Nuclear imaging Instrumentation part

Prepared by : Dr. Ali Saad,College of applied medical Science

Dept of Biomed. Technology

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Introduction

Nuclear imaging: All procedures involving the detection of rays and image formation from the emissions of radiopharmaceuticals introduced into patients for diagnostic purposes.

Gamma Camera

• gamma cameras.

The most widely used gamma cameras are the so-called Anger cameras, in which a series of phototubes detects the light emissions of a large single crystal, covering the field of view of the camera.

SPECT imaging

• SPECT imaging systems have been devised, their cost and poor flexibility have resulted in single- or multiple-head gamma cameras which rotate around the patient, thereby acquiring the projections necessary for reconstruction of axial slices.

PET imaging

PET imaging, the most recent nuclear imaging method introduced into clinical practice is also based on ring detector systems, but recently, manufacturers also have started to fit dual-head camera systems with the coincident detection circuitry necessary for PET imaging.

Radiopharmaceutical,

• Substance consisting of a molecule to which a radionuclide is bound.

• Radionuclide,It is an isotope which is radioactive and thus

undergoes radioactive decay.• Isotopes are families of atomic elements which

have a fixed atomic number (number of protons) and a variable number of neutrons and thus of nucleons.

Radioactive decay

• There are several types of radioactive decay, classified as alpha decay, beta decay and gamma decay. Another type of decay is the so-called electron capture EC . Many radioactive isotopes, particularly heavy ones such as uranium, disintegrate by a series of radioactive decays until they have been transformed into stable atoms.

radioactive decay law

• an exponential decay. If we call the initial number of radioactive isotopes N0 and the number remaining after a time t, N(t), the decay law is given by

• Nt = N0 exp(-λt) • where λ is the radioactive decay constant. • The half life t is defined as the time during which the

number of radioactive nuclei decays to half its initial value:

• N(t)/N0 =exp(- λ t), hence • t = λ ln2

Gamma decay

• radioactive decay in which a nucleus emits a high-energy photon or gamma ray. In such a decay, a nucleus that has undergone another type of radioactive decay remains in an excited or metastable state for a prolonged time eventually relaxing back to the ground state by emitting the gamma ray.

Alpha decay

• radioactive decay process during which a radioactive nucleus also called an alpha emitter emits a helium He nucleus (alpha particle, alpha ray) consisting of two protons and two neutrons.

Beta decay

• type of radioactive decay in which a nucleus ejects a beta particle, either an electron or a positron. In a beta (-) decay a neutron gets converted into a proton and an electron. Hence, the atomic number of the nucleus increases by one, the number of nucleons stays constant and the electron leaves the nucleus as a beta (-) particle.

Gamma Camera

• imaging device used in nuclear scanning. By far the most widely used gamma camera was invented by H. Anger in the 1960s and thus is also frequently called the Anger camera.

Anger camera

Photons are selected by a collimator and produce light flashes which are detected by the photomultipliers. See text

Anger Camera (cont.)

Light flash producing different responses in the detecting photomultipliers

Collimator

• device made of a highly absorbing material such as lead which selects X- or gamma-rays along a particular direction.

. In nuclear imaging, they serve to suppress scatter but also to select a ray orientation

Parallel hole collimator, typically made of lead, with "honeycomb“-like structure. Note that the thickness of lead shielding between adjacent holes is minimal every 60.

Collimator

Photomultiplier tube

SPECT imaging

• Single photon emission (computed) tomography (SPECT or SPET): tomographic nuclear imaging technique producing cross-sectional images from gamma ray emitting radiopharmaceuticals

• SPECT data are acquired according to the original concept used in tomographic imaging

SPECT (cont)

• multiple views of the body part to be imaged are acquired by rotating the Anger camera detector head(s) around a craniocaudal axis.

Triple head SPECT camera

• In SPECT attenuation degrades the images. Thus, data of the head reconstructed without attenuation correction may show substantial artificial enhancement of the peripheral brain structures relative to the deep ones.

The simplest way to deal with this problem is to filter the data before reconstruction

Head SPECT image

SPECT image (technetium-99m HMPAO), showing a normal brain perfusion

Backprojection,

PET Imaging

• Positron Emission Tomography: is a tomographic nuclear imaging procedure, which uses positrons as radiolabels and positron - electron annihilation reaction-induced gamma rays to locate the radiolabels.

• The PET principle is as follows. A low dose of a radiopharmaceutical labelled with a positron emitter such as C-11, N-13, O-15 or F-18 is injected into the patient, who is scanned by the tomographic system.

• Scanning consists of either a dynamic series or a static image obtained after an interval during which the radiopharmaceutical enters the biochemical process of interest.

• The scanner detects the spatial and temporal distribution of the radiolabel by detecting gamma rays during the so-called emission scan.

PET Principle

PET principle showing annihilation reaction between positron and electron, production of two gamma rays and detection in coincidence detection system.

PET Principle• 1. the positron is emitted by a beta decay, • 2. it is slowed down to small speeds which are necessary

for the annihilation reaction between the positron and a shell electron of a neighbouring atoms to occur. The distance the positron travels (mean free path) depends on the energetics of the beta decay but is typically one or a few millimeters.

• 4. The annihilation reaction produces two 511 keV gamma rays which travel in almost exactly opposite directions (this is due to the conservation of energy and momentum laws).

• 5. The two gamma rays are detected by a coincidence counting detection system (see below).

• 6. After proper filtering the collected raw data sinograms are reconstructed into a cross-sectional image.

Annihilation reaction

Electron and a positron meet, annihilate and form two gamma rays

Coincidence counting

• Method of counting employing a coincidence circuit so that an event is recorded only if events are detected in two sensing devices simultaneously.

• Such counting methods may be used to reduce background noise if a radioisotope emits more than one detectable radiation event in coincidence.

• The requirement for a coincidence between two detectors eliminates background counts that occur in only one detector at a time.

Sinogram,

Sinogram,

Four sinograms of four transaxial PET sections through a patient's body