Radionuclide Imaging

MII 3073

Detection of Nuclear Emission

Nuclear radiation detectors

Detectors that are commonly used in nuclear medicine: 1. Gas-filled detectors 2. Scintillation detectors 3. Semiconductor detectors 4. Film badge and Thermoluminescent dosimeters (TLD)

Gas-filled detectors: 1. Geiger-Mueller (GM) counter 2. Ionization chamber 3. Dose calibrator

Scintillation detectors: 1. Sodium iodide well counter 2. Single probe counting system 3. Dose calibrator

Gas-filled detectors

• Operational principle: measuring the ionization that radiation produces within the gas. Commonly for monitoring α and β radiations. Typical gases used are argon and helium.

• The central electrode is an anode, that has been insulated from the chamber walls and the cathode. A voltage is applied to the anode and the chamber walls.

• As a charged particle passes through, it ionizes some of the gas (air). The positive anode attracts the electrons, or negative particles. The detector wall, or cathode, attracts the positive charges.

• This movement of ions/charges is an electric current, which can be detected by a sensitive meter.

• The current between the electrodes is a measure of the amount of incoming radiation.

Gas-filled detectors

Gas-filled detectors

The amount of current produced depends on several factors: 1. The applied voltage between the two electrodes

2. Distance between the two electrodes

3. Type of gas

4. Volume, pressure and temperature of the gas

5. Geometry and shape of the electrodes

Typically for a gas-filled detector, the amount of the current produced by a single radiation is a function of the applied voltage.

Their relationship can be divided to 5 distinct regions.

Gas-filled detectors

Gas-filled detectors

Region I: recombination The voltage is low, some ion pairs are still able to recombine and

form neutral atoms or molecules. Incomplete collection of primary ion pairs by the electrodes.

As the voltage increases, more primary ion pairs are collected and more current flows.

Region II: ionization plateau The voltage is sufficiently high to attract all primary ion pairs.

Region III: proportional The higher voltage is able to attract all primary ion pairs and

sufficient to provide energy to some primary ion pairs for producing secondary ion pairs through collisions with neutral atoms and molecules of the gas (gas amplification).

The amount of secondary ion pairs produced depends on the energy acquired by primary ion pairs.

The amount of current produced by a radiation increases with voltage increasing.

Gas-filled detectors

• Region IV: Geiger Muller – As the voltage is increased, a point is reached at which most of

the gas within the detector is massively involved in the multiple, successive ionizations (no more gas amplification).

– The pulse of current is larger but becomes independent of number of primary ion pairs produced.

• Region V: Continuous discharge – The voltage is so high that radiation is not necessary to produce

discharge.

– Under this high electric field, the electrons are pulled out form the atomic shells, the atoms and molecules become ionized and a discharge may be established even without radiation (spontaneous and continuous ionization).

– This interaction stops only when voltage is lowered.

Dose calibrator

• A dose calibrator (activity meter) consists of:

– a cylindrically shaped, gas filled sealed chamber with a well,

– high voltage supply applied to electrodes,

– specific energy settings for different radionuclides,

– an activity readout (e.g. in MBq, GBq, etc).

• How to use dose calibrator?

1. Turn on the main power and wait for any self checks or

warm-up up to complete.

2. Place the syringe or vial holder in the detector well.

3. Select appropriate (nuclide, energy) settings.

4. Zero the dose calibrator.

5. Measure the activity of the radionuclide in the syringe or vial.

6. Read the activity from the display console and record.

Scintillation detectors • Scintillators are materials that emit visible or UV light

following the interaction of ionizing radiation with material.

• The most widely used crystals are made of sodium iodide

(NaI); clear glass-like structure, fragile and sealed in an air-

tight aluminum container.

• NaI crystals are doped with small amounts of stable

thallium (Tl); improve response to gamma ray photons.

• When an incoming x or gamma ray hits the scintillation

detector, it will interact with an electron (from the valence

band) in the crystal, by either a Compton or PE process

(energy transfer).

• Each of these energetic electrons distributes its energy

among electrons in the crystals, leaving them in ionized

and excited states.

Scintillation detectors

• These electrons may move to higher energy levels,

known as the conduction band, until they fall into

certain impurity centers, which act as energy traps.

• These traps are produced by the addition of chemical

impurities into the crystal at the time of manufacture,

called activators.

