Transcript NImag

Nuclear imaging
Instrumentation part
Prepared by : . Dr. Ali Saad,
College of applied medical Science
Dept of Biomed. Technology
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
Collimator
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.
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
(technetium99m 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 reactioninduced 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