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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

Health & Medicine

increasing collimator

high resolution collimator

lowenergy collimator

spatial resolution of

shahid younas nuclear

clinical imaging

selection of parallel

given hole decreases