Anatomically derived attenuation coefficients for use in quantitative single photon emission tomography studies of the thorax

Elimination of errors due to poor attenuation correction is an essential part of any quantitative single photon emission tomography (SPET) technique. Attenuation coefficients (_Tc) for use in attenuation correction of SPET data were determined using technetium 99m and cobalt 57 flood sources and using topographical information obtained from computed tomography (CT) scans and magnetic resonance (MR) images. In patients with carcinoma of the bronchus, the mean attenuation coefficient for ^99mTc was 0.096 cm^–1 when determined across a transverse section of the thorax at the level of the tumour by means of a ⁵⁷CO flood source (13 patients) and 0.093 and 0.074 cm^–1 as determined from CT scans for points in the centre of the tumour and contralateral normal lung, respectively (21 patients). In 18 patients with breast tumours, the mean attenuation coefficient for ^99mTc was 0.110 and 0.076 cm^–1 when determined from MRI cross-sections for points in the centre of the tumour and normal contralateral lung, respectively. This indicates significant overcorrection for attenuation when the conventional value of 0.12 cm^–1 is used. A value in the range 0.08–0.09 cm^–1 would be more appropriate for SPET studies of the thorax. An alternative approach to quantitative region of interest (ROI) analysis is to perform attenuation correction appropriate to the centre of each ROI (using topographical information derived from CT or MRI) on non-attenuation-corrected reconstructions.     

Collimator design for single photon emission tomography.

We discuss recent trends in collimator design and technology, with emphasis on theoretical and practical issues of importance for single photon emission tomography (SPET). The well-known imaging performance parameters of parallel-hole collimators are compared with those of fan-beam collimators, which have enjoyed considerable success in recent years, particularly for brain SPET. We review a simplistic approach to the collimator optimization problem, as well as more sophisticated "task-dependent" treatments and important considerations for SPET collimator design. Practical guidance is offered for understanding trade-offs that must be considered for clinical imaging. Finally, selective comparisons among different SPET systems and collimators are presented for illustrative purposes.     

An analytical approach to single photon emission computed tomography with the attenuation effect

The problem of transverse plane reconstruction from an ensemble of projections is considered in its general formulation and an analytically exact solution to the attenuated tomographic operator is proposed. Such a technique, called the regularizing iterative method (RIM), allows the introduction of a priori knowledge on the size and shape of the activity distribution and in principle on the exact attenuation distribution. The relaxation factor used is so named because it provides noise filtering for a small number of iterations. The effectiveness of RIM was studied in the single photon emission computed tomography (SPECT) problem with the aim of correcting for attenuation before quantitative study. Its application involves the use of a rotating scintillation camera connected to a mini-computer system.Various mathematical and physical phantoms were studied, and a satisfactory attenuation correction was always obtained in the final image with an improvement in the contrast and signal-to-noise ratio. Preliminary clinical studies on liver transverse sections seems to indicate an improvement in deep lesion detectability, compared with images obtained by the filtered (Ramp) back projection technique.     

Collimator design for single photon emission tomography

We discuss recent trends in collimator design and technology, with emphasis on theoretical and practical issues of importance for single photon emission tomography (SPET). The well-known imaging performance parameters of parallel-hole collimators are compared with those of fan-beam collimators, which have enjoyed considerable success in recent years, particularly for brain SPET. We review a simplistic approach to the collimator optimization problem, as well as more sophisticated task-dependent treatments and important considerations for SPET collimator design. Practical guidance is offered for understanding trade-offs that must be considered for clinical imaging. Finally, selective comparisons among different SPET systems and collimators are presented for illustrative purposes. Offprint requests to: S.C. Moore     

The use of single photon emission computed tomography in depressive disorders

Single photon emission computed tomography (SPECT) and positron emission tomography (PET) have advanced our understanding of the biological underpinnings of depression. There is, however, considerable variability in the literature. Depression is a complex disorder with marked heterogeneity in diagnosis and treatment. There is also evidence of heterogeneity in pathophysiology. In addition, the literature is marked by inconsistencies in the use of imaging techniques and data-analytical procedures. In this review we have attempted to focus on the SPECT studies that have used more refined methodologies and more homogenous clinical sub-groups of patients. We have focused on the main diagnostic sub-types of depression and on specific issues such as treatment response, correlates of neuroimaging abnormalities in depression, and so-called 'emotional circuitry' - the connectivity of regions implicated in depression. The future of molecular imaging in depression will be determined by the pace of the development of useful ligands and the exciting opportunities emerging in the field of imaging genomics. Future studies must attend to several key confounds including clinical heterogeneity, medication and the problems surrounding recruitment of drug-naive patients. It remains the case that longitudinal studies are the design of choice if questions relating to state and trait are to be addressed. Molecular imaging will be used increasingly to quantify neuroreceptor and transporter binding, and the activity of neurtransmitters, allowing the neurochemistry of this complex condition to be explored.     

