Nuclear Medicine Resources Manual

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Geiger counters have some disadvantages; in particular their detection

efficiency decreases rapidly below around 70 keV, although some models can

be used at lower energies. In nuclear medicine, most of the photon and beta

energies used are above 60 keV, so the energy limitation is not a major

problem. The detector should also be ‘quenched’ to allow use at higher dose

rates.

The most versatile and useful Geiger counter for nuclear medicine use

should have the following features:

—Have a thin window (but with protection against accidental damage) for

particle detection;

—Have a window shield for discrimination against particles;

—Produce an audible signal of radiation events (for contamination

detection);

—Be calibrated in dose rate units allowing a wide measurement range, say

10 Sv/h to 10 mSv/h;

—Be energy compensated to give the lowest possible detectable energy;

—Be powered by easily available batteries, or have an inbuilt battery

charger.

A detachable probe may be of use for contamination measurements, but

inbuilt probes will suffice in most cases. Some Geiger counters are minia

turized, for use as personal dosimeters. They may have a digital readout of total

dose and dose rate. Some models have an audible indication of dose rate and/or

an alarm which sounds at predetermined steps of total dose. These devices are

very useful where staff may be exposed to high levels of radiation, for example,

when using

131

I for therapy.

(b) Gaseous detectors — ionization chambers

Ionization chambers are only used to measure radiation exposure rates,

but are very sensitive and accurate and can be used at low photon energies.

They are, however, more bulky and expensive than Geiger counters and may

not be as rugged. They are usually used where more accurate measurements

are needed. While they (like Geiger counters) measure exposure, they can be

calibrated to measure absorbed dose or equivalent dose.

The detector chamber may be less than 1 cm

in size or as large as

1500

. The chamber volume determines the sensitivity. They may operate at

ambient pressure or be pressurized for higher sensitivity and stability.

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If an ionization chamber survey meter is required for radiation safety use

in nuclear medicine, it should:

—Have a chamber volume of around 100 cm

;

—Be calibrated in the SI unit of equivalent dose (deep dose, H*10);

—Have a usable range of around 10 mSv/h to 10 mSv/h.

The use of ionization chambers in larger nuclear medicine departments is

justifiable, and they may also be useful in smaller facilities.

The proportional counter, a special type of ionization chamber, produces

a pulse proportional to the energy of the detected photon or particle, and does

not produce an average current. Proportional counters are used for sensitive

radiation detection where energy discrimination is important. Their main

radiation safety use is in contamination detectors, which can be set for a

particular radionuclide.

(d) Scintillation detectors

The sodium iodide scintillation detectors used in radiation safety have

crystals of the order of 2–5 cm diameter, and are between a few millimetres and

a few centimetres long.

Scintillation detectors are used for in vitro sample counting, for probes

designed for organ counting or surgical exploration and for general counting.

Owing to their energy discrimination capability, they are used in some larger

nuclear medicine facilities for spectroscopic investigations to identify radionu

clides. These instruments have a very high sensitivity and provide a reading in

counts per minute or counts per second.

(e) Semiconductor detectors

This class of detector can be thought of as solid state ionization chambers.

Until recently, the most common type was the germanium detector — an

expensive and complicated device used for high resolution photon

spectroscopy, and rarely used in nuclear medicine.

There are now many miniaturized solid state detectors available as

personal dosimeters, with the ability to provide integrated dose, dose rate and

dose or dose rate alarms. These devices are affordable and are recommended in

situations where staff may be involved in higher radiation level work.

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Personal semiconductor dosimeters should:

—Provide an easily readable digital display of integrated absorbed or

equivalent dose that can be reset;

—Provide an audible alarm at user selectable integrated dose levels, and/ or

provide an audible indication of instantaneous dose rate;

—Be powered by an easily obtainable battery.

(f) Pocket electrometer dosimeters

Also known as quartz fibre electrometers (QFEs), these devices have

been available for many years. They allow direct reading of integrated

exposure (or calibrated as absorbed dose), and are simple and cheap.

Recharging is performed with a small battery powered charger. They are

available in full scale ranges from 1 mSv to 100 Sv.

