The typical radioisotope is a photon emitter.The typical radioisotope is a photon emitter.

The photon energy must be above EThe photon energy must be above E = 100 keV otherwise = 100 keV otherwise

the body tissue will cause attenuation of the emitted the body tissue will cause attenuation of the emitted --radiation.radiation.

Various types of detectors are being used in radioisotope imaging.Various types of detectors are being used in radioisotope imaging.

Most common ones …Most common ones …• scintillation detectorsscintillation detectors

• semiconductorsemiconductor detectorsdetectors

• multi-wire gas countersmulti-wire gas counters

Scintillation detectors Scintillation detectors are based on the conversion of are based on the conversion of radiation to visible light which is detected by phototubes (see radiation to visible light which is detected by phototubes (see section 7.4). They are characterized by large efficiency for low section 7.4). They are characterized by large efficiency for low energy energy -radiation which is limited by the thickness of the -radiation which is limited by the thickness of the crystal crystal d d 2 cm. This results in a optimum for the of efficiency 2 cm. This results in a optimum for the of efficiency attenuation conditions for attenuation conditions for energies E energies E 100 - 200 keV. 100 - 200 keV.

The scintillation detectors have a resolution of The scintillation detectors have a resolution of E/E E/E 10% - 15%. 10% - 15%.

The capability of energy resolution allows detector to The capability of energy resolution allows detector to distinguish between unscattered radiation and scattered distinguish between unscattered radiation and scattered radiation which have lost energy in the scattering process. radiation which have lost energy in the scattering process.

The photopeak corresponds to the full The photopeak corresponds to the full absorption of the absorption of the -photon in the crystal.-photon in the crystal.

Semiconductor detectors Semiconductor detectors have an efficiency of have an efficiency of E/E E/E 1% - 2% 1% - 2%

This kind of resolution allows much better separation from This kind of resolution allows much better separation from scattered scattered -radiation and therefore a much improved localization of the -radiation and therefore a much improved localization of the origin of the radiation.origin of the radiation.

However the large costs for the production of pure Germanium However the large costs for the production of pure Germanium crystals is prohibitive. crystals is prohibitive.

Nevertheless small test arrays have shown large potential!Nevertheless small test arrays have shown large potential!

Multiwire gas countersMultiwire gas counters are based on the ionization are based on the ionization effects of radiation in gas.effects of radiation in gas.

Despite their low efficiency for detecting Despite their low efficiency for detecting -radiation -radiation they have several advantages because they can cover fairly they have several advantages because they can cover fairly large areas with good resolution.large areas with good resolution.

This makes them usable for PET This makes them usable for PET applications which depend on many large detector applications which depend on many large detector arrays located at several angles around the patient. arrays located at several angles around the patient.

The production costs are very low compared to The production costs are very low compared to the costs for a scintillator system covering the same area.the costs for a scintillator system covering the same area.

The development of large area scintillator crystals led to theThe development of large area scintillator crystals led to thedevelopment of the development of the Gamma Camera Gamma Camera which allowed to scan over larger which allowed to scan over larger areas with high spatial resolution. areas with high spatial resolution.

The Gamma camera consists out of a The Gamma camera consists out of a large area NaI crystal with a lead collimator which large area NaI crystal with a lead collimator which allows only transmission of allows only transmission of radiation from a radiation from a particular point. The detector device can be moved particular point. The detector device can be moved (rectilinear or linear) to cover larger areas. (rectilinear or linear) to cover larger areas.

To obtain a reliable image the scanning speed To obtain a reliable image the scanning speed vvscanscan [cm/s] must be limited that the count rate per position [cm/s] must be limited that the count rate per position

IIpp [counts/s] (in the photopeak) is high enough to allow [counts/s] (in the photopeak) is high enough to allow

sufficient information density sufficient information density ID ID [counts/cm[counts/cm22] in the ] in the recorded image for each scan (with a line space recorded image for each scan (with a line space [cm]). [cm]).

The figure shows an image from a rectilinear The figure shows an image from a rectilinear scan showing cancer in the upper left lung using a scan showing cancer in the upper left lung using a 6767Ga citrate.Ga citrate.

The cancer is indicated by a high The cancer is indicated by a high -countrate -countrate TTpp which is translated into a high density of counts which is translated into a high density of counts ID.ID.

The thickness of the lead collimator, the orientation and size of The thickness of the lead collimator, the orientation and size of the holes as well as the distance between source and collimator define the holes as well as the distance between source and collimator define the resolution and sensitivity of the camera.the resolution and sensitivity of the camera.

For a parallel hole For a parallel hole collimator the spatial resolution collimator the spatial resolution RRcc is determined by the length of is determined by the length of

the holes L, the hole diameter d, the holes L, the hole diameter d, and the source to collimator and the source to collimator distance distance zz::

Decrease in Decrease in z z and and d d improves the spatial resolution, improves the spatial resolution, while the length of the collimator while the length of the collimator holes is less influential (typically holes is less influential (typically determined by the thickness of the determined by the thickness of the lead necessary to absorb the lead necessary to absorb the --radiation).radiation).

