Digital Imaging Flashcards
True or False?
A digital image has a continuous density across it.
False, a digital image has discrete pixels with numerical density values.
What are the two types of digital radiography?
Computed radiography (CR) and Digital radiography (DR)
What is the difference between CR and DR?
CR uses imaging plates that need developing in a processor whereas DR acquires an image directly without using a processor.
True or False?
In digital radiography, the film is the detector and storage medium
False, this is screen-film radiography. In digital radiography, the detector only generates the image which is then stored in a digital medium.
List the 4 steps of the digital imaging series
- Generation
- Processing
- Archiving
- Presentation
Name 4 types of DR (Direct Radiography) digital detectors
- Photoconductor FPD (Direct Conversion)
- Scintillator-TFT (Indirect Conversion)
- Image intensified (Indirect)
- Selenium-drum (Direct)
- Scintillator-CCD (Indirect)
What are the 2 types of digital detectors?
Indirect conversion & Storage phosphors
Name and explain the main functions of a digital image detector
Capture- Absorbing X-rays
Coupling- Linking between the capture element and the converter
Conversion- Change of X-ray energy into electrical signal which can be read (can be direct or indirect)
What photoconductor absorbs X-rays?
Amorphous Selenium
What happens to the electrical charge generated by the DR photoconductor?
It is held in storage elements (e.g. capacitors) on a 2 dimensional detector array, which produces a stored charge distribution
What are the types of CCD indirect radiography systems?
- Lens-coupled CCD based system
- Slot-scan CCD based system
What is the function of Scintillator materials?
To produce a brief burst of light when struck by high energy rays like X-rays
Finish the sentence.
“In direct digital radiography (DDR), X-ray energy is converted directly into..”
..electrical signal.
List 1 similarity and 1 difference between a Selenium-drum detector system and a Flat-panel detector system
- They both use Amorphous Selenium Photoconductors
- A flat-panel system uses TFT detector arrays to convert X-ray energy into electrical charge whereas selenium-drum detector systems use A/D converters to do so.
What does a photoconductor made of a-Se do?
Absorbs X-ray energy and generates electrical charge
True or False?
In indirect DR, each cell in the detector has a top layer of a scintillation crystal made of TI (Thallium) activated by CsI (Caesium Iodide), which produces light when hit by X-rays.
False. It is a layer of a scintillation crystal made of CsI activated by TI on top of the detector cells.
List 3 similarities between Direct and Indirect DR.
- Quick reading (about 1 second)
- The digital image is sent to an output device
- A 2D array of electrodes containing switching elements in a thin film transistor (TFT) reads the stored charge distribution
- Sampled electrical charge values are counted to produce a digital image
Give 2 features of digital imaging
- Wide exposure latitude which enable a wide range of exposures that give an acceptable image
- Wide “dynamic range” that records a lot of densities in one exposure
- CR plates that are sensitive to X-ray fogging
- Good resolution
True or False?
There is a greyscale of 10,000+ in CR and DR imaging as opposed to film imaging where there is a greyscale of 30
True.
What value is recorded that indicates whether or not the X-ray exposure of the digital image is within the recommended range?
Detector dose index (DDI)
What do the following terms mean and on which systems are they found on?
EI, logM, S value, REX, DEI, DI
- EI: Exposure Index (Kodak/Siemens/Philips)
- logM: Log median exposure (Agfa)
- S value: Sensitivity number (Fuji)
- REX: Reached exposure value (Canon)
- DEI: Detector exposure index (GE)
- DI: Deviation Index (Toshiba)
How many mAs are equivalent to a dI value of +3 (excessive)?
60
What causes a grainy (noise) image?
Under-exposure
Define what is meant by the term “Quality vs. Dose”?
“Quality versus dose” refers to radiographers over-exposing in order to get high quality images and avoid image noise.
When does the effect of variations in numbers of individual X-ray photons become noticeable?
At very low mAs values, where random fluctuations in X-ray beam intensity starts to show (Quantum Mottle)
Apart from Spatial Resolution, what 2 other factors affect digital image quality?
Contrast & Noise (SNR)
What does MTF mean?
MTF (modulation transfer function) is the capacity of the input signal at a given spatial signal to its output
List 4 sources of noise
- Random electrical fluctuations in circuits
- Detector faults
- Quantum Mottle (occurs with small numbers of X-rays and very sensitive plates
- Pressure on digital plates
What is the difference between signal from anatomical detail and signal from noise?
Signal from anatomical detail is constant over time whereas with signal from noise it is random and variable over time.
What decreases as the DQE is increased for the image SNR and exposure conditions?
The radiation exposure that the patient requires.
