Perfusion scanning

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Perfusion scanning
Purposeprocess by which perfusion can be observed

Perfusion is the passage of fluid through the lymphatic system or blood vessels to an organ or a tissue. [1] 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. [2]

Contents

Applications

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 have meant clearer images and solid data, such as graphs depicting blood flow, and blood volume charted over a fixed period of time. [2]

Methods

Microsphere perfusion

Using radioactive microspheres is an older method of measuring perfusion than the more recent imaging techniques. This process involves labeling microspheres with radioactive isotopes and injecting these into the test subject. Perfusion measurements are taken by comparing the radioactivity of selected regions within the body to radioactivity of blood samples withdrawn at the time of microsphere injection. [3]

Later, techniques were developed to substitute radioactively labeled microspheres for fluorescent microspheres. [4]

CT perfusion

The method by which perfusion to an organ measured by CT is still a relatively new concept, although the first dynamic imaging studies of cerebral perfusion were reported on in 1979 by E. Ralph Heinz et al. from the Duke University Medical Center, Durham, North Carolina, [5] itself citing a reference on a presentation on "Dynamic Computed Tomography" at the XI. Symposium Neuroradiologicum in Wiesbaden, June 4–10, 1978, which has not been submitted to the conference proceedings. [6] The original framework and principles for CT perfusion analysis were concretely laid out in 1980 by Leon Axel at University of California San Francisco. [7] 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 first described by Ken Miles, Mike Hayball and Adrian Dixon from Cambridge UK [8] and subsequently developed by many individuals including Matthias Koenig and Ernst Klotz in Germany, [9] and later by Max Wintermark in Switzerland and Ting-Yim Lee in Ontario, Canada. [10]

MRI perfusion

There are different techniques of Perfusion MRI, the most common being dynamic contrast-enhanced (DCE), dynamic susceptibility contrast imaging (DSC), and arterial spin labelling (ASL). [11]

In DSC, Gadolinium contrast agent (Gd) is injected (usually intravenously) and a time series of fast T2*-weighted images is acquired. As Gadolinium passes through the tissues, it induces a reduction of T2* in the nearby water protons; the corresponding decrease in signal intensity observed depends on the local Gd concentration, which may be considered a proxy for perfusion. The acquired time series 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).

DCE-MRI also uses intravenous Gd contrast, but the time series is T1-weighted and gives increased signal intensity corresponding to local Gd concentration. Modelling of DCE-MRI yields parameters related to vascular permeability and extravasation transfer rate (see main article on perfusion MRI).

Arterial spin labelling (ASL) has the advantage of not relying on an injected contrast agent, instead inferring perfusion from a drop in signal observed in the imaging slice arising from inflowing spins (outside the imaging slice) having been selectively saturated. A number of ASL schemes are possible, the simplest being flow alternating inversion recovery (FAIR) which requires two acquisitions of identical parameters with the exception of the out-of-slice saturation; the difference in the two images is theoretically only from inflowing spins, and may be considered a 'perfusion map'.

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. [12] All nuclear medicine scans give information to the referrering clinician on the function of the system they are imaging.

Specific techniques used are generally either of the following:

Uses of NM perfusion scanning include Ventilation/perfusion scans of lungs, myocardial perfusion imaging of the heart, and functional brain imaging.

Ventilation/perfusion scans

Ventilation/perfusion scans, sometimes called a VQ (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.

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 test.

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%), [13] and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography.

Functional brain imaging

Usually the gamma-emitting tracer used in functional brain imaging is technetium (99mTc) exametazime (99mTc-HMPAO, hexamethylpropylene amine oxime). Technetium-99m (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 proportional 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 shown accuracy of SPECT in Alzheimer diagnosis as high as 88%. [14] 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. [15] 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 generators 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 limited by its natural short 110 minute half-life.

