Perfusion scanning

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]

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]

• Microspheres
• CT
• MRI
• Nuclear medicine or NM

Using radioactive

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,

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'.

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:

• 3-dimensional
  images of the target organ or organ system.
• Scintigraphy, creating 2-dimensional images.

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

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

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 (MPI) is a form of functional cardiac imaging, used for the diagnosis of

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% (

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

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]

• Functional magnetic resonance imaging
• Ischemia-reperfusion injury of the appendicular musculoskeletal system
• MUGA scan
• Perfusion
• Positron emission tomography
• Stroke
• Ventilation/perfusion ratio