Perfusion scanning
Encyclopedia
Perfusion is defined as the passage of fluid through the lymphatic system or blood vessels to an organ or a tissue. The practice of perfusion scanning, is the process by which this perfusion can be observed, recorded and quantified. The term perfusion scanning encompasses a wide range of medical imaging
modalities.
is still a relatively new concept, although the original framework and principles were concretely laid out as early as 1980 by Leon Axel at University of California San Francisco. It is most commonly carried out for neuroimaging using dynamic sequential scanning of a pre-selected region of the brain during the injection of a bolus of iodinated contrast material as it travels through the vasculature. Various mathematical models can then be used to process the raw temporal data to ascertain quantitative information such as rate of cerebral blood flow (CBF) following an ischemic stroke
or aneurysmal subarachnoid hemorrhage
. Practical CT perfusion as performed on modern CT scanners was developed by many individuals including Matthias Koenig and Ernst Klotz in Germany, and later by Max Wintermark in Switzerland and Ting-Yim Lee in Ontario, Canada.
(V=Ventilation, Q=perfusion) scan, is a way of identifying mismatched areas of blood and air supply to the lungs. It is primarily used to detect a Pulmonary embolus.
The perfusion part of the study uses a radioisotope tagged to the blood which shows where in the lungs the blood is perfusing. If the scan shows up any area missing a supply on the scans this means there is a blockage which is not allowing the blood to perfuse that part of the organ.
Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.
.
A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview, GE healthcare), 99mTc-sestamibi (Cardiolite, Bristol-Myers Squibb now Lantheus Medical Imaging). Following this, the heart rate is raised to induce myocardial stress, either by exercise 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. Diagnosis is made by comparing stress images to a further set of images obtained at rest. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1–7 days later (although, with a Tl-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as two-hours post stress). However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.
MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%) http://jnm.snmjournals.org/cgi/content/abstract/43/12/1634, and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography
.
(99mTc) is a metastable nuclear isomer
which emits gamma rays which can be detected by a gamma camera. When it is attached to exametazime, this allows 99mTc to be taken up by brain tissue in a manner proportial to brain blood flow, in turn allowing brain 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, 99mTc-exametazime (as well as the similar 99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia
. Meta analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease, vs. 81% sensitivity for clinical exam (mental testing, etc.). More recent studies have show accuracy of SPECT in Alzheimer diagnosis as high as 88%. In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias. This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease.
99mTc-exametazime SPECT scanning competes with fludeoxyglucose (FDG) PET
scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes. SPECT is more widely available, however, for the basic reason that the radioisotope generation technology is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. The reason for this is that 99mTc is extracted from relatively simple technetium-99m generator
s which are delivered to hospitals and scanning centers weekly, to supply fresh radioisotope, whereas FDG PET relies on FDG which must be made in an expensive medical cyclotron
and "hot-lab" (automated chemistry lab for radiopharmaceutical manufacture), then must be delivered directly to scanning sites, with delivery-fraction for each trip handicapped by its natural short 110 minute half-life.
Medical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
modalities.
Applications
Being able to observe and quantify perfusion in the human body has been an invaluable step forward in medicine. With the ability to ascertain data on the blood flow to vital organs such as the Heart and the Brain, doctors are able to make quicker and more accurate choices on treatment for patients. Nuclear medicine has been leading perfusion scanning for some time, although the modality has certain pitfalls. It is often dubbed 'unclear medicine' as the scans produced may appear to the untrained eye as just fluffy and irregular patterns. More recent developments in CT and MRI has meant clearer images and solid data such as graphs depicting blood flow and blood volume charted over a fixed period of time.CT Perfusion
The method by which perfusion to an organ measured by CTComputed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
is still a relatively new concept, although the original framework and principles were concretely laid out as early as 1980 by Leon Axel at University of California San Francisco. It is most commonly carried out for neuroimaging using dynamic sequential scanning of a pre-selected region of the brain during the injection of a bolus of iodinated contrast material as it travels through the vasculature. Various mathematical models can then be used to process the raw temporal data to ascertain quantitative information such as rate of cerebral blood flow (CBF) following an ischemic stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
or aneurysmal subarachnoid hemorrhage
Subarachnoid hemorrhage
A subarachnoid hemorrhage , or subarachnoid haemorrhage in British English, is bleeding into the subarachnoid space—the area between the arachnoid membrane and the pia mater surrounding the brain...
. Practical CT perfusion as performed on modern CT scanners was developed by many individuals including Matthias Koenig and Ernst Klotz in Germany, and later by Max Wintermark in Switzerland and Ting-Yim Lee in Ontario, Canada.
MR Perfusion
There are different techniques of detecting perfusion parameters with the use of MRI, the most common being dynamic susceptibility contrast imaging (DSC-MRI) and arterial spin labelling (ASL). In DSC-MRI, Gadolinium contrast agent is injected and a time series of fast T2*-weighted images is acquired. As Gadolinium passes through the tissues, it produces a reduction of T2* intensity depending on the local concentration. The acquired data are then postprocessed to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to peak).NM Perfusion
Nuclear medicine uses radioactive isotopes for the diagnosis and treatment of patients. Whereas radiology provides data mostly on structure, nuclear medicine provides complementary information about function. All nuclear medicine scans give information to the referrering clinician on the function of the system they are imaging. However, there are only a few specific scans which are dedicated to looking at only one organ.- VQVentilation/perfusion scanA ventilation/perfusion lung scan, also called a V/Q lung scan, is a type of medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within a patient's lungs, in order to determine the ventilation/perfusion ratio...
