Instrumentation Review

What is an ionization chamber?

a gas filled chamber with both postivie and negative electrodes

What is the interaction that occurs within an ionization chamber?

Interaction of ionizing radiation with gas in the chamber creates positive and negative ions.The ions move to the electrodes producing an electrical current.

An proper voltage, the current is a direct measure of the total amount of ion pairs produced per unit time in the enclosed volume of gas.

What are the three types of ionization chambers?

1. Basic Ionization Chamber

2. Proportional Counter

3. Geiger-Muller Counter

Describe the characteristics of a basic ionization chamber

• Operate at a low voltage and collect ONLY primary ions

• Very precise and accurcate

• Very reliable with high amounts of radiation

What are the 3 types of basic ionization chambers?

1. Radiation survey meter “Cutie pie”

2. Pocket Dosimeters

3. Dose Calibrators

What are radiation survey meter or Cutie-pie best used for?

• good for accurately determining exposure rate from theraputic dose

• measured in milliroentgens per hours (mR/hr)

What is a pocket dosimeter?

• Shape and size of a pen

• measures the total radiation dose during a certain time interval

• gives immediate readings

What is a dose calibrator?

• well type of ionization chamber

• unit of radioacitivity such as millicuries or microcuries

What are Geiger-Mueller counters?

• highest applied voltage from the anode to cathode

• causes one ionization to result in an “avalanche” of electrons

• Second ionization may go undetected

What is dead time?

the time during which the GM counter cannot respond to another ionizing event, makes it inefficient

*important to reset the counter after a detection of a large amount of radiation

What type of radiation detection do GM counters NOT provide useful results?

• NOT useful in presence of large amounts of radioactivity

• good for detecting low levels of radiation (ex. contamination)

• not as accurate as a radiation survey meter but MORE sensitivie

What are the quality control steps of Survey Meters and how often are they done?

1. Battery Check - daily

2. Sealed Source Check (Constancy) - Daily: Cesium Source, must be within ±10%

3. Calibration - Annually: performed by a physicist by placing the meter in front of a high-acitivity cesium sealed source ane exposing the meter. Reading should be atleast 30 mR/hr at 100 cm. Calculated using the inverse square law. Exposed reading should be adjusted to rad the same as the calculated value, or within ±10%

What Instrumentation equipment is considered a gas filled detector?

Dose calibrators

What are the quality control tests perfromed for dose calibrators and how often are they performed?

• Constancy - Installation and Daily

• Linearity - Installation and Quarterly

• Accuracy - Installation and Annually

• Geometry - Installation

What is required for the constancy quality control test for a dose calibrator?

• determines the reproducibility of measurements of a source of known acitivity from day to day.

• 10% variations

What is required for the linearity quality control test for a dose calibrator?

• assesses ability to measure low-high acutuvrt doses accurately

• a dose of high activity, short lived radionuclide is assayed several times until it decays down to 30uCi or less

• actual measures are compared to calculated delayed acitivities

• 10% variations

What system can you use for linearity?

attenuation shields to simulate decay

What is required for the accuracy quality control test for a dose calibrator?

• assesses the ability to provide a true measure of acitivity of radionuclides at different gamma energies

• at least two reference standards are assayed 3 times each and averaged

• 10% variations

How often should reference sources be leak tested?

every 6 months

How often should reference sources be inventoried?

every 6 months

What is required for the geometry quality control test for a dose calibrator?

• assesses the ability of the instrument to accurately measure activities in different configured containers

If geometry variations are greater than ____ percent than what is used?

10%

correction factors

What is the formula for correction factor?

expected value / actual value

*multiply actual reading by correction factor

What are the principles of operation for scintillation detection systems?

• conversion of ionizing radiation energy to electrical energy

• certain materials have the property of emitting a flash of light or scintillation of ionzing radiation

What are the three types of scintillation detectors?

1. liquid scintillation systems

2. semiconductor scintillators

3. solid scintillation detectors

When are semiconductor scintillators used?

• primarily in research

• lymphatic mapping, scintimammography, and cardiac

What are some examples of solid scintillation detectors?

