Singles Transmission Scanning Research Articles

Count Rate Performance of the MicroPET Focus 220 Animal Scanner in Singles Transmission Scanning Mode

Count rate loss in singles transmission scans due to dead time can decrease the accuracy of attenuation correction. In addition, for very low transmission source activities measured attenuation may be affected by noise and background radiation emitted by radioactive lutetium-176 in the scintillator material. The aims of this study were to investigate count rate performance in singles transmission scanning for the microPET Focus 220 animal scanner, to examine the effect of energy window width on noise, scatter and dead time, and to determine the optimal range of activity for accurate attenuation correction. Transmission measurements were performed with a decaying <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">99m</sup> Tc point source and using wide (21%) and narrow (4%) energy windows. We observed count rate losses at the detector level and in the front end electronics during energy discrimination. Further losses emerged due to multiplexing at the detector head interface for activities higher than 150 MBq for the wide energy window. Substantial count rate losses during transfer of list mode data from the microPET to the PC were also observed for this condition. Lutetium-176 background caused a significant deviation of measured attenuation for activities lower than 10 MBq for both energy windows. No significant bias due to scatter was introduced when using the wide energy window. We conclude that accurate attenuation data can be obtained with single photon transmission source activity in the range 10 to 150 MBq without requiring deadtime correction.

Implementation of Histogram Based Soft-Tissue Segmentation for Single Spiral Transmission Scanning in Whole Body PET

3D continuous emission and spiral transmission (CEST) scanning provides a high-throughput whole-body PET study by using two dedicated detectors for 3D emission and singles transmission, together with continuous bed movement. To supress the amount of transmission scatter components (TSC) in singles transmission data, the transmission detector was designed to have a short axial extent with a highly collimated <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs point source, and to remove the amount of emission contamination (EC) in post-injection scanning, real-time EC correction was implemented. However, transmission images can be still affected by residual EC and TSC, depending on patient size and injected dose. This produces slight variations in attenuation coefficients, depending on the patient's axial and radial positions. In this study, we developed a new soft-tissue segmentation (STS) method based on histogram scaling at each axial position of the spiral transmission. Peaks, corresponding to soft-tissue in a histogram of attenuation coefficients, were found at each axial position and the transmission image was scaled using the ratio of soft-tissue histogram peaks to the theoretical water attenuation coefficient. In scaled transmission images, pixel values near soft-tissue peaks were replaced with their theoretical water attenuation coefficients. Quantitative evaluation of the transmission images obtained was performed under various acquisition conditions, both with and without the proposed STS method. Final imaging performance evaluations included quantification of emission images reconstructed using both STS attenuation correction and hybrid scatter correction. Results showed that the proposed STS method for spiral transmission scanning provided quantitative images that contained activity, independent of object size.

Evaluation of transmission methodology and attenuation correction for the microPET Focus 220 animal scanner

An accurate, low noise estimate of photon attenuation in the subject is required for quantitative microPET studies of molecular tracer distributions in vivo. In this work, several transmission-based measurement techniques were compared, including coincidence mode with and without rod windowing, singles mode with two different energy sources (68Ge and 57Co), and postinjection transmission scanning. In addition, the effectiveness of transmission segmentation and the propagation of transmission bias and noise into the emission images were examined. The 57Co singles measurements provided the most accurate attenuation coefficients and superior signal-to-noise ratio, while 68Ge singles measurements were degraded due to scattering from the object. Scatter correction of 68Ge transmission data improved the accuracy for a 10 cm phantom but over-corrected for a mouse phantom. 57Co scanning also resulted in low bias and noise in postinjection transmission scans for emission activities up to 20 MBq. Segmentation worked most reliably for transmission data acquired with 57Co but the minor improvement in accuracy of attenuation coefficients and signal-to-noise may not justify its use, particularly for small subjects. We conclude that 57Co singles transmission scanning is the most suitable method for measured attenuation correction on the microPET Focus 220 animal scanner.

