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Important Information About PET-CT

24 April, 2011 | PET-CT

In most modern healthcare facilities, PET scans are now carried out on instruments that combine PET and CT scanners. The combined PET-CT scans provide images that identify the exact site of irregular metabolic activity within the body. The combined PET-CT scans have been shown to provide more accurate diagnoses than the two scans performed separately. Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging.

PET-CT has revolutionized many fields of medical diagnosis, by adding the precision of anatomic localization to functional imaging, which was previously lacking in pure PET imaging.  Although the combined device of complex PET-CT parts is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.

Nuclear medicine is a branch of medical imaging that uses small amounts of radioactive material to diagnose or treat a variety of diseases, including many types of cancers, heart disease and certain other abnormalities within the body. Nuclear medicine or radionuclide imaging procedures are noninvasive and with the exception of intravenous injections, are usually painless medical tests that assist physicians in the diagnosis of medical conditions.

Depending on the type of nuclear medicine exam the patient is undergoing, a radiotracer is either injected into a vein, swallowed or inhaled as a gas and eventually accumulates in the organ or area of the body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radiotracer absorbed by the patient’s body and produces unique pictures detailing both the structure and function of organs and tissues.

Often, nuclear medicine images are placed over computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses. In addition, manufacturers are now making single positron emission tomography/computed tomography (PET-CT) units that are able to perform both imaging studies at the same time. Two and three dimensional image reconstruction may be rendered as a function of a common software and control system PET-CT part.

A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, to help doctors evaluate how well organs and tissues are functioning.

CT imaging uses special x-ray equipment, and in some cases a contrast material, to produce multiple images or pictures of the inside of the body. These images can then be interpreted by a radiologist on a computer monitor as printed images. CT imaging provides excellent anatomic information.

PET/CT scans are performed for the following reasons:

  • Cancer detection.
  • To determine if a cancer has spread in the body.
  • To assess the effectiveness of a treatment plan, i.e. to determine if a cancer has returned after treatment.
  • To find out about blood flow to the heart muscle.
  • To establish the effects of a myocardial infarction.
  • To identify areas of the heart muscle that would benefit from a procedure such as angioplasty or coronary artery bypass surgery (in combination with a myocardial perfusion scan).
  • To evaluate brain abnormalities, such as tumors, memory disorders and seizures and other central nervous system disorders.
  • To map normal brain and heart function.

One of the biggest obstacles to even wider usage of combined PET-CT is the difficulty and cost of producing and transporting radiopharmaceuticals used for PET-CT imaging, as they are very short-lived.

The reason for this is as the injected radioisotope undergoes positron emission decay, it emits a positron, and antiparticle of the electron with opposite charge. The emitted positron travels in the tissue for a short distance, during which time it loses kinetic energy, until it decelerates to a point where it can interact with an electron. The encounter destroys both electron and positron, producing a pair of annihilation photons moving in opposite directions. These are detected when they reach a scintillator, which is an exclusive PET-CT part, in the scanning device, creating a burst of light which is detected by photomultiplier tubes or solid state detector PET-CT parts. The technique depends on simultaneous or instantaneous detection of the pair of photons moving. Photons that do not arrive in temporal “pairs” are ignored.

Usually a technique much like the reconstruction of computed tomography and single photon emission computed tomography data is used, although the data set collected in PET is not as good as that of CT, so reconstruction techniques are more difficult.

Because PET imaging is most useful in combination with anatomical imaging, such as CT, up-to-date PET-CT part PET scanners are now available with integrated high-end multi-slice CT scanners. Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more precisely registered, so that the areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images.

The PET-CT system comprises two different blocks based on different technologies:

  • The PET section of the PET-CT part, which is a nuclear medicine scanner.
  • The CT section of the PET-CT part, which is an x-ray scanner. It also provides attenuation correction for the acquired PET data.
  • A single patient bed PET-CT part serves both scanners. The patient bed PET-CT part, should be very stable, robust and easy to manipulate according to individual needs. It should have large scan range for both modalities, and high horizontal speed.

A wide patient port is preferable (70cm Bore). The whole PET-CT machine should be well-cooled by the cooling system PET-CT part.
The PET Detector assembly is the most important part of the PET Machine. New generation detectors are highly recommended:

  • Large number of crystals, full detector range (360 Degrees), ability to count high photon flux, variable energy level, high sensitivity, and low noise level –ensuring the acquisition of high quality images.

The reconstruction system PET-CT part is also very important. It should be a multi-processor computer platform which provides fast and efficient processing of the acquired data. In the newer PET-CT systems, the reconstruction algorithm also takes into consideration the “time of flight” (T.O.F) of the photon to achieve better images, (lower noise).

The CT machine PET-CT part should have high rotational speed and a multi-slice (6 slices or more). A large number of slices is preferable, especially if high resolution CT scan is desirable. Flexible clinical software will maximize clinical efficiency.  The system should provide parameter registration of PET-CT data, single and multi-modality review and display and quantitative analysis software. A local working station PET-CT part for nuclear medicine is very convenient.

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