Nuclear imaging PET CT Imaging Medical Physics Nuclear Medicine

Lecture 10: Nuclear Imaging-II
Shahid Younas
NUCLEAR IMAGING
The Scintillation Camera- Design Performance

Gamma Camera –Collimators
Which of the following statements best describes the primary purpose of a
collimator on a gamma camera?
a. It prevents scattered photons from reaching the detector.
b. It prevents cosmic radiation from reaching the detector.
c. It stops pre-detector scattered photons.

 To allow photons from a given region of interest to strike the
detector and try to minimize the contribution of photons
originating from outside this region.

 Most scintillation cameras are provided with a selection of
parallel-hole collimators:
 “low-energy, high-sensitivity”
 “low-energy, all-purpose” (LEAP)
 “low-energy, high-resolution” (LEHR)

Parallel-hole collimator
If a low-energy collimator is (incorrectly) used with a high-energy
radionuclide the results would be:
a. A reduced camera sensitivity (counting efficiency).
b. A blurred image.
c. A reduced field of view.
d. Reduced image detail.

 Size of the image produced by parallel-hole collimator not
affected by distance of object from collimator.
 Spatial resolution degrades rapidly with increasing collimator-to-
object distance.

The principal disadvantage in using a high resolution collimator on a
gamma camera is that it has,
a. Limited field of view.
b. More distortion.
c. Less scatter rejection.
d. Lower sensitivity.

 R is an indicator of spatial
resolution.

 Collimator spatial resolution of multi-hole collimators is
determined by geometry of the holes .
 Spatial resolution improves as the,
 Diameters of the holes is reduced
 Lengths of the holes (thickness of the collimator) are increased

 Changing hole geometry to improve spatial resolution generally
reduces the collimator’s efficiency.
 Resultant compromise is single most significant limitation on
scintillation camera performance.

 Spatial resolution of parallel-hole collimator decreases linearly as
collimator-to-object distance increases.
 Also one of the most important factors limiting scintillation
camera performance.

Do you know why the efficiency of a parallel-hole collimator is
nearly constant over the collimator-to-object distances used for
clinical imaging.
Number of photons passing through given hole decreases with square
of distance, number of holes through which photons can pass
increases with square of distance.

 FWHM of LSF increased
linearly with distance.
 Total area under LSF
(photon fluence through
collimator) decreases
very little with s-c
distance.

Parallel-hole LEAP collimator
 LEAP collimators have holes with a large diameter.
 The sensitivity is relatively high as where the resolution is moderate.
(larger diameter holes allow more scattered photons).
 The average sensitivity of a LEAP is approx. 500 kcpm for a 1-uCi
source, and the resolution is 1.0cm at 10cm from the patent side of the
collimator.

Parallel-hole LEHR collimator
 LEHR collimators have higher resolution images than the LEAP.
 They have more holes that are both smaller and deeper.
 The sensitivity is approx. 185 kcpm for 1-uCi source, and the resolution
is higher with 0.65cm at 10cm from the patient side of the collimator.

Which is the ideal radionuclide to use with LEAP and LEHR?
99mTc

Parallel-hole Medium Energy Collimator
 Medium Energy Collimators are used for medium energy photons of
nuclides such as Krypton81, Gallium67, Indium111. These collimators
have thinner septa than LEAP and LEHR collimators.

Parallel-hole Medium Energy Collimator
 High Energy Collimators are used for Iodine131 and F-18FDG. These
collimators have thicker septa than LEAP and LEHR collimators in
order to reduce septal penetration by the higher energy photons.

Parallel-Slant -hole Collimator
 A variation of the Parallel hole which has all tunnels slanted at a specific
angle.
 It generates an oblique view for better visualization of an organ, which
view is (partly) blocked by other parts of the body.
 As an advantage, this collimator can be positioned close to the body for
the maximum gain in resolution.

Pinhole collimator
 Used to produce magnified views of small
objects, such as thyroid or hip joint.
 Consists of small (typically 3- to 5-mm
diameter) hole in a piece of lead or
tungsten.
 Mounted at apex of a leaded cone.

Pinhole collimator
 Produces a magnified image whose
orientation is reversed.
 Magnification decreases as object is
moved away from pinhole.

