RAPHEX Flashcards
A CT image consists of 200 slices, each 512 × 512 pixels, each pixel having a 16-bit pixel depth. The size of the file is _
16 bits equals 2 bytes. 2 × 512 × 512 × 200 = 104857600 bytes = 100 MB
highest to lowest energy radiation
gamma - xray- UV- infrared - microwave - radio
isomeric transition
Isomeric transition is a radioactive decay process that occurs in an atom where the nucleus is in an excited meta state (e.g. following the emission of an alpha or beta particle). The nucleus emits a gamma ray which can sometimes interact with the orbital electrons and emit an internal conversion electron. If this occurs, it can be followed by a characteristic x-ray, or an Auger electron.
secular equilibrium
Secular equilibrium defines a special case of transient equilibrium in which the half-life of the parent is very long compared with that of the product. In this case the product activity approaches but never truly achieves equilibrium.
transient equilibrium
half-life of the daughter is shorter than the half-life of the parent. Contrary to secular equilibrium, the half-life of the daughter is not negligible compared to parent’s half-life
variation of the focal spot size in the image occurs in which direction?
anode-cathode direction
typical filament current for extremity radiation
5000 mA
tube current is 1-5 mA for fluoro and 200-1000 mA for radiography
explain sensitometric strip
daily QA
exposes a film with a calibrated sensitometer similar to a screen phosphor
steps 1-8 show a greater response to increase in developer temperature
step 14 shows that almost all silver bormide grains were used and that increase in temperature will not increase the density as much as for the lower density steps
steps 21 shows very high density and is likely to be in shoulder part of characteristic curve
most appropriate image receptor for fluoro
cesium iodide scintillator, photocathode
is CsI used in screen-film radiography?
Cesium iodide is an excellent scintillator used in fluoroscopy and digital radiography; however it is too sensitive to moisture to be used for screen-film radiography
what screens are used for screen-film mammo?
Gadolinium oxysulfide
grid ratio
strip height/width of interspacing
higher grid ratio= better scatter cleanup
linear blurring tomography
-high contrast for thin sections
Wide-angle tomography produces thin sections, which have low contrast. This is because all the anatomy above and below the section in focus is blurred across the image with an effect similar to fog. The patient exposure is higher because most of the time the x-ray beam is passing through the patient at an angle, i.e., the patient is effectively thicker.
The simplest method is linear tomography, in which the X-ray tube is moved in a straight line in one direction while the film moves in the opposite direction. As these shifts occur, the X-ray tube continues to emit radiation so that most structures in the part of the body under examination are blurred by motion. Only those objects lying in a plane coinciding with the pivot point of a line between the tube and the film are in focus.
typical Pb equivalent in lead aprons in fluoro rooms
0.5 mm Pb
reduces x-ray intensity by 95-99% depending on kVp
contrast resolution of CT
0.3-0.6%
minimum mammo resolution
With a bar line-pair pattern on a 4.5 cm high phantom, the high contrast spatial resolution produced by a mammography unit should be at least 11 lp/mm with the bars perpendicular to the anode-cathode axis and 13 lp/mm with the bars parallel to the anode-cathode axis
Current commercial full field mammography units have image receptors which are capable of resolving 5 to 7 lp/mm, whereas screen-film receptors designed for mammography can often resolve 15 to 20 lp/mm.
where is resolution in mammo lowest at?
at the top of the breast because geometric blur increases with magnification
A technologist notices that a DR (or CR) spine image appears unacceptably noisy on the review workstation. The most appropriate action is
Increase the AEC density control and repeat exposure.
DR/CR systems cannot compensate for excessive noise due to quantum mottle, i.e., dose too low. If unacceptable, the view needs to be repeated with a higher exposure to the image receptor (and to the patient).
high speed mode yields?
noisy images
speed is sensitivity
spatial resolution is independent of speed for digital imaging systems
what will the appearance of an overexposed DR or CR image be?
Overexposed images are generally very good in DR/CR because they will be low in noise. The system will rescale the brightness and contrast so they will appear properly exposed.
best estimate value for radiogenic fatal cancer
10%/Sv
A patient is examined with an auto brightness, triple mode FOV, image intensifier x-ray fluoroscope. The patient’s entrance air kerma rate (EAKr) is 20 mGy/min when the 14 cm FOV is used and the x-ray beam is collimated to the full FOV, the patient’s EAKr
Increases more than 30% when the FOV is decreased to 10 cm (full field collimation).
Decreasing the FOV decreases the geometric light amplification factor of the image intensifier. More radiation is therefore needed to produce the required output light level. Also, less scatter is produced (and reaches the image intensifier) when the beam is collimated. A small increase in EAKr is needed to offset this effect.
ratio of brightness gains equals the ratio of what in an image intensifer?
ratio of magnification gains
major differences between fluoro and standard radiography
tube potentials are the same
The focal spots for fluoroscopy are typically 0.3 or 0.6 mm; those for standard radiography are usually 1.0 to 1.2 mm. The spatial resolution for fluoroscopy is usually limited by the TV system to 1.8 to 2.5 lp/mm, while radiography has resolutions of 4 to 8 lp/mm. The tube current for fluoroscopy is usually 1 to 3 mA in order to limit anode heating for the long exposure times of 3 to 10 minutes; because of the short exposure times (less than 1 second) of radiography, tube currents of 200 to 800 mA can be used. Tube potentials are the same for both procedures. SSDs in fluoroscopy are usually 18 to 20 inches, while the SSDs for radiographs are typically about 25 inches (except for chest radiographs).
how is 18F produced
bombard 18O target with proton beam in cyclotron
only accurate method for measuring activity of a pure beta source
A liquid scintillation counter, in which the sample is dissolved in a liquid scintillant is the only accurate method. The other methods are inaccurate, because the activity determination of a pure beta source is extremely position and geometry sensitive
The window setting used on a gamma camera for 99mTc is set with the center at 140 keV with a width of ±14 keV, i.e., 20%. What is the reason for this?
Photons, which impinge upon the crystal, lose energy by Compton scattering and the photoelectric effect. Both processes convert the gamma-ray energy into electron energy. On average, approximately one electron-hole pair is produced per 30 eV of gamma-ray energy deposited in the crystal. These electrons result in the release of visible light when trapped in the crystal. These light quanta are collected and amplified by photomultiplier tubes. The statistical fluctuation in the number of light quanta collected and their amplification is what causes the spread in the detected energy peak, even when most of the 99mTc photons deposit exactly 140 keV in the NaI crystal.
When counting an 125I source of unknown activity, the multi-channel analyzer shows a peak at 60 keV, in addition to the main peak at 30 keV. What is this due to?
A second peak is seen at twice the energy of the main peak. Pulse pile-up occurs when two photons impinge upon the detector at the same time, resulting in an event being recorded as if it had twice the energy of the initial photon.
difference between 2D and 3D PET
2-D scans have septa in front of the detectors to reduce events from scattered photons. The septa reduce a large fraction of the cross-plane coincidences, in order to reduce coincidence events from scattered photons. Both 2-D and 3-D scans acquire all axial slice data simultaneously, and can reformat the data to display sagittal or coronal images
In nuclear medicine imaging quality control, what is checked daily using standardized long half-life source.
dose calibrator constancy
