Planar scintigraphy produces two-dimensional images of three dimensional objects
Rowley, Laura, Financial Columnist has reference to this Academic Journal, PHwiki organized this Journal Planar scintigraphy produces two-dimensional images of three dimensional objects. It is h in addition to icapped by the superposition of active in addition to nonactive layers which restricts the accurate measurement of organ functions. Emission computed tomography (ECT) is based on the production of multi cross sectional images of tissue function which are used to produce by overlay three dimensional images.
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Two ECT techniques are currently available: Single Photon Emission Tomography (SPECT) which involves the imaging of single 7-ray activity (typically from 99Tcm). Positron Emission Tomography (PET) which involves the imaging of the 511 keV annihilation radiation originated in positron decay (typically from 18F) Image planes are derived by using two different techniques: longitudinal ECT (limited-angle technique), photons are detected within a limited angular range from several body sections simultaneously. The reconstructed image planes are positioned parallel tothe detector plane. transaxial ECT (transverse section technique), the detector moves by 360 ° around the body to sample photons from multiple body sections. The reconstructed image planes are perpendicular to the detector plane.
Longitudinal ECT allows to view the radioactivity from different angles (within a limited angle range) to obtain in as long as mation about the depth of the radiation source. This is done by using a rectilinear scanner system coupled with a highly focused collimator who creates a sharp image only from a particular plane at a depth defined by the angle range (focal point). The multiplane tomographic scanner represents an improved version which replaces the single detector by a gamma camera. It also allows to adjust (by electronic repositioning) the focal distance in addition to can there as long as e select different image planes. As example are shown twelve images of the skeleton recorded in longitudinal multiplane tomography technique. Each image represents a different plane along the body.
Single-Photon Emission Computed Tomography (SPECT) with a rotating gamma camera (transaxial ECT) allows multiple views of the three dimensional distribution of the radioactivity from different directions. The gamma camera is coupled to a parallel hole collimator (no focusing) which allows to produce a 2D image (64×64) consisting of multiple profiles (64), each profile represents a ID projection of the radioactivity in the profile (distributed over 64 channels). Each point in the profile represents the sum of the activity along the line of sight: with as detector efficiency in addition to as solid angle The camera rotates either continuously or in fixed angle steps in addition to repeats the monitoring until the completion of the 360° turn. The three dimensional image is constructed by using similar Fourier analysis techniques as designed as long as X-ray CT scanning. Positron Emission Tomography (PET) operates by using at least two opposite to each other positioned rotateable detector. PET is based on the principle of detecting annihilation radiation with coincidence techniques. The injected radionuclide must be a positron (+) emitter. The positron annihilates after about 1mm path length (depending on density of tissue material in addition to on the energy of the positron) in addition to emits two 511 keV photons in opposite directions.
Detection of both photons in coincidence defines a line along which the annihilation event has taken place. The position of the radionuclide is within 1mm distance. This distance as well as a slight deviation from the 1800 emission of the two photons limits the spatial resolution to about 1mm 2mm. The use of annihilation radiation coincidence technique in PET improves the quality of image as long as mation considerably compared to collimator techniques used in SPECT. In SPECT the intensity in addition to the resolution of the signal degrades with increasing depth, due to attenuation through body tissue of increasing thickness d, in addition to due to the degradation of collimator resolution c with increasing source collimator distance z: with constant hole diameter d in addition to hole length L as long as the collimator system. In annihilation radiation coincidence measurements the resolution remains essential constant with depth because of the uni as long as mity of the geometric response defined by the straight line between the two detection processes.
The intensity of the coincidence signal is defined by the attenuation in body material from the point of annihilation at depth d in both directions, with T being the thickness of the body along the line in addition to (x)dx . There as long as e the intensity as long as the annihilation signal along the line is independent of the depth. The absolute count rate as long as coincidence events is determined by the count rate as long as true coincidences Itrue (real coincidence events originating from one single annihilation process) in addition to as long as r in addition to om coincidences Ir in addition to om (fake coincidence events which occur when accidentally each detector records an uncorrelated signal within a time window ). The count rate as long as true coincidences from I0 annihilation events is determined by the efficiency e in addition to solid angle of each detector: as long as present PET machines the total efficiency as long as coincidence measurement,
The r in addition to om coincidence count rate is determined by the count rate as long as single events: in the two detectors in addition to by the coincidence time window r: This yields a ratio of true to r in addition to om coincidences: which is independent of efficiencies in addition to solid angle but only depends on the intensity of the emitted annihilation radiation, in addition to the coincidence time window: At these conditions the intensity of the radiation source inside the body must be at least I0 106 events/s to obtain a true to r in addition to om ratio of unity. This would require a source strength of at least 1 MBq inside the body. A 1 MBq source inside the body (neglecting attenuation effects corresponds to a r in addition to om count rate of: To improve the detection conditions in addition to to separate the true from the r in addition to om coincidences the time structure of the signals can be utilized by decreasing . Separation is based on the fact that r in addition to om events come continuously while true events come within a few nano seconds (speed of light c=31010 cm/s). St in addition to ard electronic is used to separate true coincidence events in the sharp time peak from r in addition to om events using fast timing conditions on the electronic signals. For = 10 ns at a count rate of I0 = 106 s-1 a true coincidence rate of: is obtained at a r in addition to om coincidence rate of:
Modern PET systems are based on multicrystal designs. Instead of a rotateable detector pair the patient is surrounded by a ring of individual small Nal scintillator detectors. Each detector is coupled to his own individual phototube in addition to is in electronic coincidence with any of those detectors at the opposite site of the patient. With such a device a multiple image can be obtained in one shot. This is particular important as long as monitoring physiological processes with short time scales ( t 10-3 s). Detecting coincidence events between one detector in addition to two neighboring detectors at the opposite site defines the spatial resolution of the device to 5mm.
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