AIM: Ch 7: Radiography Flashcards
It is production of a two-dimensional image from a three-dimensional object
Radiography
The procedure projects the x-ray shadows of the patient’s anatomy onto the detector and is often called ____
Projection radiography
T/F: In projection radiography, the magnification will always be greater than 1.0 but approaches 1.0 when a relatively flat object (such as a hand in radiography) is positioned in contact with the detector, where b is more or less 0.
True
For most radiographic examinations, the source to imager distance (SID) is fixed at ____
100 cm (40 inches)
An exception to the 100-cm (40 inches) SID is for UPRIGHT chest radiography, where the SID is typically set to ____. The larger SID used for chest radiography reduces the differential magnification in the lung parenchyma.
183 cm (72 inches)
T/F: In most circumstances, higher object magnification increases the width of the edge gradient and reduces the spatial resolution of the image.
True
Meaning of PACS
Picture Archiving Communication System
The ____ in the intensifying screen converts incident x-ray photons to visible light, which then exposes the silver halide emulsion on the film.
Scintillator
Name 4 most common scintillators
Film is composed of a thin plastic base coated on one or both sides with a layer of light-sensitive emulsion consisting of ____ crystals held in water-soluble gelatin
Silver halide (about 95% AgBr and 15% AgI)
The shapes of the silver halide crystals incorporated in radiographic film are engineered in a ____ (see Fig. 7-4) to increase surface area for improving light capture efficiency and reducing silver content to save cost.
Tabular “T” grain shape
Silver halide grains have a small amount of ____ that introduces defects in the ionic crystal structures, where the negative charge (from Br- and I-) builds up on the surfaces and a net positive charge (____) is more central in the crystals.
AgS
Positive is our center
A ____, induced by the lattice defects caused by AgS, is a protrusion of the positive charge that reaches the surface of the crystal.
Sensitivity speck
If greater than or equal to 5 Ag+ ions are reduced, which depends on the incident light intensity, a ____ is formed.
Stable latent image center
An exposed film that has not been chemically developed contains a latent image consisting of a pattern of invisible silver halide grains with latent image centers. There is also a slight intensity rate effect, giving rise to slight differences in film density depending on the exposure rate (called ____).
Reciprocity law failure
The gelatin layer is permeable to aqueous solutions, which is necessary for the aqueous developing chemicals to come into contact with the silver crystals. When the film is developed in an aqueous chemical bath containing a reducing agent, called the ____, the metallic Ag atoms at the latent image centers act as a catalyst, causing the remaining silver ions in that grain to be reduced. A grain of reduced metallic silver atoms appears as a black speck on the film.
Developer
After the film has passed through the developer, it passes through a bath of an aqueous oxidizing solution called ____ that dissolves the remaining (inactivated) silver halide from the emulsion layer areas that were not exposed (or were underexposed) to light.
Fixer
It is a plot of the log of the relative exposure on the x-axis and the optical density (OD) of the processed film on the y-axis. This figure illustrates that the optical density (OD) of film changes nonlinearly with the x-ray exposure level.
“H and D” curve (for Hurter and Driffield)
Screen-film systems can also be adjusted to deliver different image contrast, as seen in Figure 7-7B. Contrast is adjusted by altering the grain size and other parameters in the film emulsion. While high contrast is desirable, the compromise is that dynamic range (called ____ in radiography) is reduced.
Latitude
Chest: wide latitude (good dynamic range)
Mammography: narrow latitude (good contrast)
____ refers to photostimulable phosphor detector (PSP) systems, which are historically housed in a cassette similar to a screen-film cassette.
Computed radiography (CR)
Traditional scintillators, such as Gd2O2S and cesium iodide (CsI), emit light promptly (nearly instantaneously) when irradiated by an x-ray beam. When x-rays are absorbed by photostimulable phosphors, some light is also promptly emitted, but a fraction of the absorbed x-ray energy is trapped in the PSP screen and can be read out later using laser light. For this reason, PSP screens are also called ____
Storage phosphors
Most CR imaging plates are composed of a mixture of BaFBr and other halidebased phosphors, often referred to as ____
Barium fluorohalide
The imaging plate is translated vertically in the (y) direction by rollers across a moving stage and is scanned horizontally in the (x) direction by a laser beam of approximately ____
700 nm wavelength
____ stimulates the emission of trapped energy in a tiny area (x,y location) of the imaging plate, and ____ is emitted from the storage phosphor as energetic electrons drop down to their ground state.
