Janet Saffer

Qualification of PET Scanners for Use in Multicenter Cancer Clinical Trials: The American College of Radiology Imaging Network Experience

The PET Core Laboratory of the American College of Radiology Imaging Network (ACRIN) qualifies sites to participate in multicenter research trials by quantitatively reviewing submitted PET scans of uniform cylinders to verify the accuracy of scanner standardized uptake value (SUV) calibration and qualitatively reviewing clinical PET images from each site. To date, cylinder and patient data from 169 PET scanners have been reviewed, and 146 have been qualified. Methods: Each site is required to submit data from 1 uniform cylinder and 2 patient test cases. Submitted phantom data are analyzed by drawing a circular region of interest that encompasses approximately 90% of the diameter of the interior of the phantom and then recording the mean SUV and SD of each transverse slice. In addition, average SUVs are measured in the liver of submitted patient scans. These data illustrate variations of SUVs across PET scanners and across institutions, and comparison of results with values submitted by the site indicate the level of experience of PET camera operators in calculating SUVs. Results: Of 101 scanner applications for which detailed records of the qualification process were available, 12 (12%) failed because of incorrect SUV or normalization calibrations. For sites to pass, the average cylinder SUV is required to be 1.0 ± 0.1. The average SUVs for uniform cylinder images for the most common scanners evaluated—Siemens Biograph PET/CT (n = 43), GE Discovery LS PET/CT (n = 15), GE Discovery ST PET/CT (n = 34), Philips Allegro PET (n = 5), and Philips Gemini PET/CT (n = 11)—were 0.99, 1.01, 1.00, 0.98, and 0.95, respectively, and the average liver SUVs for submitted test cases were 2.34, 2.13, 2.27, 1.73, and 1.92, respectively. Conclusion: Minimizing errors in SUV measurement is critical to achieving accurate quantification in clinical trials. The experience of the ACRIN PET Core Laboratory shows that many sites are unable to maintain accurate SUV calibrations without additional training or supervision. This raises concerns about using SUVs to quantify patient data without verification.

Comparison of diffuse optical tomography of human breast with whole‐body and breast‐only positron emission tomography

We acquire and compare three-dimensional tomographic breast images of three females with suspicious masses using diffuse optical tomography (DOT) and positron emission tomography (PET). Co-registration of DOT and PET images was facilitated by a mutual information maximization algorithm. We also compared DOT and whole-body PET images of 14 patients with breast abnormalities. Positive correlations were found between total hemoglobin concentration and tissue scattering measured by DOT, and fluorodeoxyglucose (18F-FDG) uptake. In light of these observations, we suggest potential benefits of combining both PET and DOT for characterization of breast lesions.

First results of a dedicated breast PET imager, BPET, using NaI(Tl) curve plate detectors

We present the first imaging results from phantom measurements of a dedicated, breast-only positron emission imager, BPET, using NaI(Tl) Curve Plate detectors. The scanner uses 19 mm thick NaI(Tl) detectors in a split-ring design which surrounds the breast as the woman lies prone and the breast hangs down from the body. Because the detectors are close to the breast and the scanner detects photons that do not pass through the body, system sensitivity and spatial resolution are both optimized. The split ring design provides for flexibility for needle aspirations of masses or alternate viewing orientations. We have measured energy resolution, spatial resolution, scatter fraction, and system sensitivity. We have compared the BPET scanners performance to our clinical whole-body scanner using a breast phantom with hot spheres simulating lesions. The results show that for activity concentrations that correspond to clinical FDG doses, the dedicated scanner has better lesion detectability than the whole-body scanner for the 20 cm detector separation used.

Stability of cerebral blood flow measures using a split-dose technique with 99mTc-exametazime SPECT