Postinjection single photon transmission tomography with ordered-subset algorithms for whole-body PET imaging

We have evaluated the penalized ordered-subset transmission reconstruction (OSTR) algorithm for postinjection single photon transmission scanning. The OSTR algorithm of Erdogan and Fessler (1999) uses a more accurate model for transmission tomography than ordered-subsets expectation-maximization (OSEM) when OSEM is applied to the logarithm of the transmission data. The OSTR algorithm is directly applicable to postinjection transmission scanning with a single photon source, as emission contamination from the patient mimics the effect, in the original derivation of OSTR, of random coincidence contamination in a positron source transmission scan. Multiple noise realizations of simulated postinjection transmission data were reconstructed using OSTR, filtered backprojection (FBP), and OSEM algorithms. Due to the nonspecific task performance, or multiple uses, of the transmission image, multiple figures of merit were evaluated, including image noise, contrast, uniformity, and root mean square (rms) error. We show that: 1) the use of a three-dimensional (3-D) regularizing image roughness penalty with OSTR improves the tradeoffs in noise, contrast, and rms error relative to the use of a two-dimensional penalty; 2) OSTR with a 3-D penalty has improved tradeoffs in noise, contrast, and rms error relative to FBP or OSEM; and 3) the use of image standard deviation from a single realization to estimate the true noise can be misleading in the case of OSEM. We conclude that using OSTR with a 3-D penalty potentially allows for shorter postinjection transmission scans in single photon transmission tomography in positron emission tomography (PET) relative to FBP or OSEM reconstructed images with the same noise properties. This combination of singles+OSTR is particularly suitable for whole-body PET oncology imaging.

Singles transmission scanning for rotating positron cameras with large-area detectors

Describes the novel geometry and initial results of a singles transmission system for PETRRA, a whole-body PET camera with rotating planar detectors. Special frames mounted on to the front face of each detector support a collimated /sup 137/Cs source in various positions. Cone-beam transmission data are acquired during 360/spl deg/ gantry rotation and images are reconstructed using the Feldkamp (1984) algorithm. The radial transaxial field of view (FOV) can be increased by using two source positions displaced either side of the central position. The truncated data acquired from each source position are combined to form a complete dataset. The axial FOV depends on the amount of collimation in the axial direction and could be increased by horizontal couch motion. Preliminary singles transmission images of both real and simulated thorax phantoms have been obtained. The effects of varying the linear sampling interval, the number of projections and the axial FOV have been studied. The singles transmission scanning system developed for PETRRA could be applied to other PET cameras with large planar detectors.

A comparison of segmentation and emission subtraction for singles transmission in PET

Single photon transmission scans, using /sup 137/Cs (662 keV gamma rays), are performed post-injection for clinical whole body PET studies. Two methods of processing the transmission data are investigated. The first method segments the transmission image into lung and soft tissue volumes. Average attenuation coefficients for 511 keV are applied and this transmission image is forward projected for attenuation correction. In the second method short emission contamination (EC) scans are performed after the singles transmission scans. These are singles transmission scans without a /sup 137/Cs transmission source. These EC scans are then subtracted from the transmission scans to remove contamination of the 662 keV energy window by 511 keV emission gamma rays. From ten patients studied by this method, the EC subtracted transmission images have average soft tissue attenuation coefficients of 0.074/cm, compared to the expected value of 0.086/cm for 662 keV gamma rays. Simple scaling of these transmission images to 511 keV values (0.095/cm) results in quantitative attenuation correction. A comparison of the two methods is made for these ten patient studies. Both methods result in accurate attenuation correction. The segmentation method performs low noise attenuation correction but is more limited to torso studies with the limbs out of the FOV. The EC subtraction method is more versatile and is applicable to the entire body but results in some more noise in the fully corrected images.

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.

Attenuation correction in PET using single photon transmission measurement.

The use of single photon transmission measurement with a rotating rod source has been evaluated to measure the attenuation correction factors in positron emission tomography (PET). The singles projections are resampled into the coincidence geometry using the detector positions and the rod source location. A nonparalyzable dead time correction algorithm was developed for the block detectors used in the McMaster PET scanner. This enables accurate attenuation correction factors (ACFs) to be computed using a wide range of source strengths for transmission scanning. Transaxial resolution is approximately 6 mm, which is comparable to emission scanning performance. Axial resolution is about 25 mm, with only crude source collimation. ACFs are underestimated by as much as 10% due to increased cross-plane scatter, compared to coincidence transmission accuracy. The response of the correction factors to object density is within 15%, when comparing singles transmission measurement to current coincidence transmission measurement. The major advantage of using singles transmission measurement is a dramatically increased count rate. A factor of 7 increase in count rate over coincidence scanning is possible with a 2-mCi transmission rod source. Uniformity of 2% in the transmission images is possible with this source strength and a 2-min acquisition. There are no randoms counted in singles transmission scans, which makes the measured count rate vary linearly with source activity. Singles detector dead time losses are approximately 6% in the detectors opposite a 2-mCi rod source.

Single lens transmission scanning acoustic microscope

A single lens transmission scanning acoustic microscope is presented. The microscope contains an acoustoelectric detector as a receiving element. It is shown that the presented version of the microscope provides sufficient resolution and image contrast.