• Thallium activated sodium iodide    NAI(Tl) *primarily used in gamma cameras

• Cesium iodide     CsI(Tl)

• Bismuth germanate    BGO

• Lutetium oxyorthosilicate     LSO

• Gadolinium oxyorthosilicate         GSO

What are the advantages of NAl(TI) crystals?

• relatively inexpensive

• flexibility in size and shape

• stopping power is good for energy range used in clincial nuclear medicine

What are the disadvantages of NAl(TI) crystals?

• very fragile

• hygroscopic (readily absorbs moisture)

• must be sealed

Height of the electrical impulse coming from the PMT is _______ to the energy of the radiation absorbed in the crystal.

Proportinonal

• This allows radionuclides with different energies to be distinguished from one another by pulse height analysis.

What is an Pulse Height Analyzer?

an electronic device that enables the operator to select pulses of a certain height and to reject all pulses of a different height

What is the areas between the upper and lower discriminators called?

a window

What is an example of a single channel analyzer?

Tc-99m

What isotopes use multichannel analyzers (multiple peaks)?

Gallium, Indium

What is Comptom Scatter?

photons that undergo Compton scattering LOSE some of there energy and therefore have LOWER pulse heights

*it is critical to recognize compton scatter photons because only primary photons are desired to create the image

What are the three types of solid scintillation detectors?

1. Uptake probe

2. well counter

3. gamma camera

What is an uptake probe commonly used to perform?

thyroid uptakes

How many PM tubes does an uptake probe have?

One

What is the name of the collimator used for uptake probes?

flat field

What test is performed on an uptake probe to compare the measured variability of a group of measurements?

Chi-Square

What is the significance of a well counter over a gamma camera?

90% of emitted photons are absorbed (no dead time, the source is completely surrounded, picks up more counts)

What is the maximum amount of activity that can be placed into a well counter?

1 uCi, 37 kBql

What is the formula for detector efficiency?

number of detected events / number of source emissions

What are the two types of efficiency?

1. Geometric

2. Intrinsic

What is the quality control of an Uptake Probe and Well counter and how often are they performed?

1. voltage calibration - daily: determining the appropriate operating high voltage and amplifier gain

2. efficiency - quarterly: evaluates the ability of the instrument to detect radioactive disintegrations

3. constancy - daily: 10% variations

4. energy resolution - annually: determines the ability of the instrument to discriminate between different energies, 8-12% range

5. chi square - variable: checks for random errors greater than those predicted, acceptable values between 0.1-0.9

What is the purpose of a collimator?

• it is the first part of camera encountered by the photon after it leaves the patient

A collimator discriminates against unwanted photons based on what?

direction of travel

What occurs for a star artifact to appear?

septal penetration

What are the four types of collimators?

1. parallel hole

2. pinhole

3. converging

4. diverging

What are the characteristics of a parallel hole collimator?

• consists of lead foil with thousands of parallel holes uniformily distributed

• most used collimator

• for the use of low energy, medium energy, high energy, and high-Z

What are the characteristics of pinhole collimators?

• thick concial collimator woth a single 2-5mm hole in the bottom

• magnifies and inverts an image

• as the pinhole is moved further from the source, the image becomes SMALLER

• as it is moved closer, the image becomes LARGER

What are the characterisitcs of a converging collimator?

• used to magnify the image geometrically

• used primarily in peds imaging (hips, knees, etc.)

What are the characteristics of a diverging collimator?

• center hole is perpendicular to the crystal and then the holes begin to diverge at incresingly greater angle

• permits a LARGER area of the body to be imaged (lung scan of a larger patient)

What is a ¼ inch crystal thickness used for?

lower energy

What is a 3/8 inch crystal thickness used for?

medium energy

What is a ½ inch crystal thickness used for?

high energy (PET)

What is the most commonly used crystal thickness used?

3/8 inch

What do PMT’s convert?

changes radioactivity to light photons and magnifies it

What are the quality control tests performed on a gamma camera?