• For NaI, small amounts of thallium produce the

trapping centers; (thallium-activated sodium iodide).

• For returning to the original state, the trapped electron

may give up its energy in the form of a light photon.

• This light photon will then be detected and converted

into electrical signal by photomultiplier tube (PMT).

Scintillation detectors • The desirable properties of a scintillator are:

1. The conversion efficiency: the fraction of deposited energy

that is converted into light should be high. (The conversion

efficiency should not be confused with detection efficiency)

2. For many applications, the decay times of excited states should

be short. (Light is emitted promptly after an interaction).

3. The material should be transparent to own emissions. (Most

emitted light escapes reabsorption).

4. The frequency spectrum (color) of emitted light should match

the spectral sensitivity of the light receptor (PMT, photodiode or

film).

5. If used for x- and gamma-ray detection, the attenuation

coefficient (µ) should be large, so that the scintillation detectors

have high detection efficiency. (Materials with large atomic

numbers and high densities have large attenuation coefficients).

6. The material should be rough, unaffected by moisture and

inexpensive to manufacture.

Photomultiplier tube (PMT)

• The amount of light produced in Nai(Tl) crystals or any other

scintillator is very small in volume.

• PMT is a light sensitive device that converts light into

measurable electronic pulses.

• It consists of a photocathode facing the window through

which light enters, a series of metallic electrodes known as

dynodes arranged in special geometry and pattern, and an

anode. All of these are enclosed in vacuum in a glass tube.

• Photocathode is a clear photosensitive glass surface that

has been coupled with a light-conductive transparent gel to

the surface of the crystal.

• The transparent gel has the same refractive index as the

crystal and the PMT window.

PMT

• When the light photon hits the photocathode, it produces an

electron of low energy through PE interaction; called

photoelectron.

• This photoelectron is accelerated by a potential difference ( range

of 50-100 V) between the emitting surface and the 1st dynode.

• Upon collision with the dynode, the electron acquires sufficient

kinetic energy to create a number of secondary electrons.

• These secondary electrons are then accelerated toward a 2nd

dynode, with a similar electron multiplication.

• Eventually, at the last dynode (generally 10th) the total electron

gain of about 105-108 is produced.

• These electrons generate a current pulse of a few microamperes

in amplitude and less than a microsecond in duration at the

anode.

Visible

light

photon

Photo-cathode

Dynode

Anode

Photo-electron

Sodium iodide well counter

• Well counters are common in nuclear medicine laboratories,

for performing in vitro studies as well as QC and QC

procedures.

• Many NaI well counters are designed for counting radioactive

samples in standard test tubes.

• Generally, there is a solid cylindrical NaI crystal with a

cylindrical well cut into the crystal, into which the test tube is

placed.

• PMT is optically coupled to the crystal base. Radiation from

the sample interacts with the crystal and is detected by the

PMT, which feeds into a scalar.

• The scalar readout directly reflects the amount of radioactivity

in the sample and is usually recorded in counts for the period

of measurement.

Sodium iodide well counter

Single probe counting system

(thyroid probe) • A thyroid probe has a single NaI crystal, a PMT at the end, and a

single-hole collimator.

• Single probe counting systems using only 1 crystalline detector

are useful for measuring not only thyroid uptake of radioactive

iodine but also cardiac output.

• The probe used for thyroid counting is actually similar to the

standard well counter, although it does not have the central hole

in the NaI crystal.

• The typical crystal is 5 cm in diameter and 5 cm in thickness, with

a cone-shaped collimator. Again, a PMT is located at the crystal

base.

• When this probe is used, it is important for quantitative

consistency to maintain a fixed distance from the object being

measured to the face of the crystal and to eliminate all

extraneous sources of background radiation.

Thyroid probe

Thyroid probe

Semiconductor detectors • In metals, the valence band is partially filled. However, in

semiconductors and insulators, the valence band is

completely filled and the conduction band is completely

empty.

• The energy gap between the valence and the conduction

bands of semiconductors is smaller than that of insulators.

Thus, in semiconductors, electrons (in valence band) can be

easily excited to the conduction band.

• When a photon enters a semiconductor, the energy of the

photon is absorbed (PE, Compton or PP).

• The electrons produced by the primary interaction of

photons with the semiconductor will transfer their energy to

the valence electrons, thus elevating them into the

conduction band.