Red laser light
Blue-green visible light
The light emitted through photostimulated luminescence is collected by a fiber optic light guide and strikes a ____, where it produces an electronic signal
Photomultiplier tube (PMT)
The electronic signal is digitized and stored as a ____ value
Pixel
To eliminate detection by the PMT of the scattered excitation laser light, an ____ that is positioned in front of the PMT transmits the blue light and attenuates the red laser light
Optical filter
The nomenclature BaFBr:Eu indicates that the BaFBr phosphor is activated by europium. This activation procedure, also called ____, creates defects in the BaFBr crystals that allow electrons to be trapped more efficiently.
Doping
The defects in the crystalline lattice caused by the europium dopant give rise to so-called ____
F-centers
The ____ that is encoded on the imaging plate after x-ray exposure, but before readout, exists as billions of electrons trapped in F-centers.
Latent image
The number of trapped excited electrons per unit area of the imaging plate is proportional to the intensity of x-rays incident at each location of the detector during the x-ray exposure
They form images from visible light
Charge-coupled device (CCD) detectors
The amount of light transferred to the CCD is determined by 3 factors. What are these factors?
- Directionality of light emitted by the scintillator
- Lens characteristics (f number, the ratio of the focal length of a lens to its effective diameter)
- Demagnification factor, m, required to focus the image from the screen onto the CCD array.
In a CCD system, x-rays are converted to visible light in the scintillator, and then the light is converted to electrons in the CCD chip.
Typical scintillator material
Lambertian emitter
The amount of light recorded is inversely proportional to the square of the f number times the square of the demagnification factor.
T/F: X-ray photons, visible photons, and electrons are all forms of quanta
True
____ refers to the stage where the number of quanta is the lowest and therefore where the statistical integrity of the signal is the worst.
Quantum sink
Ideally, in a radiographic image, the quantum sink should be at the stage where the x-ray photons are absorbed in the converter (scintillator or solid state detector). This is referred to as ____
X-ray quantum limited detector
If there is a subsequent stage where the number of quanta are less than the number of absorbed x-rays, the image statistics and consequently the image noise will be dominated by this stage, and a ____ will occur.
Secondary quantum sink
Linear CCD arrays optically coupled to an x-ray scintillator by fiberoptic channel plates (a light guide made of individual light conducting fibers), often with a de-magnification taper of 2:1 to 3:1, are used in ____. These systems operate using a narrow x-ray fan beam with pre-and post-patient collimators, acquiring an image by scanning the beam over the anatomy for several seconds.
Slot-scan x-ray systems
____ is a method that electronically integrates signal information from the stationary patient anatomy by compensating for the velocity V of the x-ray/detector assembly by reading out the CCD in the opposite direction at a velocity V
TDI readout
Time-Delay and Integration
Based upon a crystalline silicon matrix, these arrays are essentially random access memory “chips” with built-in photo-sensitive detectors, storage capacitors, and active readout electronics, operating at low voltage (3 to 5 volts). Inherent in the ____ design is the ability to randomly address any detector element on the chip in a read or read and erase mode, enabling unique opportunities for built-in automatic exposure control (AEC) capabilities that are not easily performed with a CCD photo detector.
Complementary Metal-Oxide Semiconductor (CMOS) light sensitive arrays
Name 3 electronic components within each dexel
TFT
Charge collection electrode
Storage capacitor
It is an electronic switch that is comprised of three connections: gate, source, and drain.
TFT
For general radiography, this scintillator is most commonly used and grown in columnar crystals, so that the columns act as light pipes to reduce the lateral spread of light
CsI
The reduction of the lateral propagation of light helps to preserve spatial resolution
____ refers to the percent of the area of each dexel that is photosensitive
Fill factor
Fill factor issues place limitations on how small dexels in a TFT array can be made, and currently the smallest indirect detector dexel size is about 100 micrometer.
It uses a semiconductor material that produces electron-hole pairs in proportion to the incident x-ray intensity. Absorbed x-ray energy is directly converted into charge in the detector - there is no intermediate step involving the production of visible light photons
Direct x-ray conversion TFT arrays
____ is the semiconductor most widely used in direct TFT arrays, and is layered between two surface-area electrodes connected to the bias voltage and a dielectric layer
Amorphous selenium (a-Se)
Even though Se has a relatively low atomic number and consequently low absorption efficiency, the Se layer can be made thick (0.5 to 1.0 mm) to improve detection efficiency and still maintain excellent spatial resolution. Fill factor penalties are not as significant with direct conversion TFT systems com
In direct TFT, ____ prevents overcharging dexels, which could damage the TFT array.
