Bosky Ravindranath

Small animal simultaneous PET/MRI: initial experiences in a 9.4 T microMRI

We developed a non-magnetic positron-emission tomography (PET) device based on the rat conscious animal PET that operates in a small-animal magnetic resonance imaging (MRI) scanner, thereby enabling us to carry out simultaneous PET/MRI studies. The PET detector comprises 12 detector blocks, each being a 4 × 8 array of lutetium oxyorthosilicate crystals (2.22 × 2.22 × 5 mm(3)) coupled to a matching non-magnetic avalanche photodiode array. The detector blocks, housed in a plastic case, form a 38 mm inner diameter ring with an 18 mm axial extent. Custom-built MRI coils fit inside the positron-emission tomography (PET) device, operating in transceiver mode. The PET insert is integrated with a Bruker 9.4 T 210 mm clear-bore diameter MRI scanner. We acquired simultaneous PET/MR images of phantoms, of in vivo rat brain, and of cardiac-gated mouse heart using [(11)C]raclopride and 2-deoxy-2-[(18)F]fluoro-D-glucose PET radiotracers. There was minor interference between the PET electronics and the MRI during simultaneous operation, and small effects on the signal-to-noise ratio in the MR images in the presence of the PET, but no noticeable visual artifacts. Gradient echo and high-duty-cycle spin echo radio frequency (RF) pulses resulted in a 7% and a 28% loss in PET counts, respectively, due to high PET counts during the RF pulses that had to be gated out. The calibration of the activity concentration of PET data during MR pulsing is reproducible within less than 6%. Our initial results demonstrate the feasibility of performing simultaneous PET and MRI studies in adult rats and mice using the same PET insert in a small-bore 9.4 T MRI.

Limbic dysregulation is associated with lowered heart rate variability and increased trait anxiety in healthy adults

We tested whether dynamic interaction between limbic regions supports a control systems model of excitatory and inhibitory components of a negative feedback loop, and whether dysregulation of those dynamics might correlate with trait differences in anxiety and their cardiac characteristics among healthy adults.

Results from prototype II of the BNL simultaneous PET-MRI dedicated breast scanner

At Brookhaven National Laboratory, we are developing a simultaneous PET-MRI breast imaging system. A prototype II version of the PET system has been constructed. This device consists of 24 detector blocks where each block consists of a 4 × 8 array of 2.2 × 2.2 × 15 mm3 LYSO crystal directly coupled to a 4 × 8 non-magnetic APD array. The scanner has an inner diameter of 100mm and an axial extent of 18mm. Resolution measurements were carried out for the prototype system to evaluate the depth of interaction effects. Average resolution less than 2mm FWHM was maintained throughout the field of view. The prototype PET system was operated unshielded inside the RF coil of the Aurora 1.5 T dedicated breast MRI machine. Artifact free MRI images with good SNR were obtained.

A simultaneous PET/MRI breast scanner based on the RatCAP

We propose to develop a high resolution scanner for simultaneous PET and MRI breast imaging that is capable of providing highly sensitive and specific breast cancer examinations. The addition of high resolution positron emission tomography capability to an existing dedicated MRI system will give a device in which each of the modalities contributes its strengths to compensate for the weaknesses of the other. In this combined modality scanner, we have the anatomical information from the MRI to compensate for the somewhat poorer resolution in PET, and we have the predictive power of PET in identifying the type of lesion to overcome the high false positive rate of MRI. This device is based on the technical approach used in the RatCAP scanner with the innovation of detecting coincident events in separate rings of the RatCAP scanner. We are presenting the design and GATE simulations of the full breast imaging system and preliminary PET and MRI results from the prototype system.

An MRI-compatible PET insert for whole body studies in rodents at high functional and anatomical resolution

The feasibility of performing high-resolution PET and high-field MRI simultaneously in rodents has been previously demonstrated in small-scale systems capable of imaging the rat brain and mouse. We are nearing completion of a larger scale PET system which will accommodate the whole rat and perform at 9.4 T with <2 mm PET resolution. The PET insert has inner/outer diameters of 13.5/20.6 cm, compact enough to fit within the gradient set of a Varian large-bore 9.4T MRI system while accommodating on the inside a commercial Insight birdcage coil for the rat. The resulting volume capable of simultaneous PET/MRI imaging is 7 cm in diameter and 5 cm axially. The 96 PET detectors are arranged in 4 rings of modular detector blocks, each with an array of 2 × 2 × 14 mm LYSO crystal coupled to a Hamamatsu APD array and read out by the RatCAP ASIC. Data acquisition is divided into 4 sectors, each handled by a local FPGA which communicates via Ethernet to the host PC. Offline data processing software is being developed to bin coincidences and determine physical corrections. Image reconstruction follows a listmode OSEM approach. The design of all hardware components is complete and prototypes of each have been fabricated. System integration is underway and initial performance of the system will be presented.