The use of cerebral activation procedures with single photon emission tomography

Single photon emission tomography allows the imaging of dynamic brain functioning. The use of cerebral activating procedures within the scan protocol enables investigation of the mechanisms involved in specific brain functions in health and disease. Activation studies involve the comparison of at least two data sets describing brain activity generated in conditions that differ for the specific function in question. When designing an activation study, decisions regarding methodology include: the nature of the activation regime, the tracer-ligand utilized, the SPET instrument and the manner of subsequent data analysis. These issues are discussed in this review, both theoretically and with reference to published studies. Means of activating particular cerebral structures and functions are reviewed, as are the limitations of the techniques with respect to temporal and spatial resolution and the potentially confounding nature of preconceived ideas regarding the mechanisms of brain function.     

The use of cerebral activation procedures with single photon emission tomography

Single photon emission tomography allows the imaging of dynamic brain functioning. The use of cerebral activating procedures within the scan protocol enables investigation of the mechanisms involved in specific brain functions in health and disease. Activation studies involve the comparison of at least two data sets describing brain activity generated in conditions that differ for the specific function in question. When designing an activation study, decisions regarding methodology include: the nature of the activation regime, the tracer-ligand utilized, the SPET instrument and the manner of subsequent data analysis. These issues are discussed in this review, both theoretically and with reference to published studies. Means of activating particular cerebral structures and functions are reviewed, as are the limitations of the techniques with respect to temporal and spatial resolution and the potentially confounding nature of preconceived ideas regarding the mechanisms of brain function. Offprint requests to: P.J. Ell     

Reproducibility of quantitative hexakis-2-methoxyisobutylisonitrile single photon emission tomography in stable coronary artery disease.

The quantification of myocardial perfusion abnormalities is necessary to allow comparison of repeated studies, especially in the evaluation of the success of medical, interventional or combined treatment in stable coronary artery disease or in evolving myocardial infarction. The purpose of this study was to assess inter-observer reproducibility of tomographic study processing using a semi-automatic quantitative programme. Technetium 99m hexakis-2-methoxyisobutylisonitrile (99mTc-Sestamibi) was chosen for tomographic imaging of repeated rest-stress studies in patients with stable coronary artery disease. The quantification was performed using a modification of the Cedars polar coding and comparison with the normal data base. The perfusion defects were quantified separately for each standard perfusion area [left anterior descending (LAD), right coronary (RCA) and left circumflex (LCX) arteries] and total area of hypoperfused myocardium. The inter-observer variability for 40 tomographic studies was accomplished. The defects were the largest in the LAD perfusion area (average 19.7% of the normalized LAD supply area) with an inter-observer correlation of 0.84 for this region. The greatest variability was found for the LCX region (r = 0.55) and is attributed to a small average perfusion defect (7.1%), only 18 studies having abnormal perfusion in this area. In total, an average 14.3% of the left ventricular myocardium was significantly hypoperfused, and the inter-observer correlation was 0.87. These results show good inter-observer reproducibility using semi-automatic quantitation of perfusion defects. Careful interpretation of smaller defects in the evaluation of treatment results is advised when repeated 99mTc-Sestamibi single photon emission tomography studies are processed by more than one observer.     

Positron emission tomography and single photon emission computed tomography in epilepsy care

Radiopharmaceutical brain imaging is clinically applied in planning resective epilepsy surgery. Cerebral sites of seizure generation-propagation are highly associated with regions of hyperperfusion during seizures, and with glucose hypometabolism interictally. For surgical planning in epilepsy, the functional imaging modalities currently established are ictal single photon emission computed tomography (SPECT) with [(99m)Tc]technetium-hexamethylpropyleneamine oxime (HMPAO) or with [(99m)Tc]technetium-ethylene cysteine dimer (ECD), and interictal positron emission tomography (PET) with 2-[(18)F]fluoro-2-deoxyglucose (FDG). Ictal SPECT and interictal FDG PET can be used in presurgical epilepsy evaluations to reliably: (1) determine the side of anterior temporal lobectomy, and in children the area of multilobar resection, without intracranial electroencephalographic recording of seizures; (2) select high-probability sites of intracranial electrode placement for recording ictal onsets; and, (3) determine the prognosis for complete seizure control following anterior temporal lobe resection. Coregistration of a patient's structural (magnetic resonance) and functional images, and statistical comparison of a patient's data with a normal data set, can increase the sensitivity and specificity of these SPECT and PET applications to the presurgical evaluation.     

