Term
| Magnification is given by |
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Definition
M = SID/SOD
SID - Source to Image Distance
SOD - Source to Object Distance |
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Term
| For most radiographic exams, the SID is |
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Definition
|
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Term
| For upright chest radiography, the SID is |
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Definition
|
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Term
| Why does upright chest radiography use a larger SID? |
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Definition
| To reduce the differential magnification in the lung parenchyma. |
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Term
| The length of the edge gradient (Lg) is related to the length of the focal spot (Lf) by |
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Definition
| Lg = Lf * (edge-to-detector distance)/(source-to-edge distance) |
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Term
| geometric blurring is due to |
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Definition
| the finite length of the focal spot |
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Term
| Higher object magnification _______ the width of the edge gradient and ________ the spatial resolution of the image. |
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Definition
| increases the width of the edge gradient and reduces the spatial resolution |
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Term
| What is the purpose of the scintillator in the intensifying screen? |
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Definition
| It converts incident x-rays photons into visible light. |
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Term
| Thicker screens are ______ efficient absorbers, but result in ________ spatial resolution due to the _______ spread of the light. |
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Definition
|
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Term
| Intensifying screens are commonly made from |
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Definition
CaWO4 or Gd2O2S
Calcium Tungstate
Gadolinium Oxysulfide |
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Term
| The light sensitive emulsion on film consists of |
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Definition
| silver halide (95% AgBr and 15% AgI) |
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Term
| When a silver halide grain is exposed to light, what happens. |
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Definition
| Some of the Ag+ ions are reduced (gain electrons) and are converted into metallic Ag forming a latent image center. |
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Term
| What is the purpose of the film developer? |
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Definition
| It reduces the remaining silver ions at the latent image centers. |
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Term
| What is the purpose of the fixer? |
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Definition
| It removes the remaining (inactivated) silver halide from the emulsion layer. |
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Term
| What is plotted on an H and D curve? |
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Definition
| The optical density on the y-axis is plotted against the log of the exposure on the x-axis. |
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Term
| Film transmittance is defined as |
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Definition
The fraction of visible light passing through the film.
T = It /Ii |
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Term
| Optical density is defined as |
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Definition
| OD = -log10(T) = -log10(I/I0) |
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Term
| OD of 1, 2, and 3 transmit how much light intensity? |
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Definition
| 1 - 10%, 2 - 1%, 3 - 0.1% |
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Term
| High speed films require _____ radiation. |
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Definition
|
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Term
| A change in screen-film speed is shows as a ______ shift in the H and D curve. |
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Definition
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Term
| An H and D curve with a steeper slope has ______ contrast, but the ________ _________ is narrower. |
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Definition
| higher contrast, but the exposure latitude is narrower. |
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Term
| The speed of a screen-film system relates to how much exposure is required to obtain what specific OD? |
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Definition
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Term
| Computed radiography (CR) referes to |
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Definition
| a photostimulable phosphor detector system. |
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Term
| CR imaging plates are typically made from |
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Definition
| BaFBr - referred to as barium fluorohalide |
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Term
| ____ laser light is used to stimulate the CR plate and _____ light is emmitted from the CR plate. |
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Definition
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Term
| The barium fluorohalide in a CR plate is doped with _______. This gives rise to the _______. |
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Definition
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Term
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Definition
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Term
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Definition
|
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Term
| A CCD chip is limited in ______? |
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Definition
| size, typically 2.5 cm by 2.5 cm |
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Term
| How are CCD chips read out? |
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Definition
| By using a "bucket brigade" method. The bottom row is read out, then the electrical charges in each column are shifted down one row and the process is repeated. |
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Term
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Definition
| The stage in the imaging process where the number of quanta is the lowest and therefore where the statistical integrity of the signal is the worst. |
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Term
| X-ray quantum limited detector |
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Definition
| The quantum sink (lowest number of quanta) occurs where the x-rays are absorbed. |
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Term
| What is the problem with a secondary quantum sink? |
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Definition
| The image quality is not commensurate with the x-ray dose used to make the image. |
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Term
| What kind of system might a CCD detector be used in? |
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Definition
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Term
| What is an alternative to a CCD array? |
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Definition
| CMOS, Complementary Metal-Oxide Semiconductor, array |
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Term
| What is different about a CMOS array from a CCD array. |
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Definition
| The detector elements in the CMOS array can be randomly addressed. |
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Term
| What is the biggest problem with a CMOS array? |
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Definition
|
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Term
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Definition
| Flat Panel Thin-Film-Transistor Array Detectors |
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Term
| The TFT is an electronic switch that is comprised of three connections that are called the |
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Definition
|
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Term
| Each dexel in a TFT array consists of |
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Definition
| the TFT, a charge collection electrode, and a storage capacitor |
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Term
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Definition
| Indirect detection and direct detection |
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Term
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Definition
| The percentage of area in each dexel of a TFT detector that is photosensitive. Typically 80%. The remaining area is taken up by electronics. |