Dielectric layer
T/F: In indirect TFT, ion pairs are collected under applied voltage across the solid state converter (10–50 V/mum of thickness). This electric field in the converter almost completely eliminates lateral spreading of the charges during transit through the semiconductor, resulting in high spatial resolution
False
DIRECT TFT
Give the 4 principal x-ray technique factors used for radiography
Tube voltage (the kV)
Tube current (mA)
Exposure time
X-ray source-to-image distance, SID
The SID is standardized to ____ typically, and ____ for upright chest radiography
100 cm (72 in)
183 (40 in)
T/F:
For screen-film radiography, the AEC is set to deliver proper film darkening.
For a digital radiographic system, the AEC is adjusted so that the exposure levels are both in the range of the detector system and produce images with good statistical integrity based on a predetermined signal-to-noise ratio.
True
T/F: The product of the mA and time (s) is called the mAs, and at the same kV and distance, the x-ray fluence is inversely proportional to the mAs
False
The product of the mA and time (s) is called the mAs, and at the same kV and distance, the x-ray fluence is LINEARLY proportional to the mAs
In general, lower kV settings will ____ the dose to the patient compared to higher kV settings for the same imaging procedure and same body part, but the trade-off is that subject contrast is reduced with higher kV.
Increase
The kV is usually adjusted according to the examination type—lower kVs are used for bone imaging and when iodine or barium contrast agents are used; however, the kV is also adjusted to accommodate the thickness of the body part.
Low KV for wrist, higher KV for spine
Lower kVs emphasize contrast due to the photoelectric effect in the patient, which is important for higher atomic number (Z) materials such as bone (Zcalcium 20) and contrast agents containing iodine (Z 53) or barium (Z 56).
Higher KV for chest and abdomen for contrast and to reduce photoelectric interactions with the ribs
Most intensifying screens are comprised of fine-grain crystalline scintillating powders (also called ____), formed into a uniformly thick intensifying screen that is held together by a binder.
Phosphors
T/F: So, for a discussion where the same signal levels in the image are being compared, increasing the speed of the detector (and lowering the radiation dose) by increasing the absorption efficiency increases image noise, whereas increasing conversion efficiency can reduce dose but has no detrimental effect on quantum noise in the image.
False
So, for a discussion where the same signal levels in the image are being compared, increasing the speed of the detector (and lowering the radiation dose) by increasing the CONVERSION efficiency increases image noise, whereas increasing ABSORPTION efficiency can reduce dose but has no detrimental effect on quantum noise in the image.
Increase conversion efficiency»_space; increase in noise
Increase absorption efficiency»_space; no effect on noise
It is determined by the phosphor composition (its effective Z and density), and the screen thickness; it is also dependent on the x-ray beam energy.
Absorption efficiency
It relates to how efficiently the optical signal is conveyed from the scintillator to the silicon photodetector, and how it is then amplified and converted to signal in the image (i.e., gray scale value or digital number)
Conversion efficiency
Underexposed images are likely to be criticized by radiologists for excessive image noise, whereas overexposed images will likely be of high quality; this may lead to a phenomenon known as ____, whereby technologists tend to use unnecessarily high exposures.
Dose creep
Dose creep is most likely to occur in examinations such as portable radiography in which automatic exposure control is not feasible and manual technique factors must be chosen.
It indicates the amount of radiation reaching the detector and is not an indicator of dose to the patient.
Exposure index
Feedback to the user on whether an “appropriate” exposure has been achieved is given by the ____
Deviation index (DI)
0: Appropriate
+n: Overexposed
-n: Underexposed
A DI of 1 indicates an overexposure of about 26%; a value of 1 indicates an underexposure of 20% less than desired. The acceptable range of DI values is approximately from -1 to +1.
When the DI is in the desired range, the radiographic system is considered to be working well and is able to deliver the EIT (target exposure index) values set up by the institution.
These techniques have been developed, which can in some instances reduce anatomical noise and thereby produce an image that has better information content.
Dual energy radiographic techniques
Because of the different energy dependencies in tissue types, the acquisition of two radiographic images at two different effective energy levels (i.e., different kVs), enables either the bone component or the soft tissue component in the image to be removed by image post-processing
High Z»_space; photoelectric interaction
Low Z»_space; compton scattering
T/F: Scatter acts chiefly as a source of signal, improving the signal-to-noise ratio.
False
Scatter acts chiefly as a source of NOISE, DEGRADING the signal-to-noise ratio.