Testicular torsion detection

Radionuclide scanning of the scrotum is the most accurate imaging technique to diagnose testicular torsion, but it is not routinely available. [16] The agent of choice for this purpose is technetium-99m pertechnetate. [17] Initially it provides a radionuclide angiogram, followed by a static image after the radionuclide has perfused the tissue. In the healthy patient, initial images show symmetric flow to the testes, and delayed images show uniformly symmetric activity. [17]

See also

Related Research Articles

Single-photon emission computed tomography Nuclear medicine tomographic imaging technique

Single-photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera, but is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.

Nuclear medicine Medical specialty

Nuclear medicine, is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear imaging, in a sense, is "radiology done inside out" because it records radiation emitting from within the body rather than radiation that is generated by external sources like X-rays. In addition, nuclear medicine scans differ from radiology, as the emphasis is not on imaging anatomy, but on the function. For such reason, it is called a physiological imaging modality. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) scans are the two most common imaging modalities in nuclear medicine.

Technetium (<sup>99m</sup>Tc) sestamibi

Technetium (99mTc) sestamibi (INN) is a pharmaceutical agent used in nuclear medicine imaging. The drug is a coordination complex consisting of the radioisotope technetium-99m bound to six (sesta=6) methoxyisobutylisonitrile (MIBI) ligands. The anion is not defined. The generic drug became available late September 2008. A scan of a patient using MIBI is commonly known as a "MIBI scan".

Scintigraphy Diagnostic imaging test in nuclear medicine

Scintigraphy, also known as a gamma scan, is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted gamma radiation is captured by external detectors to form two-dimensional images in a similar process to the capture of x-ray images. In contrast, SPECT and positron emission tomography (PET) form 3-dimensional images and are therefore classified as separate techniques from scintigraphy, although they also use gamma cameras to detect internal radiation. Scintigraphy is unlike a diagnostic X-ray where external radiation is passed through the body to form an image.

Perfusion Passage of fluid through the circulatory or lymphatic system to an organ or tissue

Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is delivered to tissue, or volume of blood per unit time per unit tissue mass. The SI unit is m3/(s·kg), although for human organs perfusion is typically reported in ml/min/g. The word is derived from the French verb "perfuser" meaning to "pour over or through". All animal tissues require an adequate blood supply for health and life. Poor perfusion (malperfusion), that is, ischemia, causes health problems, as seen in cardiovascular disease, including coronary artery disease, cerebrovascular disease, peripheral artery disease, and many other conditions.

Dipyridamole Anticoagulant drug

Dipyridamole is a nucleoside transport inhibitor and a PDE3 inhibitor medication that inhibits blood clot formation when given chronically and causes blood vessel dilation when given at high doses over a short time.

Radionuclide angiography Nuclear medicine imaging the ventricles of the heart

Radionuclide angiography is an area of nuclear medicine which specialises in imaging to show the functionality of the right and left ventricles of the heart, thus allowing informed diagnostic intervention in heart failure. It involves use of a radiopharmaceutical, injected into a patient, and a gamma camera for acquisition. A MUGA scan involves an acquisition triggered (gated) at different points of the cardiac cycle. MUGA scanning is also called equilibrium radionuclide angiocardiography, radionuclide ventriculography (RNVG), or gated blood pool imaging, as well as SYMA scanning.

Bone scintigraphy

A bone scan or bone scintigraphy is a nuclear medicine imaging technique of the bone. It can help diagnose a number of bone conditions, including cancer of the bone or metastasis, location of bone inflammation and fractures, and bone infection (osteomyelitis).

Neuroimaging Set of techniques to measure and visualize aspects of the nervous system

Neuroimaging is the use of quantitative (computational) techniques to study the structure and function of the central nervous system, developed as an objective way of scientifically studying the healthy human brain in a non-invasive manner. Increasingly it is also being used for quantitative studies of brain disease and psychiatric illness. Neuroimaging is a highly multidisciplinary research field and is not a medical specialty.