The geometrical efficiency The geometrical efficiency €€gg of the parallel hole collimator is of the parallel hole collimator is

given by the length of the holes L, given by the length of the holes L, the hole diameter d, the thickness of the hole diameter d, the thickness of the lead between the holes the lead between the holes ll, and a , and a constant constant K K which depends on the which depends on the shape of the holes (e.g. shape of the holes (e.g. KK=0.26 for =0.26 for hexagonal holes):hexagonal holes):

The geometrical The geometrical efficiency does not depend efficiency does not depend on the source-collimator on the source-collimator distance distance z. z.

However, if However, if z z >> >> L L and and d d >> >> t t there is a direct correlation there is a direct correlation between geometrical efficiency between geometrical efficiency and resolution:and resolution:

The figure shows system resolution The figure shows system resolution and system efficiency for different and system efficiency for different collimator systems:collimator systems:

LEAP low energy parallel hole collimatorLEAP low energy parallel hole collimator

LEHR low energy high resolution parallel hole collimatorLEHR low energy high resolution parallel hole collimator

Fan Beam diverging multi hole collimator Fan Beam diverging multi hole collimator

Pin Hole single hole collimatorPin Hole single hole collimator

LEUHR low energy ultrahigh resolution collimatorLEUHR low energy ultrahigh resolution collimator

The tables give The tables give typical examples for parallel typical examples for parallel hole collimator types and the hole collimator types and the corresponding resolution and corresponding resolution and sensitivity.sensitivity.

Detectors for the Gamma-Camera Detectors for the Gamma-Camera are in most cases large area scintillator are in most cases large area scintillator detectors, typically Nal-crystals with up to detectors, typically Nal-crystals with up to 50 cm diameter and 6-12 mm thickness 50 cm diameter and 6-12 mm thickness which emit a blue green light of which emit a blue green light of =415 nm.=415 nm.

A typical spectrum for A typical spectrum for -radiation of -radiation of EE 150 keV indicates a resolution of 10-12 %. 150 keV indicates a resolution of 10-12 %. Phototubes are closely packed and optically Phototubes are closely packed and optically coupled to the scintillator crystal to achieve coupled to the scintillator crystal to achieve high light collection efficiency. The typical high light collection efficiency. The typical arrangement gives an hexagonal array for 7, arrangement gives an hexagonal array for 7, 19, 37, up to 61phototubes.19, 37, up to 61phototubes.

To process the data only the photopeak information To process the data only the photopeak information is necessary. Information about scattered is necessary. Information about scattered -rays gives -rays gives unnecessary background intensity.unnecessary background intensity.

A single channel analyzer which filters out only A single channel analyzer which filters out only signals of the right energy is therefore used to clean the data signals of the right energy is therefore used to clean the data and create a logic signal (TTL-pulse).and create a logic signal (TTL-pulse).

To obtain a two-dimensional image the data are recorded To obtain a two-dimensional image the data are recorded as a function of the x-y position of the signal. This requires a as a function of the x-y position of the signal. This requires a resistor network to obtain positional information for the intensity resistor network to obtain positional information for the intensity distribution.distribution.

The x,y data is then digitized using ADCs and form a The x,y data is then digitized using ADCs and form a 2D-matrix which represents the image. The x,y data is gated 2D-matrix which represents the image. The x,y data is gated by the logic signal to remove background events originated by by the logic signal to remove background events originated by scattered scattered -rays from the event matrix.-rays from the event matrix.

As example for the spatial As example for the spatial distribution the figure shows a distribution the figure shows a image of a uniform radionuclide image of a uniform radionuclide distribution, obtained with a fully distribution, obtained with a fully operating gamma camera and (at operating gamma camera and (at the right hand side) an image the right hand side) an image obtained with a gamma camera obtained with a gamma camera with one defect photo tube. with one defect photo tube. Uniformity images are necessary Uniformity images are necessary to test the quality of the equipment.to test the quality of the equipment.

As examples are shown As examples are shown two images of the brain, based on two images of the brain, based on the blood distribution in the brain. the blood distribution in the brain. Left the brain of a patient suffering Left the brain of a patient suffering from stroke. The arrow indicates from stroke. The arrow indicates the lack of cerebral blood volume. the lack of cerebral blood volume. Right a patient suffering from Right a patient suffering from brain tumor.brain tumor.

Chapter 3 Dr. Funk

Skip chapter 4 StatisticsChapter 5 Production of RadionuclidesChapter 6 Interaction of Radiation with MatterChapter 7 Gas detectorsChapter 8 Scintillation and

Semi-conductor detectors

Saha textbook……………….