Planar scintigraphy produces two-dimensional Planar scintigraphy produces two-dimensional images of three dimensional objects. It is handicapped by the images of three dimensional objects. It is handicapped by the superposition of active and nonactive layers which restricts superposition of active and nonactive layers which restricts the accurate measurement of organ functions.the accurate measurement of organ functions.

Emission computed tomography (ECT) is based on the Emission computed tomography (ECT) is based on the production of multi cross sectional images of tissue function production of multi cross sectional images of tissue function which are used to produce by overlay three dimensional images.which are used to produce by overlay three dimensional images.

Two ECT techniques are currently available:Two ECT techniques are currently available:

• Single Photon Emission Tomography (SPECT) Single Photon Emission Tomography (SPECT) which involves the imaging of single 7-ray activity which involves the imaging of single 7-ray activity (typically from (typically from 9999TcTcmm).).

• Positron Emission Tomography (PET) which involves Positron Emission Tomography (PET) which involves the imaging of the 511 keV annihilation radiation the imaging of the 511 keV annihilation radiation originated in positron decay (typically from originated in positron decay (typically from 1818F)F)

Image planes are derived by using two Image planes are derived by using two different techniques:different techniques:

• longitudinal ECT (limited-angle technique), photons are longitudinal ECT (limited-angle technique), photons are detected within a limited angular range from several body detected within a limited angular range from several body sections simultaneously. The reconstructed image planes sections simultaneously. The reconstructed image planes are positioned parallel tothe detector plane.are positioned parallel tothe detector plane.

• transaxial ECT (transverse section technique), the transaxial ECT (transverse section technique), the detector moves by 360 ° around the body to sample detector moves by 360 ° around the body to sample photons from multiple body sections. The reconstructed photons from multiple body sections. The reconstructed image planes are perpendicular to the detector plane.image planes are perpendicular to the detector plane.

Longitudinal ECT Longitudinal ECT allows to view the allows to view the radioactivity from different angles (within a limited angle radioactivity from different angles (within a limited angle range) to obtain information about the depth of the range) to obtain information about the depth of the radiation source.radiation source.

This is done by using a This is done by using a rectilinear scanner system coupled rectilinear scanner system coupled with a highly focused collimator who with a highly focused collimator who creates a sharp image only from a creates a sharp image only from a particular plane at a depth defined particular plane at a depth defined by the angle range (focal point).by the angle range (focal point).

The multiplane tomographic The multiplane tomographic scanner represents an improved scanner represents an improved version which replaces the single version which replaces the single detector by a gamma camera. It also detector by a gamma camera. It also allows to adjust (by electronic allows to adjust (by electronic repositioning) the focal distance and repositioning) the focal distance and can therefore select different image can therefore select different image planes.planes.

As example are shown twelve images of the skeleton As example are shown twelve images of the skeleton recorded in longitudinal multiplane tomography technique.recorded in longitudinal multiplane tomography technique.

Each image represents a different plane along the body.Each image represents a different plane along the body.

Single-Photon Emission Computed Tomography Single-Photon Emission Computed Tomography (SPECT) (SPECT) with a rotating gamma camera (transaxial with a rotating gamma camera (transaxial ECT) ECT) allows multiple views of the three dimensional distribution of the allows multiple views of the three dimensional distribution of the radioactivity from different directions.radioactivity from different directions.

The gamma camera is The gamma camera is coupled to a parallel hole collimator (nocoupled to a parallel hole collimator (nofocusing) which allows to produce a 2D focusing) which allows to produce a 2D image (64x64) consisting of multiple image (64x64) consisting of multiple profiles (64), each profile represents a profiles (64), each profile represents a ID projection of the radioactivity in the ID projection of the radioactivity in the profile (distributed over 64 channels). profile (distributed over 64 channels).

Each point in the profile represents the sum of the activity Each point in the profile represents the sum of the activity along the line of sight:along the line of sight:

with € as detector efficiency and with € as detector efficiency and as solid angle as solid angle

The camera rotates either continuously or in fixed angle steps The camera rotates either continuously or in fixed angle steps and repeats the monitoring until the completion of the 360° turn. and repeats the monitoring until the completion of the 360° turn.

The three dimensional image is constructed by using similar The three dimensional image is constructed by using similar Fourier analysis techniques as designed for X-ray CT scanning.Fourier analysis techniques as designed for X-ray CT scanning.

Positron Emission Tomography (PET)Positron Emission Tomography (PET) operates by using operates by using at least two opposite to each other positioned rotateable detector.at least two opposite to each other positioned rotateable detector.

PET is based on the principle of detecting annihilation PET is based on the principle of detecting annihilation radiation with coincidence techniques.radiation with coincidence techniques.

The injected radionuclide must be a positron (The injected radionuclide must be a positron (++) emitter. The positron ) emitter. The positron annihilates after about 1mm path length (depending on density of tissue annihilates after about 1mm path length (depending on density of tissue material and on the energy of the positron) and emits two 511 keV photons in material and on the energy of the positron) and emits two 511 keV photons in opposite directions.opposite directions.

Detection of both photons in coincidence Detection of both photons in coincidence defines a line along which the annihilation event has defines a line along which the annihilation event has taken place. The position of the radionuclide is within taken place. The position of the radionuclide is within 1mm distance. This distance as well as a slight 1mm distance. This distance as well as a slight deviation from the 180deviation from the 18000 emission of the two photons emission of the two photons limits the spatial resolution to about 1mm – 2mm.limits the spatial resolution to about 1mm – 2mm.