QFEs are still a simple and inexpensive means of instantaneous dose

assessment, but are not suitable for routine personal dosimetry in nuclear

medicine since a low dose rate is usually encountered.

(g) Film badges

The most common type of personal dosimeter for many years, the film

badge, still remains in widespread use. The film badge uses a special type of

photographic film in a special holder fitted with filters of various types to allow

discrimination between beta and photon (and in some cases neutron) radiation,

at various energy levels. As a result, the wearer’s radiation exposure can be

estimated as an effective dose.

The sensitivity of a film badge varies according to the supplier, but the

lower limit of readable dose is of the order of 200 mSv. Cheap, and widely

available, film badges still remain an effective means of assessing doses to staff.

(h) Thermoluminescent dosimeters

Thermoluminescent detectors (TLDs) operate on the principle that

certain materials, such as lithium fluoride, ‘store’ energy from incident

radiation more or less indefinitely and that the stored energy is released under

heating, in the form of a weak emission of light. The amount of light released is

in proportion to the stored radiation energy. In many personnel monitoring

services, TLDs have replaced film badges.

TLDs are more sensitive than film badges (around 10 mSv lower limit),

capable of being reused many times, more rugged and versatile, but more

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expensive. It is also possible to obtain TLD monitors for the fingers or hands,

which can be valuable for monitoring hot laboratory staff who handle large

quantities of radioactive material.

4.5.1.2. Radiation monitors

A nuclear medicine department needs to have, or have immediate access

to, at least one radiation monitor. Instruments may be designed to measure

dose rate (in mSv/h), integrated dose (mSv) or contamination level (Bq/cm

Dose rate measurement is necessary to ensure that levels of radiation in

working environments are within the limits required by legislation and also to

confirm dose rates from packages that may be despatched from the radio

pharmacy. Suitable monitors may be based on ionization chambers, Geiger–

Müller counters, scintillation detectors or proportional counters. The choice of

instrument is governed by the nature and level of radiation anticipated in the

environment. The suitabilities of the various types of dose rate monitor are

given in Table 4.2.

Contamination monitors are necessary for routine use to detect any

spillage of radioactivity that may have occurred. In view of the fact that gamma

emitting radionuclides are most commonly used, a monitor based on a scintil

lation detector will be suitable, although in situations where beta emitters are

used, a Geiger–Müller counter is also valuable.

Quality assurance

Any device used for radiation detection must be regularly calibrated, with

the calibration traceable to a recognized primary or secondary standard. Any

of the types of radiation instruments mentioned above can drift over time to

become inaccurate. Even film badges and TLD badges are calibrated. It is not

TABLE 4.2. COMPARISON OF DIFFERENT TYPES OF DETECTOR

Equipment Energy response Sensitivity Useful for

Ionization chamber,

betas 100–500 keV

Flat over a wide range Poor Photons,

betas 100–500 keV

Geiger–Müller counters Flat over a limited range Good Gammas >40 keV

Proportional counters Flat over a limited range Good Gammas >30 keV

Scintillation detectors Energy dependent Very good Gammas,

betas >500 keV

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always easy to obtain a calibration, however; sometimes an acceptable

alternative is to purchase a calibrated radiation source, and in turn to use this to

check the instrument. As far as contamination monitors are concerned, the

calibration source must be spread over a known area, and different radionu

clides should be used.

It is important to distinguish between calibration and consistency testing.

Calibration is performed to ensure that the instrument readings are as accurate

as possible for the type of instrument concerned. Consistency testing can be

performed on a calibrated instrument to check for drift. All that is required is a

radiation source that has a reliable output (

137

Cs for example) and a repro-

ducible testing geometry.

4.5.1.3. Radionuclide calibrators (dose calibrators)

(a) Choice of instrument

Dose calibrators are a special type of ionization chamber for

measurement of radionuclide activity in hot laboratories or radiopharmacies.

Such a piece of equipment is essential in order to measure the activities of

radiopharmaceuticals received, of generator eluates and kits prepared from

them and also of syringes containing individual patient injections.

A range of designs is available, but measurement is based on ionization

of a gas by the radioactive sample, which produces a proportional electric

current. These chambers are usually pressurized and have a large detector

volume. They are calibrated for a number of individual radionuclides so that

the activity can be measured directly. Alternative models designed for the

measurement of beta emitting radionuclides are also available but are less

likely to be required.

