Biomedical subjects

S R Meikle

Publications and source records attributed to S R Meikle.

24 records · Page 2Linked to original sources

PubMed1994

A transmission-dependent method for scatter correction in SPECT.

UNLABELLED: A method of scatter compensation has been developed that incorporates planar transmission measurements in the estimation of photopeak scatter in SPECT. METHODS: The scatter distribution is first estimated by convolving the planar projections with a monoexponential scatter function. The number of scattered events that subsequently reach the detector as a proportion of total events (i.e., scatter fraction) is then determined for each point in the projections based on narrow-beam transmission values, obtained using an external source. The assumptions of the method were tested using 99mTc and 201Tl point and line sources. The quantitative and qualitative impact of transmission-dependent scatter correction was assessed in realistic phantom experiments simulating blood-pool, lung and myocardial perfusion studies. RESULTS: The method accurately predicts the scatter distribution from 99mTc and 201Tl line sources in a phantom with variable density. Reconstructed counts are artificially enhanced in regions of high tissue density when scattered events are not removed from the projections prior to attenuation correction. Using convolution-subtraction with a constant scatter fraction (k = 0.4), scatter is underestimated in the heart and overestimated in the lungs, whereas transmission-dependent scatter correction enables activity to be quantified with > or = 95% accuracy in heart and lung regions. CONCLUSION: We conclude that incorporating transmission data enables accurate scatter compensation in objects with nonuniform density.

Female↗

PubMed1993

A scanning line source for simultaneous emission and transmission measurements in SPECT.

A scanning collimated line source for simultaneously acquiring emission and transmission data from a gamma camera has been developed. The line source is microprocessor-controlled and incorporates hardware to electronically window the spatial gamma camera signals in order to separate the emission signals of the subject from transmission signals from the line source. The device improves upon the previously described emission-transmission scanning technique using a flood source in three ways: (1) it overcomes the limitation that the transmission radionuclide must have a lower energy than the emission radionuclide; (2) it provides narrow-beam (scatter free) attenuation measurements of the subject being examined; and (3) it reduces the radiation exposure to staff. Attenuation coefficients for an elliptocal water-filled phantom were measured to be mu = 0.15 +/- 0.01 cm-1. The technique has been validated in phantom and human studies using a range of radionuclide combinations and imaging geometries and gives equivalent results using separate and simultaneous acquisitions.

Female↗

PubMed1993

Attenuation correction using count-limited transmission data in positron emission tomography.

Poisson noise in transmission data can have a significant influence on the statistical uncertainty of PET measurements, particularly at low transmission count rates. In this paper, we investigate the effect of transmission data processing on noise and quantitative accuracy of reconstructed PET images. Differences in spatial resolution between emission and transmission measurements due to transmission data smoothing are shown to have a significant influence on quantitative accuracy and can lead to artifacts in the reconstructed image. In addition, the noise suppression of this technique is insufficient to greatly reduce transmission scan times. Based on these findings, improved strategies for processing count-limited transmission data have been developed, including a method using segmentation of attenuation images. Using this method, accurate attenuation correction can be performed using transmission scan times as low as 2 min without increasing noise in reconstructed PET images.

Heart↗

PubMed1993

Correction and characterization of scattered events in three-dimensional PET using scanners with retractable septa.

Large sensitivity increases are realized in positron emission tomography when the interplane septa are removed and all lines of response acquired. Unfortunately, three-dimensional acquisition results in a large increase in scatter fraction which prevents accurate quantitation. By acquiring short two-dimensional scans prior to three-dimensional ones, scatter distributions can be estimated from differences between lines of response common to both datasets. This initial scatter distribution can be further modified to approximate scatter in the entire three-dimensional dataset. The method was validated with phantom measurements in which absolute activity concentrations were known in all compartments. Following scatter correction, a four-compartment phantom that was nonuniform in activity and density, both axially and transaxially, gave activity concentrations of 0.45 +/- 0.02, 0.31 +/- 0.02, 0.01 +/- 0.01 and 0.01 +/- 0.01 microCi/cc for compartments containing 0.43, 0.29, 0.0 (air) and 0.0 (water) microCi/cc, respectively. Thus, scatter distributions for complex sources can be estimated from image data without lengthy Monte-Carlo simulations. When activity distributions vary slowly with time, this method can be used to correct for scatter in three-dimensional patient studies.

Brain↗

PubMed1990

Current role of gallium scanning in the management of lymphoma.

Gallium 67 scanning in the malignant lymphomas has been done, with variable success, for over 20 years. After initial enthusiasm, the technique fell into disrepute and it was not until the early 1980s that it enjoyed a revival. There have been many major contributions to the literature, both favourable and unfavourable. The reasons for the latter include: poor instrumentation (only single-pulse height analysis), low gallium 67 doses, impatient and careless scanning techniques, timing of the study after treatment (chemotherapy, radiation) and insensitive methods of confirmation of the presence or absence of disease ("truth"). Anatomical diagnostic techniques (computed tomography, plain X-radiography, magnetic resonance imaging and others) are incapable of distinguishing viable tumour in normal-size lymph nodes or necrotic/fibrotic residual masses. With improvements in instrumentation (triple-pulse height analysis, gamma camera resolution and tomographic techniques) gallium 67 can detect active tumour in residual masses and in normal-size nodes. This is due to gallium 67's unique ability to localize in viable tumour cells. It has greater than 90% sensitivity, specificity, accuracy and positive predictive value in patients with lymphoma. Its major contributions are in: staging (changing management of mediastinal disease, obviating the need for a laparotomy and clearly identifying stage IV disease); detecting relapse or residual, progressive disease (it establishes true complete remission and is often the first and only evidence of relapse before clinical evidence); predicting response to therapy (failure to convert to a negative scan post-treatment signals a poor prognosis and alternative therapy is required); and predicting outcome--prognosis (it is the only diagnostic modality to predict outcome accurately).

Gallium Radioisotopes↗

PubMedDate not supplied

Simultaneous emission and transmission (SET) scanning in neurological PET studies.

PURPOSE: We evaluated the impact of simultaneous emission and transmission (SET) measurements of quantification and noise in neurological PET studies. METHOD: Bias in SET was measured as a function of emission count rate and used to predict distortion in simulated FDG tissue curves and its effect on model parameter estimates. Studies were performed on a brain phantom and a patient to verify predicted bias and examine the effect of SET on noise. RESULTS: In static imaging, SET underestimated tracer concentration by approximately 2%. In kinetic studies, tracer concentration was overestimated initially and underestimated during the mid to late part of the study, but bias in measurement of glucose metabolic rate was < 5% by simulation and < 10% in the patient study. SET imaging takes 10% longer than the emission part of a conventional scan to achieve comparable statistics. CONCLUSION: Accurate neurological PET studies can be performed with SET. The relatively small bias can be predicted and potentially corrected.

Adult↗