AimTo determine whether there is stability of cerebral blood flow (CBF) measures using single photon emission computed tomography (SPECT) imaging in healthy controls in a test–retest split-dose paradigm. Such a paradigm is frequently used in the clinical and research setting to assess various brain states. MethodsFive healthy volunteers underwent two brain SPECT scans after the administration of low and high doses of 99mTc-exametazime. The first SPECT scan was acquired approximately 30 min after the intravenous injection of approximately 259 MBq of 99mTc-exametazime. The second SPECT scan was acquired approximately 30 min after the intravenous injection of 925 MBq of 99mTc-exametazime. Both scans were acquired over approximately 30–45 min and the images were reconstructed using filtered backprojection, a low-pass filter and Changs first-order attenuation correction. Values were obtained for regions of interest (ROIs) in major brain structures and normalized to whole brain activity. Counts on the second SPECT scan were also decay corrected for activity from the first scan. ResultsThe results demonstrated a strong correlation between the low-dose and high-dose scans for all regions (r=0.86, P<0.0001). Symmetries were preserved with a strong correlation between low-dose and high-dose scans (r=0.70, P<0.0001). Finally, most regions demonstrated less than a 5% difference between the low-dose and high-dose scans. ConclusionsThe results of this study demonstrate that the split-dose technique can be employed for clinical and research applications to measure CBF in different brain states using two SPECT scans on the same day.

Dosimetry of 11C-carfentanil, a μ-opioid receptor imaging agent

Objective11C-carfentanil is a radiopharmaceutical that selectively binds the μ-opiate receptor of the central nervous system. However, its dosimetry throughout the body and other organs has never been reported in the literature. The purpose of this study was to measure the radiation dosimetry of 11C-carfentanil in healthy human volunteers. The study was conducted within a regulatory framework that required its pharmacological safety to be assessed simultaneously. MethodsThe sample included two male and three female participants ranging in age from 28 to 49 years. Three to four scans were obtained over approximately 2 h starting immediately after the intravenous administration of 0.03 μg/kg of [11C]carfentanil injected as a slow bolus (mean activity injected was 280±68 MBq). The fraction of the administered dose in 10 regions of interest was quantified from the attenuation-corrected counts obtained on the axial images. Monoexponential functions were fit to each time–activity curve using a nonlinear, least-squares regression algorithm. These curves were numerically integrated to yield the number of disintegrations per unit activity administered in source organs. Sex-specific radiation doses were then estimated with the medical internal radiation dose technique. ResultsA few participants reported mild pharmacological effects of the radiotracer, primarily mild drowsiness, which is an expected side effect. The dose-limiting organ was the bladder wall, which received a mean of 3.65E-02 mGy/MBq. The mean effective dose equivalent and effective dose for 11C-carfentanil were 5.38E-03 and 4.59E-03 mSv/MBq, respectively. ConclusionThe observed dosimetry values for 11C-carfentanil indicate that it is safe for imaging μ-opiate receptors in the central nervous system and periphery.

Performance of a dual-layer positron-sensitive surgical probe

A positron-sensitive surgical probe is being built based on a multi-anode PMT and a dual-layer detector, which consists of an 8/spl times/8 array of thin plastic scintillators and a matched GSO crystal array. Our probe uses three selection criteria to identify positrons and suppress background gammas, including annihilation 511 keV gammas. First an energy threshold was applied on the plastic signals; next a second energy threshold was applied on the PMT sum signal; finally, a coincidence technique between the positrons and the annihilation 511 keV gammas was applied. These selection criteria were individually tested and optimized, and have been implemented with 9 channels of electronics. Experiments were conducted using phantoms with /sup 18/F-FDG and /sup 99m/Tc, commonly used in sentinel lymph node (SLN) surgery. Measurements based on the 9-channel electronics indicate that the sensitivity of the 9-channel probe to positrons from /sup 18/F-FDG is /spl sim/69-152 cps/kBq (2.5-5.6 kcps//spl mu/LCi) at different signal selection criteria The final 64-channel probe is expected to have /spl sim/40% higher positron sensitivity. The pixel separation is /spl sim/3.2 in terms of the peak to valley ratio. The second layer of the detector gives superior rejection power for 140 keV gammas. The true to false positron count ratio in the presence of 511 keV and 140 keV background gammas is expected to be high (>10) at a tumor to background ratio of 10:1.

Diffuse Optical Tomography and Positron Emission Tomography of Human Breast

We have acquired images of the breasts of four females with suspicious masses using Diffuse Optical Tomography and Positron Emission Tomography. The images are compared, and new types of contrast for cancer imaging are proposed.

