Unidad 9Biofísica de las Radiaciones

Imágenes de Medicina Nuclear

Dr. Juan José Aranda Aboy

Profesor e Investigador Titular

Espectro de energía de la radiación gamma

Detección por cristal de centelleo

Cámara de centelleos

Tubo foto multiplicador (TFM)

Arreglo de TFM

Cámara Gamma• Gamma rays from the organ travel in all

directions, but the collimator tubes ensure that only radiation from a small region reaches the NaI detector.

• Light is detected by more than one photomultiplier tube, but the relative amounts received improve spatial resolution.

• An electronic pulse height analyzer selects events that have the proper gamma-ray energy.

• The resulting image has spatial resolution of about 1% of the image dimension

Cámara Gamma (2)

In a gamma camera system, radioisotopes emit gamma rays, which are collimated, strike a NaI crystal, which emits light measured by photomultiplier tubes.

Cámara Gamma multicristal

Sistemas de rastreo

In each successive gamma-camera picture of a thyroid phantom, the number of counts is increased by a factor of 2. The number of counts ranges from 1563 to 800,000. The Polaroid camera aperture was reduced to avoid overexposure as the number of counts was increased.

Images of a patient's skeleton obtained by a rectilinear scanner, in which a technetium-labeled phosphate compound reveals regions of abnormally high metabolism. The conventional analog image is on the left, the digitized version on the right.

Gamma-camera images of an anterior view of the right lobe of a patient's liver. A colloid labeled with radioactive technetium was swept from the blood stream by normal liver tissue. Left: conventional analog image. Right: Digitized version of the same data.

Single-Photon Emission Computed Tomography (SPECT)

• In single-photon emission computed tomography (SPECT), a scintillation assembly similar to a gamma camera is rotated around the patient.

• The gamma rays are collected from the patient in a manner similar to CT, but several slices are obtained at the same time.

• The resulting multiple slices show depth activity in the volume of interest.

• It is possible to see anomalies not observable with conventional X rays or gamma cameras.

Positron Emission Tomography (PET) • Some isotopes produce positrons that react with electrons to emit two

photons at 511 keV in opposite directions. • The two detectors on opposite sides of the patient that determine if the

two scintillation effects are coincident and have energy levels close to 511 keV.

• Additional pairs of detectors permit faster operation. • Image reconstruction is similar to that of CT, but an advantage of PET is

that all of the most common radioisotopes used, 15O, 13N, 11C and 18F, can be compounded as metabolites.

• For example, CO can be made with 11C. If a portion of the brain is active, increased blood flow carries the isotope to it, where it shows up on the image.

• Abnormal functioning, tumors, seizures and other anomalies may also be mapped this way. For example, measurement of glucose–fluorodeoxyglucose (FDG) metabolism is used to determine tumor growth.

• Because small amounts of FDG can be visualized, early tumor detection is possible before structural changes occur or would be detected by MRI or CT.

PET camera

In the PET camera, (a) the paired and (b) the hexagonal ring cameras rotate around the patient. (c) The circular ring does not rotate, but may move slightly to

fill in the gaps between the detectors.

PET image

The trapping of 60Cu-PTSM (a thiosemicarbazone) reflects regional blood flow, modulated by a nonunity extraction into the tissue.

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