An MR compatible PET scanner based on RatCAP for small animal imaging at 9.4 T

Combining PET and MRI technology in order to obtain simultaneous functional information with anatomical precision in vivo is of tremendous interest in the field of molecular imaging. PET/MRI imaging provides perfect anatomical and temporal coregistration of PET and MR images, along with clear delineation of tissue boundaries. An MR compatible PET scanner based on Rat Conscious Animal PET (RatCAP) is developed for simultaneous acquisition of PET/MRI images of the rat brain in a 9.4 T microMRI scanner. The PET tomograph is housed in a segmented G10/copper case constructed with a copper coating thickness of 5 microns, to minimize the eddy currents and provide better RF shielding. Special RF coils were designed to fit inside the PET tomograph. A custom made G10 tube assembly was constructed for accurate positioning of the rat bed, RF coil and RatCAP. Simultaneous PET/MRI images of a rat striatum phantom were acquired in the 9.4 T microMRI with minimal interference between PET electronics and MRI. PET compatible RF quadrature coils are being constructed which are capable of providing rat brain images with improved SNR. The preliminary results show the feasibility of performing PET/fMRI studies of rat brain and exploring the possibility of acquiring simultaneous PET/MRI whole body mouse images in the 9.4 T.

Massively parallel image reconstruction for the BNL breast scanner PET tomograph using CUDA

We describe the implementation of an image reconstruction algorithm using the parallel processing capabilities of graphics processors. We are designing a new breast scanner which will allow simultaneous acquisition of PET and MRI images. The breast scanner is based on the technology of the much smaller RatCAP PET detector. The image reconstruction of the breast scanner poses a significant computing challenge, which we hope to alleviate with the processing power of modern GPUs. We describe the status of the reconstruction and discuss goals and future possible improvements.

Dual-Modality Preclinical PET/MRI Instrumentation

Multimodality imaging using positron emission tomography (PET) and Magnetic resonance imaging (MRI) is emerging as an extremely valuable tool for investigating disease and physiological processes in small animal models. MRI has high spatial resolution but low sensitivity for detecting low abundance molecules and PET has very high sensitivity for the detection of radiotracers but poor spatial resolution. The combination of these two molecular imaging techniques offers synergistic advantages over any either modality alone. A fused anatomical and functional image affords complementary information that clearly improves our understanding. Images acquired simultaneously offer distinct advantages over sequential image acquisition since this gives “perfect” coregistration and observing the same process from two different vantage points can ensure the correlation of information that is impossible in separate experiments.

Readout technologies for the BNL-UPenn MRI-compatible PET scanner for rodents

We describe the prototype of a full-body PET scanner for rats that is compatible with a 9.4 T MRI system. The detector consists of 96 PET detector blocks in a cylindrical arrangement. In this paper we concentrate on a new readout technology, which takes advantage of the fact that optical fibers are insensitive to electromagnetic interference. The data are formatted in a FPGA on the motherboard and sent to a data acquisition computer through standard Gigabit Ethernet connections. We will describe the technology chosen for the system, and introduce the data acquisition adapted for the readout of the data.

Evaluation of cross-modality electromagnetic interactions in a shielded PET/MRI system

A small-animal PET/MRI scanner was developed previously that was integrated in a Bruker 9.4 T microMRI system, with which simultaneous PET/MR images of a rats brain and of a gated mouse-heart were obtained. To minimize electromagnetic interactions in our PET/MRI system, viz., the effect of radiofrequency (RF) pulses in the PET, our modular front-end PET electronics were surrounded with variously configured shields. These included a solid aluminum shield and thin segmented layers of copper shielding. It was noted that the gradient-echo RF pulses had no impact on PET data when the PET electronics were shielded with either the aluminum or copper shields. However, we observed spurious counts in the PET data resulting from high-intensity fast spin-echo RF pulses; compared to the unshielded condition, they were suppressed effectively by the aluminum shield (∼97%) and the double-layer copper shield (∼90%). Using the solid aluminum shield yielded a poorer signal in the MR images than compared to segmented copper shields. Our initial results on shielding demonstrate that we can obtain interference-free PET data during gradient-echo pulses and obtain good-quality MR images with thin copper layers covering the PET detector housing.