1. flood field uniformity

2. spatial linearity

3. spatial resolution

4. sensitivity

How often is flood field uniformity performed on a gamma camera?

daily

How many counts are required for flood field uniformity on a gamma camera?

3-5 million counts

What are the types of uniformity testing performed?

• extrinsic

• intrinsic

*source should be placed 5 FOV

What is the definition of spatial linearity?

• ability to produce a uniform image with straight lines corresponding to straight lines in a phantom

• accurate display of true organ shape

How often is spatial lineariy performed on a gamma camera?

weekly

What is spatial lineariy performed with on a gamma camera?

bar phantom

What is the definition of spatial resolution?

• the ability to differentiate between two points

• affects the ability to visualize small defects

How often is spatial resolution performed on a gamma camera?

weekly

What is used to perform spatial resolution on a gamma camera?

bar phantom

The real spatial resolution of a camera is expresse quantitatively as what?

Full width half maximum

What is full width half maximum?

the width in centimeters at 50% of the height of the line spread peak

Put the following in order from best to worse spatial resolution values: Planar camera 6-12mm, SPECT camera 12-20mm, PET scanner 4-5mm

PET, Planar, SPECT (detectors cant get as close to patient)

What is the formula for determining sensitivity (cpm/mCi)?

source cpm-background / source acitivity in uCi

What is sensitivity dependent on?

• collimator

• crystal thickness

• number of PMT’s

What are the four types of phantoms used for spatial quality control testing?

1. orthogonal hole

2. Hine duley

3. four quadrant

4. parallel-line equal spacing (PLES)

What is the most commonly used bar phantom?

4 quadrant

What are the advantages of SPECT?

• improved image contrast

• seperates overlapping structures

• has the ability to assess attenuation

• 3-D representation of the organ surface of “volume” rendered

What are the quality control procedures that are performed on SPECT cameras and how often are they performed?

• Field uniformity - daily

• Linearity (bars) - weekly

• Resolution (bars) - weekly

• Center of rotation (COR) - weekly

• Flood corrections and SPECT phantoms - variable

What is the purpose of the COR quality control test?

• used to correct for slught variations in the position of the camera head as it rotates

• any defect seen on planar images is magnifieid in a SPECT study

What is the purpose of field uniformity corrections?

• critical in SPECT imaging-flood correction is applied to SPECT during processing

What artifact is caused by detector non-uniformity?

Bulls-eye or ring artifacts

How many counts are needed for a field uniformity quality control test using a 64×64 matrix?

30 million counts

How many counts are needed for a field uniformity quality control test using a 128×128 matrix?

120 million counts

What arc degree is routinely used to acquired SPECT images?

360 degree

What arc degree is used for myocardial imaging?

180 degree

What are the two matrix sizes associated with SPECT?

128×128

64×64

Describe the term “Number of Projections”

the number of angular views or projections shuld be at least equal to the image matrix size

What is the minimum number of views for a 64×64 matrix?

64 views

What is the minimum number of views for a 128×128 matrix?

128 views

What artifact occurs on the reconstructed slices when the number of projections is less than the minimum?

streak artifacts

What are the two types of orbits associated with the radius of rotation?

• circular orbits

• non-circular orbit (NCO) or elliptical orbit

What are the two types of rotation methods?

• step and shoot

• continuous

What are the two types of SPECT processing methods and what is used to do so?

• filtered backprojections - exact mathematical solutions

• iterative approaches - approximations

What is the purpose of filters?

• elimination of the star artifact

• background subtraction

• edge enhancement

• supression of statisitical noise

What are the common types of filters used and what do they do to an image?

• ramp filter - erases blurring from filtered back projection

• hanning filter - smoothing (noise reduction)

• butterwirth filter - smoothing (noise reduction)

• gaussian filter - noise reduction

What happens to images when the cutoff is increased?

more high frequencies are retained

• grainy-sharper

What happens to images when the cutoff is decreased?

• smoother images

What are the two methods used for attentuation correction performed during an acquisition or during processing?

• sealed source transmission scan: line source (gadolinlum-153)

• conventional correction methods: prereconstruction methods (sorrenson) or postreconstrcution method (chang)