22-02-2009, 01:43 AM
Emission Computed Tomography is a technique where by multi cross sectional images of tissue function can be produced , thus removing the effect of overlying and underlying activity. The technique of ECT is generally considered as two separate modalities. SINGLE PHOTON Emission Computed Tomography involves the use single gamma ray emitted per nuclear disintegration. Positron Emission Tomography makes use of radio isotopes such as gallium-68, when two gamma rays each of 511KeV, are emitted simultaneously where a positron from a nuclear disintegration annihilates in tissue.
SPECT, the acronym of Single Photon Emission Computed Tomography is a nuclear medicine technique that uses radiopharmaceuticals, a rotating camera and a computer to produce images which allow us to visualize functional information about a patientâ„¢s specific organ or body system. SPECT images are functional in nature rather than being purely anatomical such as ultrasound, CT and MRI. SPECT, like PET acquires information on the concentration of radio nuclides to the patientâ„¢s body.
SPECT dates from the early 1960 are when the idea of emission traverse section tomography was introduced by D.E.Kuhl and R.Q.Edwards prior to PET, X-ray, CT or MRI. THE first commercial Single Photon- ECT or SPECT imaging device was developed by Edward and Kuhl and they produce tomographic images from emission data in 1963. Many research systems which became clinical standards were also developed in 1980â„¢s.
2. Single photon emission computed tomography (SPECT)
What is SPECT?
SPECT is short for single photon emission computed tomography. As its name suggests (single photon emission) gamma rays are the sources of the information rather than X-ray emission in the conventional CT scan.
Similar to X-ray, CT, MRI, etc SPECT allows us to visualize functional information about patientâ„¢s specific organ or body system.
How does SPECT manage us to give functional information?
Internal radiation is administrated by means of a pharmaceutical which is labeled with a radioactive isotope. This pharmaceutical isotope decays, resulting in the emission of gamma rays. These gamma rays give us a picture of whatâ„¢s happening inside the patientâ„¢s body.
But how do these gamma rays allow us to see inside?
By using the most essential tool in Nuclear Medicine-the Gamma Camera. The Gamma Camera can be used in planner imaging to acquire a 2-D image or in SPECT imaging to acquire a 3-D image.
How are these Gamma rays collected?
The Gamma Camera collects the gamma rays emitted from the patient, enabling to reconstruct a picture of where the gamma rays originated. From this we can how a patientâ„¢s organ or system is functioning.
3. THEORY AND INSTRMENTATION
Single â€œphoton Emission Computed tomography or what the medical world refers to as SPECT is a technology used in nuclear medicine where the patient is injected with a radiopharmaceutical which will emit gamma rays. We seek the position and concentration of radionuclide distribution by the rotation of a photon detector array around the body which acquires data from multiple angles. The radiopharmaceutical may be delivered by 1V catheter, inhaled aerosol etc. The radio activity is collected by an instrument called a gamma camera. Images are formed from the 3-D distribution of the radiopharmaceutical with in the body.
Because the emission sources are inside the body cavity, this task is for more difficult than for X-ray, CT, where the source position and strength are known at all times.
i.e. In X-ray, CT, the attenuation is measured not the transmission source. To compensate for the attenuation experienced by emission photons from injected tracers in the body, contemporary SPECT machines use mathematical reconstruction algorithms to increase resolution.
The gamma camera is made up of two or three massive cameras opposite to each other which rotate around a centre axis, thus each camera moving 180 or 120 degrees respectively. Each camera is lead-encased and weighs about 500 pounds .The camera has three basic layers â€œthe collimator (which only allows the gamma rays which are perpendicular to the plane of the camera to enter), the crystal and the detectors. Because only a single photon is emitted from the radionuclide used for SPECT, a special lens known as a collimator is used to acquire the image from multiple views around the body .The collimation of the rays facilitates the reconstruction since we will be dealing with data that comes in only perpendicular .At each angle of projection, the data will be back projected only in one direction.
When the gamma camera rotates around the supine body, it stops at interval angles to collect data. Since it has two or three heads, it needs to only to rotate 180 or 120 degrees to collect data around the entire body .The collected data is planar. Each of the cameras collects a matrix of values which correspond to the number of gamma counts detected in that direction at the one angle.
Images can be reprojected into a three dimensional one that can be viewed in a dynamic rotating format on computer monitors, facilitating the demonstration of pertinent findings to the referring physicians.
4. THE GAMMA CAMERA
Once a radiopharmaceutical has been administered, it is necessary to detect the gamma ray emissions in order to attain the functional information. The instrument used in nuclear medicine for the detection of gamma rays is known as gamma camera
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