transverse coil performance has been improved at an angle of 14°. The key coil performance increases by 20%–50% for transverse coil but decreases by 20%–40% for the longitudinal coil. However, compared with the tight cylindrical structure (e.g. 34 cm diameter), the tilting angle will provide patient-friendly space for imaging and handling, which can be critical for fMRI and other brain studies.
In pediatric magnetic resonance imaging (MRI), infants are exposed to rapid, time‐varying gradient magnetic fields, leading to electric fields induced in the body of infants and potential safety risks (e.g. peripheral nerve stimulation). In this numerical study, the in situ electric fields in infants induced by small‐sized gradient coils for a 1.5 T MRI scanner were evaluated. The gradient coil set was specially designed for the efficient imaging of infants within a small‐bore (baby) scanner. The magnetic flux density and induced electric fields by the small x, y, z gradient coils in an infant model (8‐week‐old with a mass of 4.3 kg) were computed using the scalar potential finite differences method. The gradient coils were driven by a 1 kHz sinusoidal waveform and also a trapezoidal waveform with a 250 µs rise time. The model was placed at different scan positions, including the head area (position I), chest area (position II), and body center (position III). It was found that the induced electric fields in most tissues exceeded the basic restrictions of the ICNIRP 2010 guidelines for both waveforms. The electric fields were similar in the region of interest for all coil types and model positions but different outside the imaging region. The y‐coil induced larger electric fields compared with the x‐ and z‐ coils. Bioelectromagnetics. 43:69–80, 2022. © 2021 Bioelectromagnetics Society.
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