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Term
| Most flat panel TFT detectors use what for a scintillator and why? |
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Definition
| CsI - It grows in columns that act as light pipes which reduce the lateral spread of light. |
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Term
| Direct detection TFT arrays use a semiconductor material that produces ________ when exposed to x-rays. |
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Definition
|
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Term
| The most commonly used semi-conductor in direct detection TFT array detectors is |
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Definition
| amorphous selenium - a-Se |
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Term
| The advantage of a direct detect TFT over an indirect detect TFT is |
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Definition
| almost no lateral spreading so the spatial resolution is much higher. |
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Term
| Lower kV settings _____ patient dose, but higher kV settings ______ subject contrast. |
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Definition
|
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Term
| Typical contrast agents and their atomic number. |
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Definition
| iodine (Z=53) and barium (Z=56) |
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Term
| x-ray fluence is _______ proportional to the mAs. |
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Definition
|
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Term
| Phototiming is more properly referred to as |
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Definition
| Automatic Exposure Control (AEC) |
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Term
| CsI forms in columns which helps to reduce the spread of light, why isn't CsI used in screen-film radiography? |
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Definition
| Too fragile and expensive. |
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Term
| Absorption efficiency of a screen is determined by |
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Definition
| The phosphor composition, effective Z and density, and its thickness. |
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Term
| Increased screen thickness results in |
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Definition
| Better absorption, but increased blurring and loss of spatial resolution due to the increased lateral diffusion of the light. |
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Term
| Quantum detection efficiency |
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Definition
| Describes how well an x-ray detector captures an x-ray photon beam. |
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Term
| What two factors characterize the ability of a detector system to detect x-rays? |
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Definition
| absorption efficiency and conversion efficiency |
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Term
| Conversion Efficiency refers to |
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Definition
how well absorbed x-rays are converted into signal.
This includes x-ray to light photon energy transfer in scintillators and electronic gain in digital detectors.
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Term
| For the same signal level, increasing conversion efficiency results in ________ image noise. |
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Definition
| increased image noise. (less photons are used) |
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Term
| For the same signal level, increasing absorption efficiency results in _________ image noise, but the _____ can be reduced. |
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Definition
| the same image noise, but dose can be lowered. |
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Term
| What does the exposure index indicate? |
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Definition
| The amount of radiation reaching the detector. It is not an indicator of the dose to the patient. |
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Term
| What is the deviation index? |
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Definition
It is a measure of how close an exposure was to the target value by comparing the exposure index to the target exposure index.
DI = 10log10(EI / EIT) |
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Term
| The deviation index of an overexposure is a _________ value. |
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Definition
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Term
| An acceptable range for deviation index is |
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Definition
|
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Term
| What is the algorithm for computing dual-energy subtracted images? |
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Definition
DE = α + β [Ln(IHI) - R Ln(ILO)]
R is selected to isolate bone or soft tissue. α and β scale the image for brightness and contrast. |
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Term
| In digital radiography, scatter reduces ________. |
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Definition
|
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Term
| In screen-film radiography, scatter reduces _______. |
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Definition
|
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Term
|
Definition
| Scatter-to-Primary Ratio, SPR = S/P |
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Term
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Definition
|
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Term
| SPR refers to the _______ deposited, not to the number of ________ deposited. |
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Definition
|
|
Term
| 2 things that increase SPR |
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Definition
| larger field size, thicker patient |
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Term
| Typical SPR for abdominal radiography |
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Definition
|
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Term
|
Definition
| Ratio of the height of the interspace material to the width of the interspace. |
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Term
| The best way to reduce scatter |
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Definition
|
|
Term
| Common materials used for interspace material in grids. |
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Definition
|
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Term
|
Definition
| Number of septa per centimeter. |
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Term
|
Definition
|
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Term
|
Definition
| When a grid is not located at the correct distance from the source. Grids must be aligned with the x-ray source. |
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Term
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Definition
| A grid that oscillates in a reciprocating motion during the exposure so the grid bars are blurred out. |
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Term
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Definition
| Describes the relative increase in x-ray intensity that is required when using a grid. |
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Term
| Typical Bucky Factor for abdominal radiography. |
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Definition
|
|
Term
| When would you not use a grid? |
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Definition
| When imaging thin anatomical structures because they have very little scatter. |
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|
Term
| Contrast degradation factor |
|
Definition
refers to the reduction in contrast due to scatter
CDF = 1 / (1 + SPR) |
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Term
| What are two methods for reducing scatter other than using a grid? |
|
Definition
| Air gap and slot scanning |
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Term
| What is the drawback of using an airgap to reduce scatter? |
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Definition
| Increased magnification (loss of spatial resolution from increased geometric blurring) and reduced field of view. |
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Term
| What is the drawback to using a slot-scanning system to reduce scatter? |
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Definition
| Longer scan times that lead to increased heat loading on the x-ray tube and the potential for patient movement. |
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Term
| What is a typical exposure to mAs ratio? |
|
Definition
|
|
Term
| Image quality improvement factor (Q factor) |
|
Definition
Used in digital imaging instead of Bucky factor.
It is the factor by which the patient dose can be reduced when the grid is used and still have the same CNR. |
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