Recent developments in time-of-flight PET.

While the first time-of-flight (TOF)-positron emission tomography (PET) systems were already built in the early 1980s, limited clinical studies were acquired on these scanners. PET was still a research tool, and the available TOF-PET systems were experimental. Due to a combination of low stopping power and limited spatial resolution (caused by limited light output of the scintillators), these systems could not compete with bismuth germanate (BGO)-based PET scanners. Developments on TOF system were limited for about a decade but started again around 2000. The combination of fast photomultipliers, scintillators with high density, modern electronics, and faster computing power for image reconstruction have made it possible to introduce this principle in clinical TOF-PET systems. This paper reviews recent developments in system design, image reconstruction, corrections, and the potential in new applications for TOF-PET. After explaining the basic principles of time-of-flight, the difficulties in detector technology and electronics to obtain a good and stable timing resolution are shortly explained. The available clinical systems and prototypes under development are described in detail. The development of this type of PET scanner also requires modified image reconstruction with accurate modeling and correction methods. The additional dimension introduced by the time difference motivates a shift from sinogram- to listmode-based reconstruction. This reconstruction is however rather slow and therefore rebinning techniques specific for TOF data have been proposed. The main motivation for TOF-PET remains the large potential for image quality improvement and more accurate quantification for a given number of counts. The gain is related to the ratio of object size and spatial extent of the TOF kernel and is therefore particularly relevant for heavy patients, where image quality degrades significantly due to increased attenuation (low counts) and high scatter fractions. The original calculations for the gain were based on analytical methods. Recent publications for iterative reconstruction have shown that it is difficult to quantify TOF gain into one factor. The gain depends on the measured distribution, the location within the object, and the count rate. In a clinical situation, the gain can be used to either increase the standardized uptake value (SUV) or reduce the image acquisition time or administered dose. The localized nature of the TOF kernel makes it possible to utilize local tomography reconstruction or to separate emission from transmission data. The introduction of TOF also improves the joint estimation of transmission and emission images from emission data only. TOF is also interesting for new applications of PET-like isotopes with low branching ratio for positron fraction. The local nature also reduces the need for fine angular sampling, which makes TOF interesting for limited angle situations like breast PET and online dose imaging in proton or hadron therapy. The aim of this review is to introduce the reader in an educational way into the topic of TOF-PET and to give an overview of the benefits and new opportunities in using this additional information.

Fast generation of 4D PET-MR data from real dynamic MR acquisitions.

We have implemented and evaluated a framework for simulating simultaneous dynamic PET-MR data using the anatomic and dynamic information from real MR acquisitions. PET radiotracer distribution is simulated by assigning typical FDG uptake values to segmented MR images with manually inserted additional virtual lesions. PET projection data and images are simulated using analytic forward projections (including attenuation and Poisson statistics) implemented within the image reconstruction package STIR. PET image reconstructions are also performed with STIR. The simulation is validated with numerical simulation based on Monte Carlo (GATE) which uses more accurate physical modelling, but has 150× slower computation time compared to the analytic method for ten respiratory positions and is 7000× slower when performing multiple realizations. Results are validated in terms of region of interest mean values and coefficients of variation for 65 million coincidences including scattered events. Although some discrepancy is observed, agreement between the two different simulation methods is good given the statistical noise in the data. In particular, the percentage difference of the mean values is 3.1% for tissue, 17% for the lungs and 18% for a small lesion. The utility of the procedure is demonstrated by simulating realistic PET-MR datasets from multiple volunteers with different breathing patterns. The usefulness of the toolkit will be shown for performance investigations of the reconstruction, motion correction and attenuation correction algorithms for dynamic PET-MR data.

The Swedish laparoscopic adjustable gastric banding for morbid obesity: radiologic findings in 218 patients.

OBJECTIVE: The objective of this study was to determine the prevalence and radiologic features of postoperative complications after Swedish laparoscopic adjustable gastric banding surgery and to emphasize the role of the radiologist in the follow-up of those patients, especially in the treatment of complications. MATERIALS AND METHODS: We reviewed the radiologic findings in 218 consecutive morbidly obese patients after laparoscopic placement of the Swedish gastric banding system. Radiographic studies of the stomach (obtained with liquid barium sulfate suspension) were performed before surgery and 1 month after band placement in every patient. Additional studies in symptomatic patients were performed when needed. RESULTS: Surgical complications found included misplacement of the band (five patients, 2.3%), slippage of the band (17 patients, 7.8%), and pouch enlargement (eight patients, 3.7%). Technical problems encountered were inversion of the access port (three patients, 1.4%), leakage of the device (two patients, 0.9%), and spontaneous decrease of the stoma size caused by gastritis (seven patients, 3.2%) or the hyperosmolar properties of the IV contrast material (12 patients, 5.5%). Intrinsic abnormalities of gastroesophageal tract seen included trapping of food in the stoma (four patients, 1.8%) and esophagitis (11 patients, 5%). CONCLUSION: Although, according to the available data, the gastric banding operation with the Swedish band meets the criteria of a low-risk laparoscopic alternative treatment of morbid obesity, the radiologic appearances of various complications may be seen on the images of patients who have undergone the procedure. The radiologist plays a key role in the early detection of those complications and treatment of specific abnormalities.

CT of the temporal bone in the CHARGE association.

We reviewed the CT examinations of the temporal bone, performed with 1-mm-thick contiguous sections, of seven patients with the CHARGE association. We found abnormalities of the incus and stapes, with ossicular chain fixation, absence of the stapedius muscle and oval window, hypoplasia or dysplasia of the vestibule and absence of the semicircular canals in all ears. The pyramidal eminence and tympanic sinus were absent and there were anomalies of the cochlea in 13 of 14 ears. Absence of the semicircular canals is the most specific change in patients with the CHARGE association.

Secondary superficial siderosis of the central nervous system in a patient presenting with sensorineural hearing loss.

We present a 50-year-old man who was investigated for sensorineural hearing loss. On MRI of the brain superficial siderosis of the central nervous system was seen, while MRI of the spine revealed an ependymoma of the cauda equina. This case illustrates the importance of performing T2-weighted imaging of the brain and posterior fossa when sensorineural hearing loss is present. Spine imaging is mandatory when superficial siderosis of the brain is diagnosed without identification of a bleeding source in the brain.

Effects of treating drosophila females with TEM on their mortality, and the yields of mutation recovered from x-irradiated sperm.

Ring-X-bearing Drosophila males were irradiated with 0, 1000, 2000 or 3000 R of X-rays and mated to females that had been injected with saline or 10(-4) M TEM. The mortality and the fertility of the treated females were recorded. The rate of dominant lethals, of entire sex chromosome loss and partial loss of the Y chromosome, the sex ratio, and the rate of mosaics were determined on the progency. TEM slightly increased the rate of mortality of the females. But it did not influence the yield of mutations recovered from the irradiated spermatozoa.