A complete high-efficiency transmit amplifier unit designed to be implemented in on-coil transmit arrays is presented. High power capability, low power dissipation, scalability and cost minimization were some of the requirements imposed to the design. The system is composed of a current mode class-D (CMCD) amplifier output stage and a voltage mode class-D (VMCD) preamplification stage. The amplitude information of the radio frequency pulse was added through a customized step-down DC-DC converter with current amplitude feedback that connects to the CMCD stage. Benchtop measurements and imaging experiments were carried out to analyze system performance. Direct control of B1 was possible and its load sensitivity was reduced to less than 10% variation from unloaded to full loaded condition. When using the amplifiers in an array configuration, isolation above 20 dB was achieved between neighboring coils by the amplifier decoupling method. High output current operation of the transmitter was proved on the benchtop through output power measurements and in a 1.5 T scanner through flip angle quantification. Finally, single and multiple channel excitations with the new hardware were demonstrated by receiving signal with the body coil of the scanner.

High-resolution small animal single photon emission computed tomography (SPECT) with pinhole collimation has become well-established in preclinical research. Conventional scanner designs use custom fixtures that precisely attach and align pinholes to dedicated detectors. As a lower-cost alternative, we have developed a compact small animal SPECT device with integrated pinholes that is physically detached from gamma camera detectors and uses real-time calibration. The device consists of a shielded lead box with multiple integrated tungsten pinholes on the sides and with a rotating platform at the bottom. The animal or phantom to be imaged is positioned within a hollow cylinder mounted to the platform, with point source markers attached to the cylinder's outer surface. The entire device is placed on the patient table of a standard dual-head gamma camera system with collimators removed. Projection data are acquired with multiple energy windows to separate the point source data from the emission data. The complete geometric calibration is derived through analysis of the point source data. The system matrix is computed based on the calibration parameters and then applied to reconstruction of the emission data. Real-time calibration was found to be robust and accurate, even in the presence of downscatter from 99m Tc emission data or dual-isotope 99m Tc/ 111 In emission data. Successful calibration was consistently achieved in all pinhole configurations tried (up to seven pinholes per detector). Accuracy of real-time calibration was demonstrated in phantom studies and animal studies, where structures as small as 0.8 mm could be resolved in reconstructed images. Images appeared to be free of artifacts, and no cross-contamination from the calibration point sources was observed. Real-time calibration is feasible for pinhole SPECT, thus enabling standalone pinhole devices to be used without special alignment fixtures. The calibration accuracy is sufficient for high-resolution small animal studies. The hardware is compatible with nearly all gamma camera models and represents a lower-cost entry-level solution to the field of small animal SPECT.Index Terms-Calibration, pinhole, single photon emission computed tomography (SPECT), small animal.

Purpose Single sideband amplitude modulation (SSB) is an appealing platform for highly parallel wireless MRI detector arrays because the spacing between channels is ideally limited only by the MRI signal bandwidth. However this assumes that no other sources of interference are present outside that bandwidth. This work investigates the practical interference between multiple SSB-encoded MRI signals. Methods Noise from coil preamplifiers and carrier bleed-through are identified as sources of interference. Two different SSB systems were designed for 1.5T with different noise filtering properties. We show how the differences between the filtered noise profiles impact the received MR signal’s dynamic range (DRsig) and image signal-to-noise ratio (SNR) through simulation, bench measurements, and phantom imaging experiments. Results When operating individually in the MR scanner, both SSB systems were shown to minimally impact the original DRsig and SNR. On the other hand, when all eight channels were operating simultaneously, an average SNR loss was observed to be 12% in the one system, while a second system with more complex filtering was able to achieve a 3% loss in SNR. Conclusion Successful wireless transmission of multiple SSB-encoded MRI signals is possible as long as channel interference is properly managed through design and simulation.

Purpose A prototype wireless guidance device using single sideband amplitude modulation (SSB) is presented for a 1.5T MRI system. Methods The device contained three fiducial markers each mounted to an independent receiver coil equipped with wireless SSB technology. Acquiring orthogonal projections of these markers determined the position and orientation of the device, which was used to define the scan plane for a subsequent image acquisition. Device localization and scan plane update required approximately 30 ms, so it could be interleaved with high temporal resolution imaging. Since the wireless device is used for localization and doesn’t require full imaging capability, the design of the SSB wireless system was simplified by allowing an asynchronous clock between the transmitter and receiver. Results When coupled to a high readout bandwidth, the error caused by the lack of a shared frequency reference was quantified to be less than one pixel (0.78 mm) in the projection acquisitions. Image-guidance with the prototype was demonstrated with a phantom where a needle was successfully guided to a target and contrast was delivered. Conclusion The feasibility of active tracking with a wireless detector array is demonstrated. Wireless arrays could be incorporated into devices to assist in image-guided procedures.

Multichannel transmission has the potential to improve many aspects of MRI through a new paradigm in excitation. In this study, multichannel transmission is used to address the effects that variations in B0 homogeneity have on fat-saturation preparation through the use of the frequency, phase, and amplitude degrees of freedom afforded by independent transmission channels. B1 homogeneity is intrinsically included via use of coil sensitivities in calculations. A new method, parallel excitation for B-field insensitive fat-saturation preparation, can achieve fat saturation in 89% of voxels with Mz ≤ 0.1 in the presence of ±4 ppm B0 variation, where traditional CHESS methods achieve only 40% in the same conditions. While there has been much progress to apply multichannel transmission at high field strengths, particular focus is given here to application of these methods at 1.5 T.