The use of single photon emission computed tomography in lung imaging with aerosols

The use of technetium-99m labelled diethylenetriaminopentaacetic acid (99Tcm-DTPA) aerosol and single photon emission computed tomography (SPECT) to obtain sectional ventilation images of the lung is demonstrated in a limited series of patients. SPECT perfusion and planar ventilation and perfusion are also obtained and the SPECT studies compared with the planar views to assess the efficacy of SPECT aerosol ventilation images when used in conjunction with SPECT perfusion studies. The problems encountered in SPECT aerosol lung imaging, particularly the image noise associated with the limited number of detected counts, are described. On the basis of the results it is concluded that the technique is worth further assessment.     

Brain single photon emission computed tomography in neonates

This study was designed to rate the clinical value of [123I]iodoamphetamine (IMP) or [99mTc] hexamethyl propylene amine oxyme (HM-PAO) brain single photon emission computed tomography (SPECT) in neonates, especially in those likely to develop cerebral palsy. The results showed that SPECT abnormalities were congruent in most cases with structural lesions demonstrated by ultrasonography. However, mild bilateral ventricular dilatation and bilateral subependymal porencephalic cysts diagnosed by ultrasound were not associated with an abnormal SPECT finding. In contrast, some cortical periventricular and sylvian lesions and all the parasagittal lesions well visualized in SPECT studies were not diagnosed by ultrasound scans. In neonates with subependymal and/or intraventricular hemorrhage the existence of a parenchymal abnormality was only diagnosed by SPECT. These results indicate that [123I]IMP or [99mTc]HM-PAO brain SPECT shows a potential clinical value as the neurodevelopmental outcome is clearly related to the site, the extent, and the number of cerebral lesions. Long-term clinical follow-up is, however, mandatory in order to define which SPECT abnormality is associated with neurologic deficit.     

A comparative study of attenuation correction algorithms in single photon emission computed tomography (SPECT)

A computer based simulation method was developed to assess the relative effectiveness and availability of various attenuation compensation algorithms in single photon emission computed tomography (SPECT). The effect of the nonuniformity of attenuation coefficient distribution in the body, the errors in determining a body contour and the statistical noise on reconstruction accuracy and the computation time in using the algorithms were studied. The algorithms were classified into three groups: precorrection, post correction and iterative correction methods. Furthermore, a hybrid method was devised by combining several methods. This study will be useful for understanding the characteristics, limitations and strengths of the algorithms and searching for a practical correction method for photon attenuation in SPECT.     

A comparative study of attenuation correction algorithms in single photon emission computed tomography (SPECT)

A computer based simulation method was developed to assess the relative effectiveness and availability of various attenuation compensation algorithms in single photon emission computed tomography (SPECT). The effect of the nonuniformity of attenuation coefficient distribution in the body, the errors in determining a body contour and the statistical noise on reconstruction accuracy and the computation time in using the algorithms were studied. The algorithms were classified into three groups: precorrection, post correction and iterative correction methods. Furthermore, a hybrid method was devised by combining several methods. This study will be useful for understanding the characteristics, limitations and strengths of the algorithms and searching for a practical correction method for photon attenuation in SPECT.     

Brain single photon emission computed tomography: newer activation and intervention studies

Single-photon emission computed tomography (SPECT) regional cerebral blood flow (rCBF) findings using non-xenon 133 tracers in combination with activation and intervention techniques are reviewed. Examination of the currently available data indicates that it is possible to detect the effects of a variety of activations and interventional procedures using SPECT rCBF with non-xenon 133 tracers. There are still many issues to be resolved before SPECT can reach the level of sophistication attained by xenon 133 and positron emission tomography in studying rCBF during activation or intervention. However, research to date indicates that SPECT rCBF studied with tracers other than xenon 133 has an excellent potential for increasing the ability to differentiate normal and pathological states.     