It is defined as the amount of energy deposited in a specific location in the detector by scattered photons, divided by the amount of energy deposited by primary (non-scattered) photons in that same location
Scatter-to-primary ratio (SPR)
SPR = S/P
The SPR increases typically as the volume of tissue that is irradiated by the x-ray beam increases.
The terms S and P in this discussion refer to the energies absorbed in the detector from the scattered and primary photons, respectively.
It is used to characterize the amount of scatter
Scatter fraction
F = S/(P+S)
The terms S and P in this discussion refer to the energies absorbed in the detector from the scattered and primary photons, respectively.
It is the most widely used technology for reducing scatter in radiography, fluoroscopy, and mammography
Anti-scatter grid or scatter grid
The scatter grid has a simple geometric design, in which open interspace regions and alternating x-ray absorbing septa are aligned with the x-ray tube focal spot. This alignment allows x-ray photons emanating from the focal spot (primary radiation) to have a high probability of transmission through the grid (thereby reaching the detector), while more obliquely angled photons (scattered x-rays emanating in the patient) have a higher probability of striking the highly absorbing grid septa
It is the ratio of the height of the interspace material to its width—the septa dimensions do not affect the grid ratio metric
Grid ratio
Grid ratios in general diagnostic radiology are generally 6, 8, 10, 12, or 14, with the central three values used most commonly. Grid ratios are lower (,5) in mammography.
It is the most fundamental descriptor of the grid’s construction
Grid ratio
Most commony used grid ratio in general diagnostic radiology, except:
a. 6
b. 8
c. 10
d. 12
c. 6
The grid septa in a grid are typically manufactured from
Lead
Ideal interspace material:
More commonly used interspace material in state-of-the art imaging systems:
Ideal: Air
More common: Carbon fiber
Number of grid septa per centimeter
Grid frequency
____ grids are widely used in mammography
Crossed
The focal length of a grid is typically ____ for most radiographic suites and is ____ for most upright chest imaging units.
100 cm, 183 cm
If the x-ray tube is accidentally located at a different distance from the grid, then ____ will occur
Grid cutoff
The focal distance of the grid is more forgiving for lower grid ratio grids, and therefore high grid ratio grids will have more grid cutoff if the source to detector distance is not exactly at the focal length
For fluoroscopy, better to have lower grid ratio so to suffer less from grid cut off
It is a grid that moves with a reciprocating motion during the x-ray exposure, causing the grid bars to be blurred by this motion and not visible in the image. The motion is perpendicular to the long axis of the linear septa in the grid.
Bucky grid
It describes the relative increase in x-ray intensity or equivalently, mAs, needed when a grid is used, compared to when a grid is not used
Bucky factor
The Bucky factor essentially describes the radiation dose penalty of using the grid
It should be noted that for thin anatomical structures, there is very little scattered radiation, and the use of a grid is not necessary.
Typical values of the Bucky factor for abdominal radiography range from ____
3 to 8
Digital systems, however, have much wider dynamic ranges than screen-film systems in general, and therefore the x-ray technique does not have to be increased to replace the lost x-ray exposure to the detector, which was mostly scattered radiation anyway.
Nevertheless, these considerations suggest that DR systems may be used to provide lower dose examinations than in the screen-film era.
It is the fraction of primary photons that are transmitted through the grid, and ideally it would be 1.0.
Primary transmission factor (Tp)
Values of Tp typically run from 0.50 to 0.75 with modern grids, and this value is ____-dependent due to the penetration of the interspace material.
kV
It is the fraction of scattered radiation that penetrates the grid. Ideally, it would be 0.
Scatter Transmission Factor (Ts)
The value of Ts can range substantially depending on the amount of scatter in the field, the x-ray energy, and the grid design.
Typical values of Ts run approximately from ____ in general diagnostic radiography.
0.05 to 0.20
It is simply defined as
Selectivity
It refers to the reduction in contrast due to scattered radiation
Contrast degradation factor (CDF)
However, because digital images can be adjusted to enhance contrast, CDF is of less importance in digital radiography than in film-screen radiography.
The most ubiquitous tool used for scatter reduction
Anti-scatter grid
It is another method for scatter reduction where by moving the patient away from the detector, less of the scatter that is emitted from the patient will strike the detector
Air gap technique
The scattered x-ray intensity with air gap distances is not proportional to 1/r^2, but rather approaches 1/r
Thus, practical factors limit the utility of the air gap method for scatter reduction—as magnification of the patient anatomy increases, the coverage of a given detector dimension is reduced, and there is a loss in spatial resolution due to the increased blurring of the finite focal spot with magnificatio
Gold standard in scatter reduction methods
Scan-slot system
