NUCMED Flashcards
__ unit of exposure. The quantity exposure is a measure of ionization produced in air by photons.
roentgen
(1) defines exposure (X) as the quotient of dQ by dm where dQ is the absolute value of the total charge of the ions of one sign produced in air when all the electrons (negatrons and positrons) liberated by photons in air of mass dm are completely stopped in air
The International Commission on Radiation Units and Measurements (ICRU)
(SI) unit for exposure ___ but the special unit is roentgen (R).’
coulomb per kilogram(C/kg)
1R = __
2.58 x 10 -4 C/kg air
An x-ray beam in passing through air sets in motion electrons by
photoelectric effect, Compton effect, or pair production.
These high-speed electrons produce ___ along their tracks. Because of the __ produced by the __ applied across the ion-collection plates, the positive charges move toward the negative plate and the negative charges move toward the positive plate.
ionization; electric field ; voltage
This constitutes a current. The collected charge of either sign can be measured by an
ELECTROMETER.
Deals with the measurement of the absorbed dose or dose rate resulting from the interaction of ionizing radiation with matter.
DOSIMETRY
It also refers to the determination of radiologically relevant quantities such as:
- Exposure
- Kerma
- Fluence etc.
Any device that is capable of providing a reading that is a measure of the absorbed dose D, deposited in its sensitive volume V by ionizing radiation. __ is a device that measures __ OR __
Dosimeters ; directly or indirectly
Dosimeter is a device that measures directly or indirectly:
Absorbed dose
Exposure
Kerma
Equivalent dose
Or other related quantities.
Dosimeters Divided into Two:
- Absolute dosimeters
- Secondary dosimeters
the dose is determined without reference to another dosimeter. g: free air ionization chamber, specially designed spherical chambers of known volume, calorimeter, Fricke dosimeter.
Absolute Dosimeters
these dosimeters require calibration against a primary standard. Eg: thimble chambers, plane parallel ion chambers, TLD’s, Diodes and Films.
Secondary Dosimeters
consists of a cylindrical chamber containing air at atmospheric pressure.
A moderate voltage (__) is applied between two electrodes, the anode and cathode
Ion chamber; 100 volts
is an instrument used in the measurement of the roentgen according to its definition.
free-air, or standard, ionization chamber
chambers are too delicate and bulky for routine use.
Free-air ionization chamber
Their main function is in the standardizing laboratories where they can be used to calibrate field instruments such as a thimble chamber.
Free-air ionization chamber
__ is originating from a focal spot S, is defined by the diaphragm D, and passes centrally between a pair of parallel plates. A high-voltage (field strength of the order of 100 V/cm) is applied between the plates to collect ions produced in the air between the plates. The ionization is measured for a length L defined by the limiting lines of force to the edges of the collection plate C.
free-air chamber
The lines of force are made straight and perpendicular to the collector by a
guard ring
It works based on the Bragg -Gray Cavity theory
3.THIMBLE CHAMBERS
relates the radiation dose in a cavity volume of material to the dose that would exist in a surrounding medium in the absence of the cavity volume. The absorbed dose in the cavity containing is deposited entirely by charged particles crossing it.
Bragg-Gray cavity theory
is spherical volume of air compressed into a solid shell with an air cavity at the center.
Thimble Chamber
is when the number of electrons entering the cavity is the same as that leaving the cavity,
Electronic Equilibrium,
In practice, however, a thimble chamber is constructed with wall thicknesses of __ and this is supplemented with close-fitting caps of __ or other plastic to bring the total wall thickness up to that needed for electronic equilibrium for the radiation in question
1mm or less; Plexiglas
is shaped like a sewing thimble-hence the name.
Chamber Wall
Most commonly used wall materials are made either of ___ coated on the inside by a conducting layer of __or of a conducting mixture of __
graphite (carbon), Bakelite, or a plastic; graphite ; Bakelite and graphite.
The effective atomic number of the wall is generally a little less than that of air. It is closer to that of
carbon (Z = 6)
PRACTICAL THIMBLE CHAMBERS
- Condenser Chambers
- Farmer Chamber
is a thimble ionization chamber connected to a condenser.
A condenser chamber
are suitable for measuring exposure rate in air for relatively lower energy beams (≤2 MeV).
Condenser chambers
manufactured by Victoreen Instrument Company.
Victoreen condenser chamber
designed a chamber which provided a stable and reliable secondary standard for x-rays and y rays for all energies in the therapeutic range. This chamber connected to a specific electrometer (to measure ionization charge) is known as the __
Baldwin-Farmer substandard dosimeter
The thimble wall is made of__and the central electrode is of __. The insulator consists of __
pure graphite ; pure aluminum ;Polytrichlorofluorethylene (PTCFE)
There are three electrodes in a well-guarded ion chamber: __
the central electrode or the collector, the thimble wall and the guard electrode.
SPECIAL CHAMBERS
Extrapolation Chamber
Parallel-plate Chambers
__ designed an ionization chamber for measuring surface dose in an irradiated __in 1937
Failla; phantom
are parallel-plate chambers with a variable electrode separation. They can be used in absolute radiation dosimetry (when embedded into a tissue equivalent phantom)
Extrapolation Chamber
are similar to the extrapolation chambers except for the variable electrode spacing. The electrode spacing of the parallel-plate chambers is small __ but fixed.