Semiconductor detectors • This leaves equal numbers of holes in the valence band.

• These holes act as positively charged particles.

• If a voltage is applied across the semiconductor, the

electrons in the conduction band will move towards the

positive electrode and the holes in the valence band are

for the negative electrode.

• Since the number of electron-hole pairs produced is

proportional to the energy of the incident photon, the

collection of charges on the respective electrodes results in

a pulse whose height is proportional to the photon energy.

• This pulse can be amplified and energy-discriminated for

counting purposes.

Film badge

• External radiation monitoring system.

• Film badge is the most common and economical, although

not the most accurate.

• It consists of a small film enclosed in a plastic container

with 4 windows of the covered with different radiation

filters to identify the nature and energy exposing radiation.

• When the badge is exposed to ionizing radiation, the film

emulsion darkens in proportion to the degree of radiation

exposure received. The resultant optical density can be

measured with a densitometer and calibrated to the degree

of radiation exposure received.

• Film badge is capable of measuring exposures ranging

from 0.1-20 mSv. The film is normally changed each

month.

Thermoluminescent Dosimeter

(TLD) • Contains small chips of a thermoluminescent material,

usually lithium fluoride (LiF).

• When exposed to radiation, a portion of the absorbed

energy is stored in the crystal structure of the LiF chips in

metastable states.

• If the LiF chips are heated, the absorbed energy is

released as visible light.

• The heating and measurement of LiF chips are carried out

in a device called a reader.

• The amount of measured light is proportional to the

absorbed radiation dose.

Collimators

• The collimator is made of perforated or

folded lead and is interposed between the

patient and the scintillation crystal.

• It allows the gamma camera to accurately

localize the radionuclide in the patient’s

body.

• Collimators perform this function by

absorbing and stopping most radiation

except that arriving perpendicular to the

detector face.

• Collimator is the ‘rate limiting’ step in the

imaging chain of gamma camera

technology.

• Four types of collimators are commonly

used with the gamma camera:

Collimators • The collimator is made of perforated or folded lead and is

interposed between the patient and the scintillation crystal.

• Nuclides emit gamma ray photons in all directions. The

collimator allows only those photons travelling directly along

the long axis of each hole to reach the crystal. Photons

emitted in any other direction are absorbed by the septa

between the holes.

• Without a collimator in front of the crystal, the image would

be indistinct.

• Collimator is the ‘rate limiting’ step in the imaging chain of

gamma camera technology. Thus, by appropriate choice of

collimator, it is possible to magnify of minify images and to

select between imaging quality (resolution) and imaging

speed (sensitivity).

Collimators

• Four types of collimators are commonly used with the

gamma camera:

1. Parallel-hole

2. Pinhole

3. Converging

4. Diverging

• A parallel-hole collimator is made of a large number

(many thousands) of small holes in a lead disc. The

diameter of the lead disc is the same as the scintillation

crystal used.

• Thickness of the lead disc and diameter of the holes

depend on the desired spatial resolution and sensitivity

of the collimators.

Collimators

• Pinhole collimator consists of a single hole, usually 2-4

mm in diameter.

• The image is projected upside down and reversed right to

left at the crystal. However, it is usually corrected

electronically on the viewing screen.

• A pinhole collimator generates magnified images of a

small organ like the thyroid or a joint.

• In converging collimator, the holes are angled inward,

toward the organ/patient.

• All holes focus at an axial point, outside the collimator.

Therefore, the organ appears larger at the face of the

crystal.

• A converging collimator may be used for examination of

small areas.

Collimators

• A diverging collimator, has holes and septa that begin to

diverge from the crystal face.

• Generally, use of a diverging collimator increases the

imaged are by about 30% over that obtained with a parallel-

hole. However, the image itself is slightly minified.

• Diverging collimator is used particularly on cameras with

small crystal faces to image large organs, such as the

lungs.

• Commercially, the collimators are also classified according

to their spatial resolution or sensitivity as high sensitivity

(for dynamic studies), all purpose (for most clinical

applications), or high spatial resolution (for fine details)

collimators and according to the energies of rays low (0-

200 keV), medium (200-400 keV) and high (400-600 keV)

collimators.

Effect of septal length on collimator sensitivity and resolution

Effect of different source-to-camera distances