Ventilation/perfusion scan

A ventilation/perfusion lung scan, also called a V/Q lung scan, or ventilation/perfusion scintigraphy, 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. The ventilation part of the test looks at the ability of air to reach all parts of the lungs, while the perfusion part evaluates how well blood circulates within the lungs. As Q in physiology is the letter used to describe bloodflow the term V/Q scan emerged.

Technetium-99m Metastable nuclear isomer of technetium-99

Technetium-99m (99mTc) is a metastable nuclear isomer of technetium-99, symbolized as 99mTc, that is used in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope in the world.

Functional imaging is a medical imaging technique of detecting or measuring changes in metabolism, blood flow, regional chemical composition, and absorption.

Myocardial perfusion imaging Nuclear medicine imaging method

Myocardial perfusion imaging or scanning is a nuclear medicine procedure that illustrates the function of the heart muscle (myocardium).

Technetium (<sup>99m</sup>Tc) exametazime

Technetium (99mTc) exametazime is a radiopharmaceutical sold under the trade name Ceretec, and is used by nuclear medicine physicians for the detection of altered regional cerebral perfusion in stroke and other cerebrovascular diseases. It can also be used for the labelling of leukocytes to localise intra-abdominal infections and inflammatory bowel disease. Exametazime is sometimes referred to as hexamethylpropylene amine oxime or HMPAO, although correct chemical names are:

Nuclear medicine physicians, also called nuclear radiologists or simply nucleologists, are medical specialists that use tracers, usually radiopharmaceuticals, for diagnosis and therapy. Nuclear medicine procedures are the major clinical applications of molecular imaging and molecular therapy. In the United States, nuclear medicine physicians are certified by the American Board of Nuclear Medicine and the American Osteopathic Board of Nuclear Medicine.

Rubidium-82 (82Rb) is a radioactive isotope of rubidium. 82Rb is widely used in myocardial perfusion imaging. This isotope undergoes rapid uptake by myocardiocytes, which makes it a valuable tool for identifying myocardial ischemia in Positron Emission Tomography (PET) imaging. 82Rb is used in the pharmaceutical industry and is marketed as Rubidium-82 chloride under the trade names RUBY-FILL and CardioGen-82.

Cardiac magnetic resonance imaging perfusion

Cardiac magnetic resonance imaging perfusion, also known as stress CMR perfusion, is a clinical magnetic resonance imaging test performed on patients with known or suspected coronary artery disease to determine if there are perfusion defects in the myocardium of the left ventricle that are caused by narrowing of one or more of the coronary arteries.

Cardiac imaging

Cardiac imaging refers to non-invasive imaging of the heart using ultrasound, magnetic resonance imaging (MRI), computed tomography (CT), or nuclear medicine (NM) imaging with PET or SPECT. These cardiac techniques are otherwise referred to as echocardiography, Cardiac MRI, Cardiac CT, Cardiac PET and Cardiac SPECT including myocardial perfusion imaging.

Brain positron emission tomography Form of positron emission tomography

Brain positron emission tomography is a form of positron emission tomography (PET) that is used to measure brain metabolism and the distribution of exogenous radiolabeled chemical agents throughout the brain. PET measures emissions from radioactively labeled metabolically active chemicals that have been injected into the bloodstream. The emission data from brain PET are computer-processed to produce multi-dimensional images of the distribution of the chemicals throughout the brain.

Cerebral blood volume is the blood volume in a given amount of brain tissue.

