An 8 channel parallel transmit system with current sensor feedback for MRI-guided interventional applications.

Background.Parallel transmit (pTx) has introduced many benefits to magnetic resonance imaging (MRI) with regard to decreased specific absorption rates and improved transmit field homogeneity, of particular importance in applications at higher magnetic field strengths. PTx has also been proposed as a solution to mitigating dangerous RF induced heating of elongated conductive devices such as those used in cardiac interventions. In this work we present a system that can augment a conventional scanner with pTx, in particular for use in interventional MRI for guidewire safety, by adjusting the amplitude and phase of each channel right before the start of the imaging pulses.Methods.The pTx system was designed to work in-line with a 1.5 T MRI while the RF synthesis and imaging control was maintained on the host MR scanner. The add-on pTx system relies on the RF transmit signal, unblanking pulse, and a protocol driven trigger from the scanner. The RF transmit was split into multiple fully modulated transmit signals to drive an array of custom transceiver coils. The performance of the 8-channel implementation was tested with regards to active and real-time control of RF induced currents on a standard guidewire, heating mitigation tests, and anatomical imaging in sheep.Results. The pTx system was intended to update RF shims in real-time and it was demonstrated that the safe RF shim could be determined while the guidewire is moved. The anatomical imaging demonstrated that cardiac anatomy and neighbouring superficial structures could be fully characterized with the pTx system inline.Conclusion.We have presented the design and performance of a real-time feedback control pTx system capable of adding such capabilities to a conventional MRI with the focus of guidewire imaging in cardiac interventional MRI applications.

Interventional cardiac MRI using an add-on parallel transmit MR system: In vivo experience in sheep.

PURPOSE: We present in vivo testing of a parallel transmit system intended for interventional MR-guided cardiac procedures. METHODS: The parallel transmit system was connected in-line with a conventional 1.5 Tesla MRI system to transmit and receive on an 8-coil array. The system used a current sensor for real-time feedback to achieve real-time current control by determining coupling and null modes. Experiments were conducted on 4 Charmoise sheep weighing 33.9-45.0 kg with nitinol guidewires placed under X-ray fluoroscopy in the atrium or ventricle of the heart via the femoral vein. Heating tests were done in vivo and post-mortem with a high RF power imaging sequence using the coupling mode. Anatomical imaging was done using a combination of null modes optimized to produce a useable B1 field in the heart. RESULTS: Anatomical imaging produced cine images of the heart comparable in quality to imaging with the quad mode (all channels with the same amplitude and phase). Maximum observed temperature increases occurred when insulation was stripped from the wire tip. These were 4.1℃ and 0.4℃ for the coupling mode and null modes, respectively for the in vivo case; increasing to 6.0℃ and 1.3℃, respectively for the ex vivo case, because cooling from blood flow is removed. Heating < 0.1℃ was observed when insulation was not stripped from guidewire tips. In all tests, the parallel transmit system managed to reduce the temperature at the guidewire tip. CONCLUSION: We have demonstrated the first in vivo usage of an auxiliary parallel transmit system employing active feedback-based current control for interventional MRI with a conventional MRI scanner.

Feasibility of dual-phase 99mTc-MDP SPECT/CT imaging in rheumatoid arthritis evaluation.

BACKGROUND: To prospectively demonstrate the feasibility of performing dual-phase SPECT/CT for the assessment of the small joints of the hands of rheumatoid arthritis (RA) patients, and to evaluate the reliability of the quantitative and qualitative measures derived from the resulting images. METHODS: A SPECT/CT imaging protocol was developed in this pilot study to scan both hands simultaneously in participants with RA, in two phases of 99mTc-MDP radiotracer uptake, namely the soft-tissue blood pool phase (within 15 minutes after radiotracer injection) and osseous phase (after 3 hours). Joints were evaluated qualitatively (normal vs. abnormal uptake) and quantitatively [by measuring a newly developed metric, maximum corrected count ratio (MCCR)]. Qualitative and quantitative evaluations were repeated to assess reliability. RESULTS: Four participants completed seven studies (all four were imaged at baseline, and three of them at follow-up after 1-month of arthritis therapy). A total of 280 joints (20 per hand) were evaluated. The MCCR from soft-tissue phase scans was significantly higher for clinically abnormal joints compared to clinically normal ones; P<0.001, however the MCCR from the osseous phase scans were not different between the two joint groups. Intraclass Correlation Coefficient (ICC) for MCCR was excellent [0.9789, 95% confidence interval (CI): 0.9734-0.9833]. Intra-observer agreement for qualitative SPECT findings was substantial for both the soft-tissue phase (kappa =0.78, 95% CI: 0.72-0.83) and osseous-phase (kappa =0.70, 95% CI: 0.64-0.76) scans. CONCLUSIONS: Extracting reliable quantitative and qualitative measures from dual-phase 99mTc-MDP SPECT/CT hand scans is feasible in RA patients. SPECT/CT may provide a unique means for assessing both synovitis and osseous involvement in RA joints using the same radiotracer injection.

Safe guidewire visualization using the modes of a PTx transmit array MR system.

