- Single-photon emission computed tomography.
- Why it's done;
- Evolution of brain imaging instrumentation..
Amen Clinics has provided education and insight on the clinical use of brain imaging in psychiatry. It is a nuclear medicine procedure widely used to study heart, liver, thyroid, bone and brain problems. Because brain activity is directly related to blood flow, SPECT effectively shows us the patterns of activity in the brain. SPECT allows physicians to look deep inside the brain to observe three things: areas of the brain that work well, areas of the brain that work too hard and areas of the brain that do not work hard enough.
The procedure guidelines of the Society of Nuclear Medicine list the evaluation of suspected brain trauma, evaluation of patients with suspected dementia, presurgical location of seizures, and the detection and evaluation of cerebral vascular disease as common indications for brain SPECT.
The guidelines also say that many additional indications appear promising. At Amen Clinics, because of our experience, we have added the indications of evaluating violence, substance abuse, the subtypes of ADD, anxiety and depression, and complex or resistant psychiatric problems for brain SPECT. MRI does not give any information on function. This gives a total scan time of 15—20 minutes. Multi-headed gamma cameras can accelerate acquisition.
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For example, a dual-headed camera can be used with heads spaced degrees apart, allowing two projections to be acquired simultaneously, with each head requiring degrees of rotation. Triple-head cameras with degree spacing are also used. Triggered by electrocardiogram EKG to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle, and also to allow calculation of left ventricular ejection fraction , stroke volume, and cardiac output.
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful, such as tumor imaging, infection leukocyte imaging, thyroid imaging or bone scintigraphy.
Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs, such as functional cardiac or brain imaging. Myocardial perfusion imaging MPI is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease.
The underlying principle is that under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test. A cardiac specific radiopharmaceutical is administered, e. Following this, the heart rate is raised to induce myocardial stress, either by exercise on a treadmill or pharmacologically with adenosine , dobutamine , or dipyridamole aminophylline can be used to reverse the effects of dipyridamole.
SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium.
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Diagnosis is made by comparing stress images to a further set of images obtained at rest which are normally acquired prior to the stress images. Attaching it to HMPAO allows 99m Tc to be taken up by brain tissue in a manner proportional to brain blood flow, in turn allowing cerebral blood flow to be assessed with the nuclear gamma camera.
Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99m Tc-HMPAO tracer as well as the similar 99m Tc-EC tracer is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia.
SPECT is more widely available, because the radioisotope used is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. While 99m Tc is extracted from relatively simple technetiumm generators , which are delivered to hospitals and scanning centers weekly to supply fresh radioisotope, FDG PET relies on FDG, which is made in an expensive medical cyclotron and "hot-lab" automated chemistry lab for radiopharmaceutical manufacture , and then delivered immediately to scanning sites because of the natural short minute half-life of Fluorine In the nuclear power sector, the SPECT technique can be applied to image radioisotope distributions in irradiated nuclear fuels.
The Clinical Use of SPECT and PET Molecular Imaging in Alzheimer’s Disease
These may be imaged using SPECT in order to verify the presence of fuel rods in a stored fuel assembly for IAEA safeguards purposes,  to validate predictions of core simulation codes,  or to study the behavior of the nuclear fuel in normal operation,  or in accident scenarios. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to artifacts.
Scanning is time consuming, and it is essential that there is no patient movement during the scan time.
SPECT Imaging of the Brain | R. Duncan | Springer
Movement can cause significant degradation of the reconstructed images, although movement compensation reconstruction techniques can help with this. A highly uneven distribution of radiopharmaceutical also has the potential to cause artifacts. A very intense area of activity e.
This is a limitation of the filtered back projection reconstruction algorithm. Iterative reconstruction is an alternative algorithm that is growing in importance, as it is less sensitive to artifacts and can also correct for attenuation and depth dependent blurring. Furthermore, iterative algorithms can be made more efficacious using the Superiorization methodology.
Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity, and optimal correction is obtained with measured attenuation values. It also provides a precisely registered CT image, which can provide additional anatomical information. Scatter of the gamma rays as well as the random nature of gamma rays can also lead to the degradation of quality of SPECT images and cause loss of resolution.
For example, for cardiac SPECT studies, where Tl or 99m Tc may be used, Tl delivers a much higher radiation dose per study than for a 99m Tc study even though the amount of activity administered is lower.
Care should be taken to select the appropriate protocol and radionuclide to fit the situation. The reconstruction method used will also affect image quality. Most SPECT system manufacturers now offer iterative reconstruction methods that provide features such as resolution recovery, scatter and other corrections. These features may lead to sufficiently improved image quality to allow for a lower administered activity compared to traditional reconstruction algorithms such as filtered backprojection.
Optimizing the SPECT acquisition to achieve the best spatial resolution and image noise for a particular clinical task involves selecting the proper instrumentation and reconstruction technique and performing the study properly. This assures the most efficient use of the administered activity and, therefore, provides the best opportunity for radiation dose optimization. References Chandra R: Nuclear medicine physics: the basics, 7th ed. Malden, MA, Blackwell Science, J Nucl Med, ; Nuclear Medicine-2 Volumes, 2nd ed.