A 64-pixel positron-sensitive surgical probe

We report on the continued development of a 64-pixel positron-sensitive surgical probe with a dual-layer detector and a multi-anode PMT. An 8 /spl times/ 8 array of thin plastic scintillators in the first layer detects positrons and a matched GSO crystal array in the second layer detects annihilation 511 keV gammas, which are required to be in coincidence with the detected positrons. Also, the 64 PMT anode signals are differentiated and an overshoot threshold is applied to separate the fast decay plastic anode signals from the slower GSO anode signals. Finally, an energy threshold is applied to the summed anode signal to distinguish 511 keV gammas from the 140 keV gammas commonly used in sentinel lymph node (SLN) surgery. Previously we reported on how these signal selection criteria were individually tested and optimized based on 9 channels of prototype electronics. Currently the electronics have been upgraded to Xilinx/spl reg/ programmable components, allowing on-the-fly alteration of signal selection criteria, and all 64 channels are operational. Initial measurements of the complete 64-pixel probe were conducted using /sup 18/F-FDG positron sources and /sup 18/F-FDG and /sup 99m/Tc phantoms (background 511 keV and 140 keV gammas), simulating lesions in the SLN surgery environment. The average positron sensitivity is measured to be 3.0-7.0 kcps//spl mu/Ci at different signal selection criteria. The lower bound on sensitivity corresponds to settings optimized for high image resolution and high background rejection ability. The upper bound on sensitivity corresponds to settings optimized for high sensitivity at the cost of lower image resolution and lower background rejection ability. The measured true-to-background contrast in the presence of clinically observed levels of 511 keV and 140 keV background gammas is /spl sim/3:1 for a tumor-to-background uptake ratio of 5:1. Performance measurements of the complete 64-pixel probe including sensitivity, true-to-background ratio, and the pixel separation ability are presented.

SU‐FF‐I‐66: A Method for Validation of Image Fusion Software for PET and CT Coregistration for Brain Radiotherapy

Purpose: A technique is described to quantitatively evaluate several methods for coregistering PET and CTimages: 1) manual registration based on fiducial markers, and 2) automated registration using several commercial algorithms that maximize mutual information.Method and Materials:CT and PET scans of an Alderson Striatal head phantom were obtained with and without three rectangular fiducial markers (1 mm × 1 mm × 3 mm). Each marker contained 10 μCi of Na‐22 along with contrast agent so markers were visible on both CT and PET scans. Several CT scans were acquired with a pixel size of 0.7 mm × 0.7mm of the phantom rotated by known amounts along the principle axes. PET scans were obtained at two resolutions, 2 mm× 2 mm and 4 mm × 4 mm pixels, at a single orientation (supine, zero rotation). For these scans the main compartment of the phantom was filled with F‐18‐FDG at clinically‐observed concentrations. Transmission scans also were obtained. Results: The accuracy of both automatic and manual coregistration of the emission PET to the CT was evaluated by comparing the transformation matrices (TM) to the known displacements. Image fusion between different CT and the PET scans show TM within 2–4 degrees of the known rotation for manual fusions and 1–3 degrees for automated registration. A subjective qualitative evaluation of the results based on the structures in the phantom and intensity pattern showed agreement to within 2 mm for higher resolution PET scans. Results were similar for registration of the transmission PET to the CT. Because these results are based on rigid‐body motions, they establish an upper‐bound on the accuracy obtainable from these registration techniques. Conclusion: A method has been developed to assess and quantitatively evaluate image coregistration software for CT and PETimages.

Factors affecting non-uniform attenuation compensation for SPECT myocardial perfusion imaging

The purpose of this study was to evaluate factors affecting the quality of the measured attenuation map and nonuniform attenuation compensation (AC) for SPECT myocardial perfusion imaging. Acquiring this map with low noise and accurate attenuation coefficients is important for simultaneous transmission-emission (STE) systems using fan beam geometry. The authors have investigated several factors affecting the measured attenuation map and AC using phantom and patient data. Myocardial uniformity decreased as extent of truncation increased. Finer sampling (128/spl times/128) in acquisition and reconstruction provided more accurate myocardial wall thickness and better contrast compared to 64/spl times/64 matrix. Downscatter correction improved accuracy of attenuation coefficients and myocardial uniformity. Reliable myocardial uniformity required scan durations of at least 14 minutes for myocardial activity of 350 /spl mu/Ci. Over a broad range of ML-EM iterations (20 to 50) in transmission (TCT) reconstruction measured attenuation coefficients for water were similar. Scatter correction improved normal myocardial wall contrast and moderate or severe defect contrast. In conclusion, methods to achieve accurate and low noise AC in myocardial imaging should use the best affordable sampling and scan duration, adequate activity of Gd-153 TCT source, proper positioning of patients and scatter correction.