A comparison of normalization effects on three whole-body cylindrical 3D PET systems.

Normalization coefficients in three-dimensional positron emission tomography (3D PET) are affected by parameters such as camera geometry and the design and arrangement of the block detectors. In this work, normalization components for three whole-body 3D-capable tomographs (the GE Advance, the Siemens/CTI962/HR+ and the Siemens/CTI951R) are compared by means of a series of scans using uniform cylindrical and rotating line sources. Where applicable, the manufacturers' normalization methods are validated, and it is shown that these methods can be improved upon by using previously published normalization protocols. Those architectural differences between the three tomographs that affect normalization are discussed with a view to drawing more general conclusions about the effect of machine architecture on normalization. The data presented suggest that uniformity of system response becomes easier to achieve as the uniformity of crystal response within the detector block is improved.

Evaluation of myocardial perfusion using positron emission tomography in infants following a neonatal arterial switch operation.

This study was performed to examine the use of positron emission tomography (PET) as a method of evaluating myocardial perfusion after the arterial switch operation for correction of transposition of the great arteries. Eleven asymptomatic patients (median age 2.3 years, range 1.3-4.3 years) post successful neonatal arterial switch repair for transposition underwent cardiac PET scanning using N(13) ammonia before and after dipyridamole infusion. Reconstructed data from static scans were analyzed for regional perfusion defects before and after pharmacological stress. Simultaneous assessment of coronary flow before and after stress was performed using a Patlak graphical analysis of data from dynamic scans. Results obtained from PET scanning were correlated with patterns of coronary artery anatomy, electrocardiogram (ECG) recordings, and echocardiographic evaluation. PET scanning demonstrated normal distribution of myocardial perfusion before and after stress in all but one patient, who was found to have a discrete inferior transmural perfusion defect. The defect was well correlated with perioperative ECG changes and a complicated postoperative course. Myocardial blood flow before dipyridamole (0.690 ml/min/g) was similar to reported adult rest values. There was a small but significant (p < 0.002) increase in myocardial blood flow after dipyridamole stress with a mean coronary flow reserve of 1.19 (+/-0.103). Echocardiographic evaluation failed to demonstrate significant wall motion abnormalities in any of the patients. Cardiac PET scanning is a reliable noninvasive method for evaluation of myocardial perfusion in small children. In this study, the incidence of myocardial perfusion defects after the arterial switch operation is lower than previously reported. The data obtained concerning coronary flow and coronary flow reserve after the arterial switch need to be interpreted with caution because normal data in children are not available.

Randoms variance reduction in 3D PET.

In positron emission tomography (PET), random coincidence events must be removed from the measured signal in order to obtain quantitatively accurate data. The most widely implemented technique for estimating the number of random coincidences on a particular line of response is the delayed coincidence channel method. Estimates obtained in this way are subject to Poisson noise, which then propagates into the final image when the estimates are subtracted from the prompt signal. However, this noise may be reduced if variance reduction techniques similar to those used in normalization of PET detectors are applied to the randoms estimates prior to use. We have investigated the effects of randoms variance reduction on noise-equivalent count (NEC) rates on a whole-body PET camera operating in 3D mode. NEC rates were calculated using a range of phantoms representative of situations that might be encountered clinically. We have also investigated the properties of three randoms variance reduction methods (based on algorithms previously used for normalization) in terms of their systematic accuracy and their variance reduction efficacy, both in phantom studies and in vivo. Those algorithms investigated that do not make assumptions about the spatial distribution of random coincidences give the best estimates of the randoms distribution. With the camera used, which has a limited axial extent (10.8 cm) and a large ring diameter (102 cm), the gains in image signal-to-noise ratio obtained with this technique ranged from approximately 5% to approximately 15%, depending on object size, activity distribution and the amount of activity in the field of view. Larger gains would be expected if this technique were to be employed on cameras of greater axial extent and smaller ring diameter.

Developments in component-based normalization for 3D PET.

Normalization in positron emission tomography (PET) is the process of ensuring that all lines of response joining detectors in coincidence have the same effective sensitivity. In three-dimensional (3D) PET, normalization is complicated by the presence of a large proportion of scattered coincidences, and by the fact that cameras operating in 3D mode encounter a very wide range of count-rates. In this work a component-based normalization model is presented which separates the normalization of true and scattered coincidences and accounts for variations in normalization effects with count-rate. The effects of the individual components in the model on reconstructed images are investigated, and it is shown that only a subset of these components has a significant effect on reconstructed image quality.

A clinical evaluation of the quantitative accuracy of simultaneous emission/transmission scanning in whole-body positron emission tomography.