Pinhole collimator
 If the object is as far from the
pinhole as the pinhole is from the
crystal of the camera, the object is
not magnified.

Pinhole collimator
 This creates pitfall in clinical usage.
 A thyroid nodule deep in the mediastinum can appear to be
in the thyroid itself.
 Extensive use is in pediatric nuclear medicine.

Converging collimator
 In a Converging collimator the holes are
not parallel but focused toward the organ.
 The focal point is normally located in the
center of the field of view (FOV).

 Magnifies the image.
 Magnification increases as the object is
moved away from the collimator.

 Converging collimator is seldom used.
 Imaging characteristics are superior, in theory, to the parallel-
hole collimators but in practice owing to,
 Decreasing FOV and varying magnification with distance

Diverging collimator
 Many holes, all aimed a focal point behind
the camera.
 Produces a minified image.
 Amount of mini-fication increases as
object is moved away from the collimator

 May be used to image a large portion of a patient on a small (25-cm
diameter) or standard (30-cm diameter) FOV camera.
 You can perform lung study using a mobile gamma camera in
intensive care unit.

 Compared to a diverging collimator, a converging collimator will
produce:
a. A increase in sensitivity when distance is increased.
b. Better image detail.
c. A reduced FOV as distance is increased.

What collimator will you get if you reverse a diverging collimator on
a camera?
Converging

 Its seldom used because of,
 Inferior imaging characteristics to the parallel hole
 Large crystal sizes of modern cameras.

Fan Beam collimator
 They are designed for a rectangular
camera head or SPECT to image smaller
organs like the brain and heart.
 When viewed from one direction, the
holes are parallel. When viewed from the
other direction, the holes converge.
It’s a hybrid of parallel-hole
and converging collimator.

collimator
Image magnification is changed if you change,
A. Radionuclide
B. Imaging time
C. Type of collimator
D. Patient-collimator distance
E. PHA window level

Image formation
 Photons from each point in the patient are emitted isotropically.
 Some photons escape the patient without interaction, some scatter
within the patient before escaping, and some are absorbed in the
patient

Image formation
 Many of the escaping photons are not detected because they are
emitted in directions away from the detector.
 Only a tiny fraction of emitted photons has trajectories permitting
passage through the collimator holes.

Image formation
 Of those reaching the crystal, some are absorbed in the crystal,
some scatter from the crystal, and some pass through the crystal
without interaction

Image formation
 Relative probabilities
of these events
depends on the
energies of the
photons and the
thickness of the crystal

Image formation
 Of those photons absorbed in the crystal, some are absorbed by a
single photoelectric absorption, others undergo one or more
Compton scatters before a photoelectric absorption.

Image formation
Is it possible for two photons to simultaneously interact with the
crystal?

Image formation
 If the energy signal (Z) from the coincident interactions is within
the energy window of the energy discrimination circuit, the result
will be a single count mis-positioned in the image.
 Fraction of simultaneous interactions increases with the interaction
rate of the camera.

Image formation
 Spatial resolution and image contrast reduced by:
 Interactions in crystal of photons that have been scattered in the
patient.
 Photons that have penetrated the collimator septa.
 Photons that undergo one or more scatters in the crystal.
 Coincident interactions.

Image formation
Energy discrimination circuits reduce this by rejecting photons that
scatter in the patient or result in coincident interactions.

Image formation
If the PHA window on a gamma camera is (incorrectly) set below the
photo-peak energy you would expect to get:
 Decreased sensitivity.
 An image of primarily scattered radiation.
 Decreased lesion contrast.

Image formation
 Pulse Height Analyzer (PHA) reduce this loss of resolution and
contrast by rejecting scattered photons.
 However, low energy photons can scatter through large angles with
only a small energy loss.

Image formation
 140 keV photon scattering 45 degree will only lose 7.4% of its
energy.
 This causes a wide photo-peak because PHA accepts most of the
significant fraction of the scattered photons and coincident
interactions.

Image formation
The full width at half maximum (FWHM) of a photo-peak is a
measure of,
a. PHA window setting.
b. Camera sensitivity.
c. Field of view.
d. Detector energy resolution.

Image formation
Performance

Nuclear imaging PET CT Imaging Medical Physics Nuclear Medicine

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