Parallel-plate Chambers ; ~2 mm
Parallel Plate/Plane parallel chamber is recommended for:
– Dosimetry of electron beams with energies below 10 MeV.
– Depth dose measurements in photon and electron beams.
– Surface dose measurements of photon beams. Depth dose measurements in the build-up region of megavoltage photon beams. They have a fixed electrode spacing (1- 2mm).
Sensitive volume =
0.35 cm3
produce projection images of the distribution of radioactivity in patients.
Scintillation cameras
Scintillation cameras are sometimes called
gamma cameras or Anger cameras.
are essential for providing spatial information in planar NM imaging.
Collimators
Gamma rays that pass through the collimator are incident on a
NaI scintillator
Scintillators absorb incident gamma photons and produce many __
light photons
Light output from the NaI scintillator is detected by an array of __ photomultiplier tubes(PMTs) and converted to an __
photomultiplier tubes(PMTs); electrical signal.
Approximately __ of the absorbed gamma ray energy is converted to __
10%; light
Scintillation cameras typically use __
55 PMTs
The position of the gamma ray interaction is determined by a ___ based on the relative strength of signals from each PMT.
pulse arithmetic circuit
__ refers to the registration of a single gamma ray by the detector, and__ are acquired for a typical scintillation camera image.
Count; ∼500,000 counts
are typically made of lead and contain multiple holes.
Collimators
The lead strips between the holes are called __
septa
The most used collimator is the __, which contains thousands of parallel holes. The holes may be round, square, or triangular; however, most state-of the-art collimators have hexagonal holes.
parallel-hole collimator
project the same object size onto the camera, and the field of view (FOV) does not change with distance.
Parallel-hole collimators
has many holes, all aimed at a focal point in front of the camera and produce a magnified image, and FOV decreases with distance.
- The magnification increases as the object is moved away from the collimator
Converging collimators
project an image size that is smaller than the object size, and FOV increases with distance.
Diverging collimators
are cone shaped with a single hole at the apex.
– Images generated using a __ are normally magnified and inverted.
pinhole collimator
- a hybrid of the parallel-hole and converging collimator, called a fan-beam collimator, is used in single photon emission computed tomography (SPECT)
Fan Beam Collimators
High-sensitivity collimators have __.
larger holes and lower resolution
High-resolution collimators have __
smaller holes and lower sensitivity.
is degraded with increasing distance from the collimator.
Resolution
Low-energy collimators used with
99mTc and
201TI have thin septa.
Medium-energy collimators used with __have thicker septa and therefore fewer holes and lower sensitivity.
67Ga and 111In (indium)
High-energy collimators are required for __ imaging and have the thickest septa.
131I (iodine)
__ provides computed tomographic views of the three-dimensional distribution of radioisotopes in the body.
– __ collimators are commonly used for__ imaging.
– Scintillation cameras rotate __ around the patient.
– Projection images are obtained at selected angles, typically every__
– Each projection takes __
– Cardiac SPECT images make use of a __
– Noncardiac SPECT imaging likely uses a __
Single photon emission computed tomography (SPECT)
Parallel-hole
180 or 360 degrees
3 or 6 degrees.
∼30 seconds with a total scan time of ∼15 minutes.
64 × 64 matrix size.
128 × 128 matrix size
__ algorithms are now used.
– is more accurate and minimizes artifacts/ refines the images.
Iterative reconstruction
are used to increase system sensitivity and reduce scan times.
Multi-headed cameras
The use of __ (i.e., body contouring) for scintillation camera traveling around the patient allows the distance to the patient to be minimized.
elliptical orbits
Common clinical SPECT applications include __
myocardial ischemia or infarctions and evaluation of abnormalities seen on planar bone scans.
SPECT studies are also performed with __ octreotide for neuroendocrine imaging,
111In
are inorganic scintillators that emit more light than BGO.
GSO and LSO
A __ camera contains rings of detectors (scintillators) surrounding the patient.
– Detectors are coupled to __ to detect light produced in each detector.
– Most early-generation PET scanners had detectors made of __
– Modern PET scanners use detectors made of__ or .
– __is also used, which is LSO doped with a small amount of yttrium.
Positron emission tomography (PET)
photomultiplier tubes (PMTs)
bismuth germanate (BGO).
Lutetium yttrium oxyorthosilicate (LYSO)
Two interactions occurring within a specified time interval τ (coincidence timing window) are called a __
coincidence event
There are three types of coincidences:
1.true coincidences,
2. scatter coincidences, and
3. random (accidental) coincidences.
The most common positron emitter used for PET imaging is __
in the form of __is the most commonly used agent.
18F (T1/2 = 110 min)
fluorodeoxyglucose
Three-dimensional scanner sensitivity (without septa) is__than 2D scanner sensitivity (with septa).
six times higher
__-slice scanners are targeted for cardiac applications.
Sixty-four
__slice CT scanners are adequate for most PET/CT applications.
Sixteen-
__ scanning from the eyes to the upper thigh can be performed in 15 to 20s
Spiral CT
Axial coverage in PET is __
15 cm to 22 cm.
Typical PET scans take __ at each detector position.
2 to 3 minutes
Up to__ would be required for head to toe PET imaging of melanoma patients.
11 positions