The instrument chosen will be influenced by the range, geometry and

activities of the nuclides handled. Recent publications that deal with design,

calibration and use of calibrators should be consulted for further information.

(b) Quality assurance

The aspects of the calibrator that need to be assessed for acceptance are

considerably more extensive than those which should be assessed on a daily

basis once the calibrator has been brought into use. The important features are

listed in Table 4.3. Radionuclide calibrators should be tested daily. Consistency

testing is useful, particularly when calibration is not easily performed, or can be

done only occasionally. The department will need a long lived comparison

source such as

137

Cs (half-life 30 years). Alternatively

Co can be used since its

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gamma energy is similar to that of

99m

Tc. However, the half-life of

Co of

271

days means the source will need to be replaced every few years.

High voltage checks are necessary to ensure the supply to the ionization

chamber is adequate. Background measurements and adjustment to zero

ensure that any unnoticed radioactive contamination of the calibrator can be

detected so that artefacts can be eliminated from measurements.

The accuracy of the instrument should be tested with a reference source

of activity whose activity has been certified by an appropriate authority. This

same source can be used to test the precision of the instrument by performing

at least ten repeated measurements of its activity.

The constancy of response can be determined using the comparison

source to ensure that the calibration factors used are appropriate and do not

vary. The value of a reading on the individual settings should decline according

to the half-life of the radionuclide in the comparison source.

The linearity of the instrument should be checked by measuring a source

99m

Tc whose initial activity is as high as possible, over a period of several

half-lives, in order to check that the response of the instrument is linear over

the range of giga- to kilobecquerels.

4.5.1.4. Minimum recommended monitoring equipment

From all the monitoring devices described, the minimum requirements

for a nuclear medicine department are given below. It should be recalled that in

TABLE 4.3. RADIONUCLIDE CALIBRATOR PERFORMANCE TESTING

Test Acceptance Daily Monthly Annually

High voltage X

X X X

Background X X X X

Zero adjustment X X X X

Accuracy X X

Precision X X X

Geometry X X X X

Constancy X

(all settings)

(nuclides in use)

(all settings)

Linearity X X

Electrical safety X X

X indicates ‘Yes’ and a blank ‘Not required’.

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most hospitals the nuclear medicine department is often called upon to deal

with radiation accidents or incidents involving radionuclides (or even radiation

sources) that may occur outside the department. Monitoring equipment is

therefore a resource for the whole institution.

(a) Small departments

Survey meter/contamination monitor

A Geiger counter is required with a removable beta shield, audible count

rate signal, dose rate calibration and a range of around 10 mSv/h to 10 mSv/h.

Radionuclide calibrator

A simple device is required with a digital readout and preset settings for

common radionuclides.

Scintillation counter

A simple counter with a single sample well for in vitro tests is required.

Personnel monitors

Film or TLD badges (usually from a national radiation regulatory body)

are required. Finger badges are recommended for staff working in a hot

laboratory.

(b) Medium sized departments (additional items)

Medium sized departments should have the same equipment as small

departments plus the following items:

Survey meter, contamination monitor

A portable ionization chamber meter with a flat energy response from

about 30 keV and a dose rate range as for a small department meter is required.

An integrating mode option is suggested.

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Radionuclide calibrator

A simple device with a digital readout and preset settings for common

radionuclides, as well as a variable user defined setting, is required — a

dedicated label printer is desirable.

Personnel monitors

At least one direct reading dosimeter (QFE or electronic), with a

measurement range of 0–1 mSv, is required.

Large departments should have the same equipment as for a medium

department plus the following items:

Survey meter, contamination monitor

A dedicated contamination monitor, ideally calibrated for common

radionuclides, is required.

Scintillation counter

A gamma spectroscopy system with a well or cylindrical scintillation

detector is required.

Personnel monitors

Direct reading dosimeters, multiple QFEs and/or at least two electronic

personnel meters, with range 0–1 mSv, are required.