Reproducibility of quantitative hexakis-2-methoxyisobutylisonitrile single photon emission tomography in stable coronary artery disease

The quantification of myocardial perfusion abnormalities is necessary to allow comparison of repeated studies, especially in the evaluation of the success of medical, interventional or combined treatment in stable coronary artery disease or in evolving myocardial infarction. The purpose of this study was to assess inter-observer reproducibility of tomographic study processing using a semi-automatic quantitative programme. Technetium 99m hexakis-2-methoxyisobutylisonitrile (^99mTc-Sestamibi) was chosen for tomographic imaging of repeated rest-stress studies in patients with stable coronary artery disease. The quantification was performed using a modification of the Cedars polar coding and comparison with the normal data base. The perfusion defects were quantified separately for each standard perfusion area [left anterior descending (LAD), right coronary (RCA) and left circumflex (LCX) arteries] and total area of hypoperfused myocardium. The inter-observer variability for 40 tomographic studies was accomplished. The defects were the largest in the LAD perfusion area (average 19.7% of the normalized LAD supply area) with an inter-observer correlation of 0.84 for this region. The greatest variability was found for the LCX region (r=0.55) and is attributed to a small average perfusion defect (7.1%), only 18 studies having abnormal perfusion in this area. In total, an average 14.3% of the left ventricular myocardium was significantly hypoperfused, and the inter-observer correlation was 0.87. These results show good inter-observer reproducibility using semi-automatic quantitation of perfusion defects. Careful interpretation of smaller defects in the evaluation of treatment results is advised when repeated ^99mTc-Sestamibi single photon emission tomography studies are processed by more than one observer.The work was performed at Nuclear Medicine Department in Ulm. Offprint requests to: M. Milinski     

Quality controls for the gamma camera with single photon emission tomography

Some tests and quality controls to assess the initial conditions and the working performance of a SPET system (Single Photon Emission Tomography), are described. Suitable tomographic phantoms were designed and built. We outline the methods to check MTF, stability of the photoelectric peak, centre of rotation, spatial uniformity, spatial resolution, sensitivity. NEMA tests should first be carried out to check the planar scintigraphic performance of the gamma-camera. This study allows to comprehensively evaluate the qualitative and quantitative performance of the SPET system and the correct check frequency of investigated parameters.     

Prognostic impact of myocardial perfusion single photon emission computed tomography in patients with major extracardiac findings by computed tomography for attenuation correction

Background

Attenuation correction computed tomography (CT) contributes to an improvement in the diagnostic accuracy of myocardial perfusion imaging (MPI) by single photon emission tomography (SPECT). The aim of this study was to explore the prognosis of patients with major findings by CT according to the results of MPI.

Methods and Results

1506 patients who underwent MPI by SPECT were retrospectively included. Attenuation correction CT images were systematically analyzed for major and minor abnormalities. 830 (55.1%) and 212 (14.1%) patients had minor and major extracardiac findings, respectively. Among patients with major extracardiac findings, the abnormality was previously unknown in 113 (53.3%) patients. 90 (41.9%) had abnormal MPI, 73 (34.4%) had a myocardial infarction scar, 55 (25.9%) had myocardial ischemia, and 38 (17.7%) patients had both myocardial infarction scar and myocardial ischemia. Among the 201 patients available for survival analysis, there were 67 (31.2%) deaths over a follow-up period of 3.2±1.3 years. There was no significant impact on survival arising from MPI, whatever the result. The results were the same among the 103 patients with previously unknown major extracardiac findings.

Conclusion

Extracardiac findings by CT during MPI are frequent. Patients with major extracardiac findings have a poor mid-term outcome, whatever the results of the myocardial perfusion imaging. Extracardiac findings should be systematically checked when attenuation correction CT is performed.

     

Instrumentation and computers for brain single photon emission computed tomography

Cerebral single-photon emission computed tomography (SPECT) requires attention to the instrumentation because of the anatomical location of the head at one end of the body, with a generally narrower diameter than the rest of the body. For a number of years, there have been SPECT units designed especially for head work, as well as general-purpose units that have performed well in imaging the head. The current emphasis on cerebral perfusion, using either agents that wash in and out with blood flow or agents that reflect blood flow in their static distribution, has allowed a concentration on imaging hardware and computer hardware and software for this purpose.