Eugene E. Gualtieri

Singles transmission scans performed post-injection for quantitative whole body PET imaging

Post-injection singles transmission scanning has been implemented in the septumless PENN PET 240H scanner. The method uses a 6 mCi point transmission source of /sup 137/Cs at the axial center and 36 cm off transaxial center of the camera field of view. Singles transmission scans of 1.8 minutes per bed axial position provide similar scan count densities to 15 minute coincidence transmission scans with a 0.5 mCi /sup 68/Ge rod transmission source. Scatter and emission contamination suppression are achieved by applying a narrow 652 keV transmission photopeak energy window. The residual 511 keV emission contamination constitutes a background of uniform spatial distribution. Accurate and low noise attenuation correction is achieved by segmenting the singles transmission images into lung and soft tissue volumes. Established 511 keV gamma ray attenuation coefficients are then applied and these images are forward projected for attenuation correction. Expectation maximisation with ordered subsets (OS-EM) reconstruction is performed for the emission data to improve image quality. Both the segmentation and OS-EM reconstruction maintain quantitative accuracy in the fully corrected emission images compared to measured coincidence transmission correction. Thus, a clinical protocol involving 40 minutes of emission scans followed by 20 minutes of singles transmission scans allow the 60 cm of the human torso to be fully scanned within 60 minutes. These quantitative whole body FDG PET images may then be used for tumor grading and assessment of tumor response to treatment.

Spatial sensitivity and temporal response of spin echo and gradient echo bold contrast at 3 T using peak hemodynamic activation time

Recent theoretical and experimental work has suggested that spin echo (SE) functional MRI (fMRI) has improved localization of neural activity compared to gradient echo (GE) fMRI at high field strengths, albeit with a decrease in blood oxygenation level-dependent (BOLD) contrast. The present study investigated spatial and temporal variations in GE and SE fMRI at 3 T in response to a brief visual stimulus. The results demonstrate that SE BOLD contrast reaches its maximum amplitude more quickly than does GE contrast at long echo times. We have called this metric the peak hemodynamic activation time (PHAT). Because BOLD changes in response to increased neuronal activity occur earlier in the microvasculature and then later propagate into the venous compartment, these results provide further evidence that SE-based BOLD contrast provides superior localization to the site of activation at 3 T. Spatial overlay of SE and GE PHAT maps onto structural images reveal markedly different spatial profiles and further support the interpretation that shorter peak times correlate to improved spatial sensitivity.

A pulse sequence for rapid in vivo spin-locked MRI.

To develop a novel pulse sequence called spin‐locked echo planar imaging (EPI), or (SLEPI), to perform rapid T1ρ‐weighted MRI.

T1ρ Contrast in Functional Magnetic Resonance Imaging

The application of T1 in the rotating frame (T1ρ) to functional MRI in humans was studied at 3 T. Increases in neural activity increased parenchymal T1ρ. Modeling suggested that cerebral blood volume mediated this increase. A pulse sequence named spin‐locked echo planar imaging (SLEPI) that produces both T1ρ and T2* contrast was developed and used in a visual functional MRI (fMRI)experiment. Spin‐locked contrast significantly augments the T2* blood oxygen level‐dependent (BOLD) contrast in this sequence. The total functional contrast generated by the SLEPI sequence (1.31%) was 54% larger than the contrast (0.85%) obtained from a conventional gradient‐echo EPI sequence using echo times of 30 ms. Analysis of image SNR revealed that the spin‐locked preparation period of the sequence produced negligible signal loss from static dephasing effects. The SLEPI sequence appears to be an attractive alternative to conventional BOLD fMRI, particularly when long echo times are undesirable, such as when studying prefrontal cortex or ventral regions, where static susceptibility gradients often degrade T2*‐weighted images. Magn Reson Med, 2005.