References

  1. ↑ American Psychological Association (APA): perfusion. (n.d.). Dictionary.com Unabridged (v 1.1). Retrieved March 20, 2008, from Dictionary.com website: http://dictionary.reference.com/browse/perfusion
  2. 1 2 http://www.webmd.com/heart-disease/cardiac-perfusion-scan#1 www.webmd.com/
  3. ↑ Wagner HN, Rhodes BA, Sasaki Y, Ryan JP (1969). "Studies of the circulation with radioactive microspheres". Invest Radiol. 4 (6): 374–86. doi:10.1097/00004424-196911000-00004. PMID   4904709. S2CID   10017854.
  4. ↑ "Fluorescent Microspheres" (PDF). Fluorescent Microsphere Resource Center. Archived from the original (PDF) on 2012-10-02.
  5. ↑ Heinz ER, Dubois P, Osborne D, Drayer B, Barrett W (1979). "Dynamic computed tomography study of the brain". Journal of Computer Assisted Tomography. 3 (5): 641–649. doi:10.1097/00004728-197910000-00013. PMID   113434. S2CID   867095.
  6. ↑ Wende, Sigur, ed. (1978). Proceedings of the XI. Symposium Neuroradiologicum. Springer Verlag. pp. X. ISBN   978-3-642-66959-0.
  7. ↑ Axel L (1980). "Cerebral blood flow determination by rapid-sequence computed tomography: theoretical analysis". Radiology. 137 (3): 679–86. doi:10.1148/radiology.137.3.7003648. PMID   7003648.
  8. ↑ Miles KA, Hayball M, Dixon AK (1991). "Colour perfusion imaging: a new application of computed tomography". Lancet. 337 (8742): 643–5. doi:10.1016/0140-6736(91)92455-b. PMID   1671994. S2CID   9681671.
  9. ↑ Koenig M, Klotz E, Luka B, Venderink DJ, Spittler JF, Heuser L (1998). "Perfusion CT of the brain: diagnostic approach for early detection of ischemic stroke". Radiology. 209 (1): 85–93. doi:10.1148/radiology.209.1.9769817. PMID   9769817.
  10. ↑ Konstas AA, Goldmakher GV, Lee TY, Lev MH (2009). "Theoretic basis and technical implementations of CT perfusion in acute ischemic stroke, part 2: technical implementations" (PDF). AJNR Am J Neuroradiol. 30 (5): 885–92. doi: 10.3174/ajnr.A1492 . PMC   7051660 . PMID   19299489.
  11. ↑ Jahng, Geon-Ho; Li, Ka-Loh; Ostergaard, Leif; Calamante, Fernando (2014). "Perfusion Magnetic Resonance Imaging: A Comprehensive Update on Principles and Techniques". Korean Journal of Radiology. 15 (5): 554–77. doi:10.3348/kjr.2014.15.5.554. PMC   4170157 . PMID   25246817.
  12. ↑ Prvulovich EM, Bomanji JB (1998). "Fortnightly review: The role of nuclear medicine in clinical investigation". BMJ. 316 (7138): 1140–1146. doi:10.1136/bmj.316.7138.1140. ISSN   0959-8138. PMC   1112941 . PMID   9552956.
  13. ↑ Elhendy A, Bax J, Poldermans D (2002). "Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease*". J Nucl Med. 43 (12): 1634–46. PMID   12468513.
  14. ↑ Bonte, F. J.; Harris, T. S.; Hynan, L. S.; Bigio, E. H.; White, C. L. (2006). "Tc-99m HMPAO SPECT in the Differential Diagnosis of the Dementias with Histopathologic Confirmation". Clinical Nuclear Medicine. 31 (7): 376–8. doi:10.1097/01.rlu.0000222736.81365.63. PMID   16785801. S2CID   39518497.
  15. ↑ Dougall, N. J.; Bruggink, S. .; Ebmeier, K. . (2004). "Systematic Review of the Diagnostic Accuracy of 99mTc-HMPAO-SPECT in Dementia". American Journal of Geriatric Psychiatry. 12 (6): 554–70. doi:10.1176/appi.ajgp.12.6.554. PMID   15545324.
  16. ↑ Sexually Transmitted Diseases Treatment Guidelines, 2010 from Centers for Disease Control and Prevention, Recommendations and Reports. December 17, 2010 / Vol. 59 / No. RR-12
  17. 1 2 Medscape > Testicular Torsion Imaging by David Paushter. Updated: May 25, 2011
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