EXAM 1
Creating the Digital Image
Creating the Digital Image
What index tells you how far your exposure is from the target exposure index?
Deviation index (DI)
DI is a standardized index that tells you how far you missed the target exposure: positive is over, negative is under
What does TFT stand for?
Thin film transistor
After the x-ray signal is stored, the gate line activates the TFT, allowing the charge to flow from the capacitor onto the data line for readout
What is the maximum number of hours a CR imaging plate should be left before being placed in the processor?
8 hours
What term describes how light or dark an image appears to the viewer?
Brightness
Note: Luminance is what the monitor emits. Brightness is how light/dark it appears to the viewer
What is the active element in a CR imaging plate?
Barium fluorohalide doped with europium
The PSP plate is barium fluorohalide, but it’s the europium doping that allows it to trap electrons and store the latent image
What is the term that describes the number of pixels in a specific area?
Pixel Density
What term describes how efficiently a digital imaging system converts x-ray input into a useful image?
Detective quantum efficiency (DQE)
DQE is how efficiently the detector turns x-rays into a useful image, higher DQE means better image quality with less dose
What component in a digital detector converts x-ray energy directly into an electrical signal?
Photoconductor
Changing the slope of the look-up table curve will affect what image property?
Contrast
The LUT is a table, but we picture it as a curve... the steeper the curve, the higher the contrast
What term describes the movement of the imaging plate through the CR reader?
Slow scan
Slow scan is the plate moving through the reader while fast scan is the laser moving across it
During digitization, which step assigns a numerical value to each pixel?
Quantization
Quantization gives the pixel a number, the LUT decides what shade of gray that number becomes
In a scintillator-based DR system, what is the correct sequence of signal conversion?
_____ -> ______ -> ________
X-rays -> Light -> Electrical signal
Indirect DR uses a two-step process: x-rays become light, then light becomes an electrical signal
What measure describes how well a digital imaging system preserves fine detail and sharpness?
Modulation Transfer Function (MTF)
MTF tells you how sharp the image is... how well fine detail is preserved
What process adjusts for exposure differences so a digital image displays with proper brightness?
Automatic rescaling
Rescaling fixes brightness after the fact... it doesn’t fix the exposure, it just makes it look normal
What term describes a digital system’s ability to display subtle differences in shades of gray?
Contrast Resolution
What term describes a digital system’s ability to detect a wide range of x-ray exposures, allowing more tissue densities to be visible?
Dynamic range
Dynamic range is how many different exposures the system can handle, more range means you can see more tissues in one image
What active material is used in indirect DR systems to convert light into an electrical signal?
Amorphous silicon
In indirect DR, the scintillator makes light, while the active material (a-Si) turns that light into signal.
Which type of digital imaging system uses amorphous selenium?
Direct conversion DR
Amorphous selenium = direct conversion: x-rays go straight to an electrical signal, no light step
What term describes the range of pixel values selected from the histogram that represents the anatomy of interest?
VOI (Values of Interest)
The VOI tells the system what to focus on, and the LUT maps those values to the image
What program focuses on patients and quality of service?
QA
QA focuses on the overall patient experience and quality of service, not just the equipment
What term describes the range of exposures that still produce a diagnostic-quality image?
Exposure latitude
Dynamic range is what the system can capture, latitude is the range that actually gives you a good image
What process releases trapped electrons to reveal the latent image in a CR imaging plate?
Laser scanning of the PSP plate
The image is stored as trapped electrons; scanning the plate releases them to create the signal.
Increase the time between exposures
Image lag happens when the detector hasn’t fully reset, giving it more time prevents carryover
As speed class decreases in digital imaging, what happens to image noise?
Noise decreases
Lower speed means more x-rays, and more x-rays means less noise, but higher dose
Increasing bit depth increases what property an individual pixel has the ability to display?
Number of gray shades
Bit depth sets how many shades are possible, but processing decides how many are actually used