Zone (area) monitor

An ionization chamber, Geiger counter or scintillation counter at a fixed

position is required, with either a visual or an audible alarm (or both) at

variable preset values.

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4.5.2. Probe systems

4.5.2.1. Probes for external organs

In vivo counting probe systems are used for measurement of thyroid

uptake and kidney function, as well as for other more specialized counting.

Whatever the application, however, counting systems have a common specifi

cation:

(a) A large volume NaI scintillation detector, typically 5 cm diameter by 5 cm

thick, to allow both good sensitivity and high detection efficiency for a

range of radionuclide photon energies.

(b) A collimator to limit the FOV of the detector, with some form of distance

indication to ensure a repeatable geometry.

movement to set the probe up to count at a variable height and orien

tation.

(d) Counting electronics including a high voltage supply, pre-amplifier, pulse

amplifier, single channel spectrum analyser (with variable counting

window and threshold); if available, a multichannel analyser is ideal.

(e) A pulse counter and timer, preferably with a printer, are required.

The collimator is made of lead or another high density material; it is

designed to allow a reasonably sized sensitive area, whilst minimizing inter

ference from other sources of radiation in the body. For thyroid examinations,

a special thyroid collimator can be used.

For measurement of kidney function, the ideal counting system has three

probes: one for each kidney plus one to measure the blood and tissue

background. Each probe has its own set of electronics.

When purchasing a probe system, one must ensure that a set of low

activity spectrum calibration sources is provided. These are crucial, as they are

used to set the single channel spectrum analyser to the required counting

energy. The sources should have a relatively long half-life, a distinct photon

energy (or energies), and cover the required energy range, usually 60–511 keV.

The following sources are suitable:

241

Am,

109

Cd,

Co,

Na,

137

Cs and

133

Ba.

The system should be checked before each use.

4.5.2.2. Surgical gamma probes

The use of surgical probes for localization of activity which can be traced

and surgically excised has been of interest since 1949. Recently, surgical probes

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have grown in importance for localization of sentinel nodes in early stage

malignant melanoma and breast cancer surgery, using

99m

Tc labelled radiophar-

maceuticals. Attempts to use surgical probes in other applications, including

monoclonal antibody studies and even

F FDG studies, are under investi-

gation. In this section emphasis is placed on the use of probes employed for

detection of

99m

Tc radiopharmaceuticals.

The technique of sentinel node localization needs multidisciplinary co-

operation between nuclear medicine physicians, surgeons and pathologists.

Success in detecting the sentinel node depends on many factors related to the

sensitivity of the detector, the spatial and energy resolution and geometric

efficiency of the detector, the radiopharmaceutical injected, the rate of

clearance from the site of injection and the uptake in the sentinel node.

Detectors are made by various manufacturers, and the preference between

different types requires knowledge of the basic physics principles, which are

summarized below.

It is important to know the reason why the surgical probe is purchased. Is

it for the purpose of sentinel node localization only for early breast cancers and

malignant melanomas or for the intraoperative detection of residual or

recurrent tumours such as colorectal cancer, thyroid cancer or parathyroid

adenoma? The energy of the radionuclides used and the expected depth of the

target from the skin or the surface of the probe are important because of the

attenuation of the photons, the FOV of the detector, the angular scatter and the

geometric efficiency. The user must familiarize him/herself with different

probes and have experience in operating them.

A surgical gamma probe is based on either a scintillation detector or a

semiconductor detector. Scintillation detectors consist of either NaI or CsI,

either 14 or 19 mm in diameter, with a photomultiplier tube and amplifier. The

signal intensity of scintillation detectors is higher than that of semiconductor

detectors, but their energy discrimination is inferior. Scintillation detectors are

also bulkier.

Semiconductor detectors consist of either cadmium telluride or, more

recently, cadmium zinc telluride. These have high stopping power materials and

accordingly are more sensitive. They are significantly more compact than

scintillation detectors and therefore more suitable for intraoperative use. Their

spatial resolution is also better. However, the useful energy range for these

detectors is limited to 200–300 keV.

When purchasing a probe system for use in surgery, the following factors

should be taken into consideration:

(a) Shielding (collimation) from scattering is important for improved local-

ization and improved spatial resolution. However, shielding will reduce