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Evaluation of attenuation and scatter corrections in lung and brain SPECT
Department of Radiations Physics, Avdelningen för radiofysik, Göteborgs universitet.
2001 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Single Photon Emission Computed Tomography (SPECT) is used to image functional processes in the human body. The image process is affected by physical effects such as attenuation, scatter, spatial resolution and statistical noise. The aim of this work was to investigate how attenuation and scatter effects and their associated correction methods affect the image quality in lung and brain SPECT.

The effects of attenuation and scattering on the image of a uniform activity distribution in the lungs was investigated using Monte Carlo simulated data and the attenuation effect was evaluated in healthy volunteers. The homogeneity was measured as the CV inside a well-defined lung contour. The attenuation effect in lung SPECT was estimated to be about 13-14% expressed as the CV. The homogeneity improved with increasing accuracy of the attenuation correction method. After attenuation correction the remaining inhomogeneity in healthy subjects was considerable and could not be explained by statistical noise and camera non-uniformity. A non-uniform attenuation correction was thus required and a TCT-based density map was found to be adequate in most instances.

The accuracy of the attenuation correction methods was studied in Monte Carlo simulated brain SPECT using the normalised mean square error, NMSE. The different degrees of accuracy in the methods were also reflected in the absolute deviation of the relative regional cerebral blood flow (rCBF) according to the min-max method. The NMSE value improved with the accuracy of the attenuationcorrection method. The difference in relative rCBF value was generally less than 5%. Therefore, it is unlikely that the choice of attenuation correction method will affect the diagnostic accuracy.

The detectability, expressed as the contrast-to-noise-ratio dependence on the choice of energy window, was evaluated using SPECT studies of a thorax phantom containing cold lesions inside the lungs and a realistic brain phantom. The effects of subtractive scatter correction methods such as the dual-window method (DW), the triple-energy-window method (TEW) and the Klein-Nishina method (KN) were also evaluated. An optimal photopeak window setting was found to be 128-154 keV in lung SPECT for a gamma camera with 10% energy resolution, and 130-154 keV in rCBF SPECT for a gamma camera with 9% energy resolution. The detection limit for lung SPECT for spherical lesions is about 2 cm in diameter when normal variations in the lungs are relatively small compared with the statistical noise level. Under these conditions the detectability is degraded by using scatter correction, except when the TEW scatter correction is used for small lesions (<3 cm in diameter), when about the same detectability is achieved.

Place, publisher, year, edition, pages
Göteborg: Göteborgs universitet , 2001. , 64 p.
Keyword [en]
SPECT, attenuation, scatter, detectability, Monte Carlo simulation
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-28146Local ID: 12959ISBN: 91-638-4850-X OAI: oai:DiVA.org:liu-28146DiVA: diva2:248697
Public defence
2001-06-01, Sal F3 Odontologen, Medicinaregatan 12 D, Göteborg, 09:15 (Swedish)
Opponent
Note

Doktorsavhandling framlagd vid Göteborgs universitet 2001-06-01

Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2013-02-27
List of papers
1. Evaluation of attenuation corrections using Monte Carlo simulated lung SPECT
Open this publication in new window or tab >>Evaluation of attenuation corrections using Monte Carlo simulated lung SPECT
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1998 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 43, no 8, 2325-2336 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-79205 (URN)10.1088/0031-9155/43/8/023 (DOI)9725607 (PubMedID)
Available from: 2012-07-03 Created: 2012-07-03 Last updated: 2017-12-07
2. Evaluation of various attenuation corrections in lung SPECT in healthy subjects
Open this publication in new window or tab >>Evaluation of various attenuation corrections in lung SPECT in healthy subjects
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2003 (English)In: Nuclear medicine communications, ISSN 0143-3636, E-ISSN 1473-5628, Vol. 24, no 10, 1087-1095 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
attenuation correction, SPECT, lung and thorax region
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-47749 (URN)10.1097/00006231-200310000-00009 (DOI)
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2013-02-27
3. Scatter and detecability in lung SPECT: a Monte Carlo study
Open this publication in new window or tab >>Scatter and detecability in lung SPECT: a Monte Carlo study
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

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.

Keyword
Scatter correction, detectability, SPECT, Monte Carlo, lung
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-89581 (URN)
Available from: 2013-02-27 Created: 2013-02-27 Last updated: 2013-02-27
4. Attenuation correction in quantitative SPECT of cerebral blood flow: a Monte Carlo study
Open this publication in new window or tab >>Attenuation correction in quantitative SPECT of cerebral blood flow: a Monte Carlo study
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2000 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 45, no 12, 3847-3859 p.Article in journal (Refereed) Published
Abstract [en]

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 µeff = 0.07 (0.09) cm-1 for the uniformpre method, the effective mass attenuation coefficient µeff/ρ = 0.08 (0.10) cm2 g-1 for the uniformpost method and µeff/ρ = 0.12 (0.13) cm2 g-1 for the non-uniformpost method. Values in parentheses represent the case of dual-window scatter correction. The non-uniformpost method performed better, as measured by the NMSE, both with and without scatter correction. Furthermore, the non-uniformpost 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.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2000
National Category
Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-79208 (URN)10.1088/0031-9155/45/12/324 (DOI)11131204 (PubMedID)
Available from: 2012-07-03 Created: 2012-07-03 Last updated: 2017-12-07
5. Dual-window scatter correction and energy window setting in cerebral blood flow SPECT: a Monte Carlo study
Open this publication in new window or tab >>Dual-window scatter correction and energy window setting in cerebral blood flow SPECT: a Monte Carlo study
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2000 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 45, no 11, 3431-3440 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Institute of Physics Publishing, 2000
National Category
Medical Image Processing
Identifiers
urn:nbn:se:liu:diva-79207 (URN)10.1088/0031-9155/45/11/323 (DOI)11098915 (PubMedID)
Available from: 2012-07-03 Created: 2012-07-03 Last updated: 2017-12-07

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