Transcript Radioisotope imaging equipment

The typical radioisotope is a photon emitter.
The photon energy must be above E = 100 keV otherwise
the body tissue will cause attenuation of the emitted radiation.
Various types of detectors are being used in radioisotope imaging.
Most common ones …
• scintillation detectors
• semiconductor detectors
• multi-wire gas counters
Scintillation detectors are based on the conversion of
radiation to visible light which is detected by phototubes (see
section 7.4). They are characterized by large efficiency for low
energy -radiation which is limited by the thickness of the crystal
d  2 cm. This results in a optimum for the of efficiency
attenuation conditions for  energies E  100 - 200 keV.
The scintillation detectors have a resolution of E/E  10% - 15%.
The capability of energy resolution allows detector to
distinguish between unscattered radiation and scattered
radiation which have lost energy in the scattering process.
The photopeak corresponds to the full
absorption of the -photon in the crystal.
Semiconductor detectors have an efficiency of E/E  1% - 2%
This kind of resolution allows much better separation from
scattered -radiation and therefore a much improved localization of the
origin of the radiation.
However the large costs for the production of pure Germanium
crystals is prohibitive.
Nevertheless small test arrays have shown large potential!
Multiwire gas counters are based on the ionization
effects of radiation in gas.
Despite their low efficiency for detecting -radiation
they have several advantages because they can cover fairly
large areas with good resolution.
This makes them usable for PET
applications which depend on many large detector
arrays located at several angles around the patient.
The production costs are very low compared to
the costs for a scintillator system covering the same area.
The development of large area scintillator crystals led to the
development of the Gamma Camera which allowed to scan over larger
areas with high spatial resolution.
The Gamma camera consists out of a
large area NaI crystal with a lead collimator which
allows only transmission of  radiation from a
particular point. The detector device can be moved
(rectilinear or linear) to cover larger areas.
To obtain a reliable image the scanning speed
vscan [cm/s] must be limited that the count rate per
position Ip [counts/s] (in the photopeak) is high enough to
allow sufficient information density ID [counts/cm2] in the
recorded image for each scan (with a line space  [cm]).
The figure shows an image from a rectilinear
scan showing cancer in the upper left lung using a
67Ga citrate.
The cancer is indicated by a high -countrate
Tp which is translated into a high density of counts ID.
The thickness of the lead collimator, the orientation and size of
the holes as well as the distance between source and collimator define
the resolution and sensitivity of the camera.
Decrease in z and d
improves the spatial resolution,
while the length of the collimator
holes is less influential (typically
determined by the thickness of
the lead necessary to absorb the
-radiation).
For a parallel hole
collimator the spatial resolution
Rc is determined by the length of
the holes L, the hole diameter d,
and the source to collimator
distance z:
The geometrical efficiency
€g of the parallel hole collimator is
given by the length of the holes L,
the hole diameter d, the thickness of
the lead between the holes l, and a
constant K which depends on the
shape of the holes (e.g. K=0.26 for
hexagonal holes):
The geometrical
efficiency does not depend
on the source-collimator
distance z.
However, if z >> L and d >> t
there is a direct correlation
between geometrical efficiency
and resolution:
The figure shows system resolution and system efficiency
for different collimator systems:
LEAP
low energy parallel hole collimator
LEHR
low energy high resolution parallel hole collimator
Fan Beam diverging multi hole collimator
Pin Hole single hole collimator
LEUHR
low energy ultrahigh resolution collimator
The tables give
typical examples for parallel
hole collimator types and the
corresponding resolution and
sensitivity.
Detectors for the Gamma-Camera
are in most cases large area scintillator
detectors, typically Nal-crystals with up to
50 cm diameter and 6-12 mm thickness
which emit a blue green light of =415 nm.
A typical spectrum for -radiation of
E 150 keV indicates a resolution of 10-12 %.
Phototubes are closely packed and optically
coupled to the scintillator crystal to achieve
high light collection efficiency. The typical
arrangement gives an hexagonal array for 7,
19, 37, up to 61phototubes.
To process the data only the photopeak information
is necessary. Information about scattered -rays gives
unnecessary background intensity.
A single channel analyzer which filters out only
signals of the right energy is therefore used to clean the data
and create a logic signal (TTL-pulse).
To obtain a two-dimensional image the data are recorded
as a function of the x-y position of the signal. This requires a
resistor network to obtain positional information for the intensity
distribution.
The x,y data is then digitized using ADCs and form a
2D-matrix which represents the image. The x,y data is gated
by the logic signal to remove background events originated by
scattered -rays from the event matrix.
As example for the spatial
distribution the figure shows a
image of a uniform radionuclide
distribution, obtained with a fully
operating gamma camera and (at
the right hand side) an image
obtained with a gamma camera
with one defect photo tube.
Uniformity images are necessary
to test the quality of the
equipment.
As examples are shown
two images of the brain, based on
the blood distribution in the brain.
Left the brain of a patient suffering
from stroke. The arrow indicates
the lack of cerebral blood volume.
Right a patient suffering from
brain tumor.
Saha textbook……………….
Chapter 3
Dr. Funk
Skip chapter 4
Statistics
Chapter 5
Production of Radionuclides
Chapter 6
Interaction of Radiation with Matter
Chapter 7
Gas detectors
Chapter 8
Scintillation and
Semi-conductor detectors