We present a clinical evaluation of the quantitative bias which is introduced during simultaneous emission/transmission (SET) acquisition for the application of whole-body positron emission tomography (PET) with fluorine-18 2-fluoro-2-deoxy-d-glucose. The quantitative accuracy of the SET technique was assessed by means of a clinical study involving 28 patients and a realistic phantom experiment. In the clinical study, SET overestimated the activity concentration in the tumours by a factor of approximately 1.10, but in the phantom study, where the tumours were smaller, the bias was found to increase to a value of 1.39. The bias in the soft tissue regions of the patient studies varied between 1.03 and 1.36, and close agreement was observed with the corresponding phantom results. The extent of the bias increased as the local activity concentration decreased and we attribute the effect to scattered photons from the transmission source which are detected in the emission window during SET.

Experimental procedure for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry.

A simple methodology for the manufacture and calibration of polyacrylamide gel (PAG) for magnetic resonance imaging (MRI) radiation dosimetry is presented to enable individuals to undertake such work in a routine clinical environment. Samples of PAG were irradiated using a linear accelerator and imaged using a 0.5 T (22 MHz) Philips Gyroscan MRI scanner. The mean spin-lattice relaxation rate was measured using a 'turbo-mixed' sequence, consisting of a series of 90 degrees pulses, each followed by acquisition of a train of spin echoes. The mean sensitivity for five different batches of PAG in the range up to 10 Gy was calculated to be 0.0285 s-1 Gy-1 for the mean spin-lattice relaxation rate with a percentage standard deviation of 1.25%. The overall reproducibility between batches was calculated to be 2.69%. This methodology, which introduces the novel use of pre-filled nitrogen vials for calibration, has been used to develop techniques for filling anatomically shaped anthropomorphic phantoms.

Algorithms for calculating detector efficiency normalization coefficients for true coincidences in 3D PET.

Accurate normalization of lines of response in 3D PET is a prerequisite for quantitative reconstruction. Most current methods are component based, calculating a series of geometric and intrinsic detector efficiency factors. We have reviewed the theory behind several existing algorithms for calculating detector efficiency factors in 2D and 3D PET, and have extended them to create a range of new algorithms. Three of the algorithms described are 'fully 3D' in that they make use of data from all detector rings for the calculation of the efficiencies of any one line of response. We have assessed the performance of the new and existing methods using simulated and real data, and have demonstrated that the fully 3D algorithms allow the rapid acquisition of crystal efficiency normalization data using low-activity sources. Such methods enable the use of scatter-free scanning line sources or the use of very short acquisitions of cylindrical sources for routine normalization.

Optimization of noise-equivalent count rates in 3D PET.

We have used noise-equivalent count (NEC) rates to optimize count rate performance for 3D acquisition in PET in a wide range of situations, with particular reference to imaging of the torso. We have also compared NEC performance for 2D and 3D acquisition in order to establish the conditions under which 3D mode offers an improvement over 2D mode. Measurements were performed on four tissue-equivalent phantoms ranging in size from that of an infant's head (13 cm diameter) to that of an obese adult's chest (37 cm x 48 cm). Count rate data were acquired as a function of phantom size, activity in the field of view, lower energy discriminator level (LLD) and acquisition mode, and NEC rates were derived as a function of these variables. The LLD at which the highest NEC rate is obtained shows a dependence both on phantom size and on the activity in the field of view both for 2D and for 3D acquisition. The relative advantage of 3D mode over 2D mode, at the optimum LLD setting, is also strongly dependent both on activity in the field of view (FOV) and on the phantom size. The main limiting factors for 3D NEC rates are detector dead-time for small phantoms and random coincidences for large phantoms. The 3D NEC rate is more than twice as great as the 2D NEC rate when less than 60 MBq is present in the FOV for all phantoms except the largest, in which case a ratio of two is only achieved for activities less than 25 MBq. For the smallest phantom, 3D/2D NEC ratios of greater than 3.5 are obtained when the activity in the FOV falls below 10 MBq.

Radiation risks to personnel and public during the treatment of malignant glioma using interstitial brachytherapy.

125I seeds are used in brachytherapy for the treatment of malignant gliomas. The use of such radioactive sources is associated with a certain radiological hazard to both personnel and members of the public. This hazard should be quantified so that the ALARA principle of radiological protection may be implemented satisfactorily. A study was undertaken to derive isodose rate contours in the vicinity of an anthropomorphic phantom with 125I seeds positioned at typical tumour sites within the cranial cavity. These contours are illustrated for seed positions appropriate to deep and superficial temporal tumours. Results indicate that the annual doses to personnel and public should not exceed those recommended by the International Commission on Radiological Protection.