Scans - SPECT
VQ Scans
Ventilation Perfusion Scans, sometimes called a VQVentilation/perfusion scan
A ventilation/perfusion lung scan, also called a V/Q lung scan, is a type of medical imaging using scintigraphy and medical isotopes to evaluate the circulation of air and blood within a patient's lungs, in order to determine the ventilation/perfusion ratio...
(V=Ventilation, Q=perfusion) scan, is a way of identifying mismatched areas of blood and air supply to the lungs. It is primarily used to detect a Pulmonary embolus.
The perfusion part of the study uses a radioisotope tagged to the blood which shows where in the lungs the blood is perfusing. If the scan shows up any area missing a supply on the scans this means there is a blockage which is not allowing the blood to perfuse that part of the organ.
SPECT
SPECT is a branch of Nuclear Medicine and can be used to complement any gamma imaging study, where a true 3D representation can be helpful. E.g. tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone imaging.Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.
Myocardial perfusion 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 testCardiac stress test
Cardiac stress test is a test used in medicine and cardiology to measure the heart's ability to respond to external stress in a controlled clinical environment....
.
A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview, GE healthcare), 99mTc-sestamibi (Cardiolite, Bristol-Myers Squibb now Lantheus Medical Imaging). Following this, the heart rate is raised to induce myocardial stress, either by exercise or pharmacologically with adenosine, dobutamine
Dobutamine
Dobutamine is a sympathomimetic drug used in the treatment of heart failure and cardiogenic shock. Its primary mechanism is direct stimulation of β1 receptors of the sympathetic nervous system. Dobutamine was developed by a laboratory led by Drs...
or dipyridamole
Dipyridamole
Dipyridamole is a drug that inhibits thrombus formation when given chronically and causes vasodilation when given at high doses over a short time.-Mechanism and effects:...
(aminophylline
Aminophylline
Aminophylline is a bronchodilator. It is a compound of the bronchodilator theophylline with ethylenediamine in 2:1 ratio. The ethylenediamine improves solubility, and the aminophylline is usually found as a dihydrate-Properties:...
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. Diagnosis is made by comparing stress images to a further set of images obtained at rest. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1–7 days later (although, with a Tl-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as two-hours post stress). However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.
MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%) http://jnm.snmjournals.org/cgi/content/abstract/43/12/1634, and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography
Echocardiography
An echocardiogram, often referred to in the medical community as a cardiac ECHO or simply an ECHO, is a sonogram of the heart . Also known as a cardiac ultrasound, it uses standard ultrasound techniques to image two-dimensional slices of the heart...
.
Functional brain imaging
Usually the gamma-emitting tracer used in functional brain imaging is technetium (99mTc) exametazime (99mTc-HMPAO, hexamethylpropylene amine oxime). Technetium-99mTechnetium-99m
Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e., that its half-life of 6 hours is considerably longer than most nuclear isomers that undergo gamma decay...
(99mTc) is a metastable nuclear isomer
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
which emits gamma rays which can be detected by a gamma camera. When it is attached to exametazime, this allows 99mTc to be taken up by brain tissue in a manner proportial to brain blood flow, in turn allowing brain 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, 99mTc-exametazime (as well as the similar 99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia
Dementia
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging...
. Meta analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease, vs. 81% sensitivity for clinical exam (mental testing, etc.). More recent studies have show accuracy of SPECT in Alzheimer diagnosis as high as 88%. In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias. This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease.
99mTc-exametazime SPECT scanning competes with fludeoxyglucose (FDG) PET
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes. SPECT is more widely available, however, for the basic reason that the radioisotope generation technology is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. The reason for this is that 99mTc is extracted from relatively simple technetium-99m generator
Technetium-99m generator
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope 99mTc of technetium from a source of decaying molybdenum-99...
s which are delivered to hospitals and scanning centers weekly, to supply fresh radioisotope, whereas FDG PET relies on FDG which must be made in an expensive medical cyclotron
Cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
and "hot-lab" (automated chemistry lab for radiopharmaceutical manufacture), then must be delivered directly to scanning sites, with delivery-fraction for each trip handicapped by its natural short 110 minute half-life.
See also
- Functional magnetic resonance imagingFunctional magnetic resonance imagingFunctional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
- Positron emission tomographyPositron emission tomographyPositron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
- PerfusionPerfusionIn physiology, perfusion is the process of nutritive delivery of arterial blood to a capillary bed in the biological tissue. The word is derived from the French verb "perfuser" meaning to "pour over or through."...
- Ventilation/perfusion ratioVentilation/perfusion ratioIn respiratory physiology, the ventilation/perfusion ratio is a measurement used to assess the efficiency and adequacy of the matching of two variables: It is defined as: the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli.* "V" – ventilation – the air...
- StrokeStrokeA stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
- MUGA scanMUGA scanA MUGA scan is a time-proven yet dated nuclear medicine test designed to evaluate the function of the right and left ventricles of the heart, thus allowing informed diagnostic intervention in heart failure. It is also called radionuclide angiography, as well as gated blood pool imaging...