SPECT (single photon emission computed tomography) images are distorted by photon attenuation. The effect is complex in the thoracic region due to different tissue densities. This study compares the effect on the image homogeneity of two different methods of attenuation correction in lung SPECT; one pre-processing and one post-processing method. This study also investigates the impact of attenuation correction parameters such as lung contour, body contour, density of the lung tissue and effective attenuation coefficient. The Monte Carlo technique was used to simulate SPECT studies of a digital thorax phantom containing a homogeneous activity distribution in the lung. Homogeneity in reconstructed images was calculated as the coefficient of variation (CV). The isolated effect of the attenuation correction was assessed by normalizing pixel values from the attenuation corrected lung by pixel values from the lung with no attenuation effects. Results show that the CV decreased from 12.8% with no attenuation correction to 4.4% using the post-processing method and true densities in the thoracic region. The impact of variations in the definition of the body contour was found to be marginal while the corresponding effect of variations in the lung contour was substantial.

The effect of increasingly more sophisticated attenuation correction methods on image homogeneity has been studied in seven healthy subjects. The subjects underwent computed tomography (CT), single photon emission computed tomography (SPECT) and transmission computed tomography (TCT) of the thorax region in the supine position. Density maps were obtained from the CT and TCT studies. Attenuation corrections were performed using five different methods: (1) uniform correction using only the body contour, (2) TCT based corrections using the average lung density, (3) TCT based corrections using the pixel density, (4) CT based corrections using average lung density, and (5) CT based corrections using the pixel density. The isolated attenuation effects were assessed on quotient images generated by the division of images obtained using various attenuation correction methods divided by the non-uniform attenuation correction based on CT pixel density (reference method). The homogeneity was calculated as the coefficient of variation of the quotient images (CVatt), showing the isolated attenuation effects. Values of CVatt were on average 12.8% without attenuation correction, 10.7% with the uniform correction, 8.1% using TCT map using the average lung density value and 4.8% using CT and average lung density corrections. There are considerable inhomogeneities in lung SPECT slices due to the attenuation effect. After attenuation correction the remaining inhomogeneity is considerable and cannot be explained by statistical noise and camera non-uniformity alone.

The image quality in SPECT is degraded by scattered photons. The finite energy resolution of the gamma camera makes the detection of scattered photons unavoidable. The effect on the image is impaired contrast and a reduction in the possibilities of detecting small lesions.

The detectability of cold lesions above statistical noise and normal variations in the activity distribution was evaluated using the Monte Carlo technique. A SPECT study of a digital thorax phantom was simulated with cold lesions of different sizes positioned inside the homogeneous activity distribution in the lungs. The contrast-to-noise for a number of energy window settings were assessed, with and without three different scatter correction methods: the dual-window, the triple-energy-window and the Klein-Nishina method.

The contrast was improved by using scatter corrections and the TEW and KN scatter corrections showed the best result. The detectability was not improved by using scatter corrections when normal variations in the lung activity are small compared with the statistical noise level. Lesions of about 2 cm in diameter are detectable. The optimum energy window was found to be 128-154 keV, both with and without scatter corrections.

Monte Carlo simulation has been used to produce projections from a voxel-based brain phantom, simulating a ^99mTc-HMPAO single photon emission computed tomography (SPECT) brain investigation. For comparison, projections free from the effects of attenuation and scattering were also simulated, giving ideal transaxial images after reconstruction. Three methods of attenuation correction were studied: (a) a pre-processing method, (b) a post-processing uniform method and (c) a post-processing non-uniform method using a density map. The accuracy of these methods was estimated by comparison of the reconstructed images with the ideal images using the normalized mean square error, NMSE, and quantitative values of the regional cerebral blood flow, rCBF. A minimum NMSE was achieved for the effective linear attenuation coefficient µ_eff = 0.07 (0.09) cm^-1 for the uniform_pre method, the effective mass attenuation coefficient µ_eff/ρ = 0.08 (0.10) cm² g^-1 for the uniform_post method and µ_eff/ρ = 0.12 (0.13) cm² g^-1 for the non-uniform_post method. Values in parentheses represent the case of dual-window scatter correction. The non-uniform_post method performed better, as measured by the NMSE, both with and without scatter correction. Furthermore, the non-uniform_post method gave, on average, more accurate rCBF values. Although the difference in rCBF accuracy was small between the various methods, the same method should be used for patient studies as for the reference material.

The image quality in SPECT studies of the regional cerebral blood flow (rCBF) performed with ^99mTc-HMPAO is degraded by scattered photons. The finite energy resolution of the gamma camera makes the detection of scattered photons unavoidable, and this is observed in the image as an impaired contrast between grey and white matter structures.

In this work, a Monte Carlo simulated SPECT study of a realistic voxel-based brain phantom was used to evaluate the resulting contrast-to-noise ratio for a number of energy window settings, with and without the dual-window scatter correction. Values of the scaling factor k, used to obtain the fraction of scattered photons in the photopeak window, were estimated for each energy window.

The use of a narrower, asymmetric, energy discrimination window improved the contrast, with a subsequent increase in statistical noise due to the lower number of counts. The photopeak-window setting giving the best contrast-to-noise ratio was found to be the same whether or not scatter correction was applied. Its value was 17% centred at 142 keV. At the optimum photopeak-window setting, the contrast was improved by using scatter correction, but the contrast-to-noise ratio was made worse.