Temporal resolving power of spin echo and gradient echo fMRI at 3T with apparent diffusion coefficient compartmentalization

The temporal resolving power of blood oxygenation level‐dependent (BOLD) functional magnetic resonance imaging (fMRI) at 3T was investigated in the visual and auditory cortices of the human brain. By using controlled temporal delays and selective visual hemifield stimulation, regions with similar (left vs. right occipital cortex) and different (occipital cortex vs. auditory cortex) vascular architectures were compared. Estimates of the time‐to‐peak (TTP) of the BOLD hemodynamic response function (hrf) were obtained using a spin echo (SE) sequence and compared to those acquired using a traditional gradient echo (GE) sequence. The hrf TTP in the visual cortex was found to be 4.73 s and 4.21 s for GE and SE, respectively. The auditory cortex response was significantly delayed, with TTPs of 4.95 s and 4.51 s for GE and SE, respectively. The GE response was able to resolve visual stimuli separated by 250 ms, whereas SE could resolve stimuli 500 ms apart. Apparent‐diffusion‐coefficient (ADC) compartmentalization of the BOLD signal was applied to restrict the vascular sensitivity of the SE and GE sequences. Limiting the response to voxels with ADCs < 0.8 × 10−3 mm2/s improved the temporal resolving power of GE and SE BOLD to 125 ms and 250 ms, respectively. Hum Brain Mapp 25:247–258, 2005.

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 &#60;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.

First results from the BNL/Penn PET-MRI system for whole body rodent imaging at 9.4T

A new PET system has been developed for the purposes of simultaneous PET/MR whole body rodent imaging in conjunction with a Varian large-bore 9.4T MRI system with a commercial Insight birdcage coil. The detector and readout technology is based on that developed for the RatCAP PET system, resulting in a highly robust and compact design that is compatible with MRI systems at the highest field strength. Testing of the full readout chain has shown a successful implementation of both modalities, with indications of only modest cross-interference on MRI image quality or PET detection efficiency. The final design cycle is complete, and supports operation of the full system with upgrades to the data acquisition electronics, firmware, and software. First results from the complete PET system will be presented, including preliminary imaging and tests of interference between modalities.

Statistical decision making in emission tomography using emission-count posteriors

We present a new approach to decision making based on the concept of emission counts (EC), i.e., the number of events emitted per voxel during the scan. The approach allows direct computation of posterior probabilities of hypotheses defined in terms of EC, and is applicable to any type of emission tomographic list-mode projection data, e.g., SPECT, PET, or time-of-flight (TOF)-PET, as well as binned data which can be considered a special case of list-mode data. Conditional Bayesian (CB) decision theory utilizing values of the posterior probability of hypotheses as test statistics are used to derive decision rules. We demonstrate that the derived decision principle is equivalent to the likelihood-ratio ideal observer for binary hypothesis testing. We use data acquired in list-mode format for an anthropomorphic torso phantom using a lanthanum-bromide time-of-flight PET scanner to provide examples of application of EC-CB observer. For data-specific decisions, we demonstrate examples of multiple-hypotheses decision making. For imaging system evaluation, we define two regions of interest (ROls) on the image, and two hypotheses, H1 and H2, that one ROI emitted on average at least r times more events than the other region. The posterior probabilities of H1 and H2 are determined. ROC curves are constructed using 50 projection data sets (list-mode tiles), each including 16 million prompts, from which 25 data sets correspond to H1 and the other 25 to H2. The areas and partial areas under the ROC curves are used as figures of merit evaluating the performance of the LaBr3 PET system in discriminating between H1 and H2 with and without TOF information. Summary: A new optimal numerical observer which makes decisions based on posterior probability of emission counts is presented. The utility of the observer for hypothesis testing is demonstrated on TOF list-mode phantom data acquired on PENN LaPET scanner.

Attenuation Correction in PET Using CS-137 Singles Transmission and Iterative Reconstruction

Attenuation correction in PET using a coincidence method typically leads to 10–20 minute acquisition time per axial bed position in order to obtain adequate counts. This makes it impractical for whole body tumor surveys requiring 7–12 axial positions in a typical dedicated PET scanner. In order to reduce the acquisition time we employ a Cs-137 point source in singles mode (gamma ray energy 662 keV).

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.