PURPOSE: MRI-guided cardiovascular intervention using standard metal guidewires can produce focal tissue heating caused by induced radiofrequency guidewire currents. It has been shown that safe operation is made possible by using parallel transmit radiofrequency coils driven in the null current mode, which does not induce radiofrequency currents and hence allows safe tissue visualization. We propose that the maximum current modes, usually considered unsafe, be used at very low power levels to visualize conductive wires, and we investigate pulse sequences best suited for this application. METHODS: Spoiled gradient echo, balanced steady-state free precession, and turbo spin echo sequences were evaluated for their ability to visualize a conductive guidewire embedded in a gel phantom when run in maximum current modes at very low power level. Temperature at the guidewire tip was monitored for safety assessment. RESULTS: Excellent guidewire visualization could be achieved using maximum current modes excitation, with the turbo spin echo sequence giving the best image quality. Although turbo spin echo is usually considered to be a high-power sequence, our method reduced all pulses to 1% amplitude (0.01% power), and heating was not detected. In addition, visualization of background tissue can be achieved using null current mode, also with no recorded heating at the guidewire tip even when running at 100% (reported) specific absorption rate. CONCLUSION: Parallel transmit is a promising approach for both guidewire and tissue visualization using maximum and null current modes, respectively, for interventional cardiac MRI. Such systems can switch excitation mode instantaneously, allowing for flexible integration into interactive sequences.

Development of an Ultra High Resolution PET Scanner for Imaging Rodent Paws: PawPET.

A positron emission tomography (PET) scanner with submillimeter spatial resolution, capable of in vivo imaging of murine extremities was built based on two dual ended readout, hybrid depth of interaction (DOI) PET detectors. Each was composed of a 36 × 36 array of 0.43 mm × 0.43 mm × 8 mm unpolished lutetium oxyorthosilicate crystals separated by a 50 µm white diffuse reflector. The array was coupled to a position-sensing photomultiplier tube at one end and to an avalanche photodiode at the other end. The detector characterization included crystal identification accuracy, DOI, energy, and timing resolution measurements. The scanner was characterized in terms of its spatial resolution and its sensitivity and mouse images were acquired of a mouse paw injected with 18-F-NaF. Out of the 36 × 36 crystals only 33 × 33 crystals were identified. The coincidence timing, DOI, and energy resolution of the scanner was measured to be 2.8 ns, 1.4 mm, and 27%, respectively. The scanner's spatial resolution was measured with a line source and determined from an ordered subsets expectation maximization reconstruction to be 0.56 mm. The sensitivity of the scanner was measured to be 0.6% at the center of the field of view.

High-resolution (18)F-FDG PET/CT for assessing disease activity in rheumatoid and psoriatic arthritis: findings of a prospective pilot study.

OBJECTIVE: Rheumatoid arthritis (RA) and psoriatic arthritis (PsA) commonly affect the small joints of the wrist and hand. We evaluated the performance of a new, high-resolution extremity positron emission tomography (PET)/CT scanner for characterizing and quantifying pathologies associated with the two arthritides in the wrist and hand joints. METHODS: Patients with RA or PsA underwent fluorine-18 fludeoxyglucose ((18)F-FDG) PET/CT wrist and hand imaging, respectively, on the high-resolution scanner. Calibrated CT images and co-registered PET images were reconstructed. PET/CT was derived for the radiocarpal and pisiform-triquetral compartments, joints with erosive changes, sites of synovitis or tenosynovitis and the nail bed and were correlated with clinical and MRI findings. RESULTS: Significantly elevated (18)F-FDG uptake was measured for the radiocarpal and pisiform-triquetral compartments and at sites of bone erosion, synovitis, pannus and oedema, compared with unaffected joints (p < 0.05) in patients with RA, consistent with their clinical findings. In patients with PsA, significantly elevated (18)F-FDG uptake was measured for joints with synovitis compared with unaffected joints (p < 0.05), with patterns of (18)F-FDG uptake along the tendons, at the enthesis and in the nail bed, consistent with tenosynovitis, enthesitis and nail dystrophy, respectively. CONCLUSION: High-resolution (18)F-FDG PET/CT imaging of the wrist and hand is feasible in an RA or PsA patient cohort and is capable of providing quantifiable measures of disease activity (synovitis, enthesitis, oedema and bone destruction). ADVANCES IN KNOWLEDGE: High-resolution PET/CT imaging shows promise as a tool for understanding the pathogenesis of the arthritic process and for non-invasive, objective assessment of RA or PsA severity and therapy selection.

Characterization of a high-resolution hybrid DOI detector for a dedicated breast PET/CT scanner.

The aim of this study is to design and test a new high-resolution hybrid depth of interaction (DOI) detector for a dedicated breast PET/CT scanner. Two detectors have been designed and built. The completed detectors are based on a 14 × 14 array of 1.5 × 1.5 × 20 mm(3) unpolished lutetium orthosilicate scintillation crystals, with each element coated in a 50 µm layer of reflective material. The detector is read out from both ends using a position-sensitive photomultiplier tube (PSPMT) and a large active area (20 × 20 mm(2)) avalanche photodiode (APD) to enable acquisition of DOI information. Nuclear instrumentation modules were used to characterize the detectors' performances in terms of timing, intrinsic spatial resolution (ISR) and energy resolution, as well as DOI resolution with a dual-ended readout configuration. Measurements with the APD were performed at a temperature of 10 °C. All crystals were identified at all depths, even though the signal amplitude from the PSPMT decreases with depth away from it. We measured a timing resolution of 2.4 ns, and an average energy resolution of 19%. The mean ISR was measured to be 1.2 mm for crystals in the central row of the array for detectors in the face-to-face position. Two off-center positions were measured corresponding to 26° and 51° oblique photon incidence, and the mean ISR at these positions was 1.5 and 1.7 mm, respectively. The average DOI resolution across all crystals and depths was measured to be 2.9 mm (including the beam width of 0.6 mm). This detector design shows good promise as a high-resolution detector for a dedicated breast PET/CT scanner.