The use of annihilation radiation coincidence technique The use of annihilation radiation coincidence technique in PET improves the quality of image formation considerably in PET improves the quality of image formation considerably compared to collimator techniques used in SPECT.compared to collimator techniques used in SPECT.

In SPECT the intensity and the resolution of the In SPECT the intensity and the resolution of the signal signal degrades with increasing depth, due to attenuation through body degrades with increasing depth, due to attenuation through body tissue of increasing thickness tissue of increasing thickness dd,,

and due to the degradation of collimator resolution and due to the degradation of collimator resolution €€cc with with

increasing source collimator distance increasing source collimator distance zz::

with constant hole diameter with constant hole diameter d d and hole length and hole length L L for the collimatorfor the collimatorsystem. system.

In annihilation radiation coincidence measurements In annihilation radiation coincidence measurements the resolution remains essential constant with depth because the resolution remains essential constant with depth because of the uniformity of the geometric response defined by the of the uniformity of the geometric response defined by the straight line between the two detection processes.straight line between the two detection processes.

The intensity of the coincidence signal is defined by the The intensity of the coincidence signal is defined by the attenuation in body material from the point of annihilation at depth attenuation in body material from the point of annihilation at depth d d in in both directions,both directions,

with T being the thickness of the body along the line and with T being the thickness of the body along the line and (x)dx (x)dx . .

Therefore the intensity for the annihilation signal along the Therefore the intensity for the annihilation signal along the line is independent of the depth.line is independent of the depth.

The absolute count rate for coincidence events is determined The absolute count rate for coincidence events is determined by the count rate for true coincidences by the count rate for true coincidences IItruetrue (real coincidence events (real coincidence events

originating from one single annihilation process) and for random originating from one single annihilation process) and for random coincidences coincidences IIrandomrandom (fake coincidence events which occur when (fake coincidence events which occur when

accidentally each detector records an uncorrelated signal within a time accidentally each detector records an uncorrelated signal within a time window window ).).

The count rate for true coincidences from The count rate for true coincidences from II00

annihilation events is determined by the efficiency e and annihilation events is determined by the efficiency e and solid angle solid angle of each detector: of each detector:

for present PET machines the total efficiency for for present PET machines the total efficiency for coincidence measurement,coincidence measurement,

The random coincidence count rate is determined by the The random coincidence count rate is determined by the count rate for single events:count rate for single events:

in the two detectors and by the coincidence time window r: in the two detectors and by the coincidence time window r:

This yields a ratio of true to random coincidences:This yields a ratio of true to random coincidences:

which is independent of efficiencies and solid angle but only depends which is independent of efficiencies and solid angle but only depends on the intensity of the emitted annihilation radiation, and the on the intensity of the emitted annihilation radiation, and the coincidence time window:coincidence time window:

At these conditions the intensity of the radiation source inside the body At these conditions the intensity of the radiation source inside the body must be at least must be at least II00 101066 events/s to obtain a true to random ratio of unity. events/s to obtain a true to random ratio of unity.

This would require a source strength of This would require a source strength of at least 1 MBq inside the body. A 1 MBq source at least 1 MBq inside the body. A 1 MBq source inside the body (neglecting attenuation effects inside the body (neglecting attenuation effects corresponds to a random count rate of:corresponds to a random count rate of:

To improve the detection conditions and to separate the true To improve the detection conditions and to separate the true from the random coincidences the time structure of the signals can be from the random coincidences the time structure of the signals can be utilized by decreasing utilized by decreasing ..

Separation is based on the fact that random events come continuouslySeparation is based on the fact that random events come continuouslywhile true events come within a few nano seconds (speed of light c=3*10while true events come within a few nano seconds (speed of light c=3*101010 cm/s). cm/s).

Standard electronic Standard electronic is used to separate true is used to separate true coincidence events in the coincidence events in the sharp time peak from random sharp time peak from random events using fast timing events using fast timing conditions on the electronic conditions on the electronic signals.signals.

For For = 10 ns at a count rate of = 10 ns at a count rate of II00 = = 101066 s s-1-1 a true coincidence rate of: a true coincidence rate of:

is obtained at a random coincidence rate of:is obtained at a random coincidence rate of:

Modern PET systems are based on multicrystal designs.Modern PET systems are based on multicrystal designs.

Instead of a rotateable detector pair Instead of a rotateable detector pair the patient is surrounded by a ring of individual the patient is surrounded by a ring of individual small Nal scintillator detectors. Each detector is small Nal scintillator detectors. Each detector is coupled to his own individual phototube and is coupled to his own individual phototube and is in electronic coincidence with any of those in electronic coincidence with any of those detectors at the opposite site of the patient. detectors at the opposite site of the patient.

With such a device a multiple image can be obtained in one shot.With such a device a multiple image can be obtained in one shot.

This is particular important for monitoring This is particular important for monitoring physiological processes with short time scales physiological processes with short time scales ( t ( t 10 10-3-3 s). s).

Detecting coincidence events between one detector and two Detecting coincidence events between one detector and two neighboring detectors at the opposite site defines the spatial neighboring detectors at the opposite site defines the spatial resolution of the device to resolution of the device to 5mm. 5mm.