Digital Radiography Flashcards
What are the advantages of Digital Radiography? (3, 1+3)
- Improved dynamic range of images
- Digital processing
- Digital storage, retrieval & transport (PACS)
- Potential for:
- Dose reduction to patients
- Low running costs
- Computer aided detection
How do magers and Receptors work in Digital Radiography? (2)
- In all digital ragiography systems, the image is first captured using a sensor.
- The image is then transformed into a series of binary numbers (0 & 1’s), that typically has 12 – 16 bit accuracy, before being displayed on a monitor.
What are the two types of digital radiography sensors? (1+2)
- In the first step, two main types of sensors are used in dental radiography:
- Computed radiography (CR)
- CCD/CMOS.
What is the most common method for digital dental radiography and why? (2)
- Computed Radiography
- As it can be retrofitted to existing equipment
How does Computed Radiography work? What is this commonly called? (2)
- Uses a storage phosphor that stores the energy of x-ray photons and then releases it as light in response to stimulation with a laser.
- Commonly referred to as photostimulable phosphor (PSP).
What is the most common PSP? What is it made up of? (1)
- Barium fluorohalide doped with Europium (BaFX:Eu), where the halide (X) is a mixture of bromide (85%) and iodide (15%).
What does the Image Receptor look like? (1)
- What does the Image Receptor look like? (1)
What happens to the exposed image receptor? (1)
- The exposed image receptor is scanned by a red laser beam in the CR reader, which releases the trapped energy as visible light.
Describe the emitted light and its intensity (1)
- An array of optical fibres direct the emitted light (blue) to one or more photomultiplier tubes to measure its intensity. The intensity of the light that is released from the PSP is directly related to the intensity of the x-ray photons.
What happens to the receptor after being exposed to emitted light? (1)
- The receptor is scanned sequentially across its width as it moves progressively through the scanning beam.
Describe the sensors in Digital Radiography (4)
- Sensors are solid state devices (CCD, charge coupled devices; CMOS, complementary metal oxide semiconductors).
- These sensors cannot be manufactured in sizes bigger than about 5 cm2, limiting their use to intra-oral radiography.
- The sensors are slightly bigger than conventional film, but are much thicker – up to 10mm.
- Most are attached to a docking station with a wire cable, but wireless sensors that use WiFi are now available
What is the role of CCD/CMOS sensors in Digital Radiography? (1)
- CCD/CMOS sensors use indirect conversion of the x-ray pattern into an electronic signal using a light emitting phosphor (caesium iodide).
What does the silicon of a digital radiographic receptor do? (1)
- The silicon of the receptor acts as a photodiode and converts the light emitted by the phosphor to electrical charge.
What forms the radiographic image? (1)
- The charge pattern formed from the pixels in the sensor matrix forms the radiographic image.
Compare the three different radiographic methods in terms of resolution, image quality, processing quality and cost. (Table)
Film Resolution: 8-15 Image Quality: Good Processing Time: 10min Cost: Cheapest
CR Resolution: 3.5-5.5 Image Quality: Better Processing Time: 10-30s Cost: Cheap
CCD/CMOS Resolution: 4-5 Image Quality: Better Processing Time: 5s Cost: Expensive
What is the main advantage of digital imaging? What tools are available for this? (2)
- The ability to manipulate the image in order to increase its diagnostic value.
- E.g. Enhancement and noise reduction, Windowing
How many pixels are there usually in digital images? (1)
- Usually 512 x 512
How do the pixels make up the image? (2)
- Each pixel is assigned a numerical value that is related to the intensity of the signal in that part of the image.
- Because in film radiography the image is a negative (black = high radiation), this is maintained in digital images so that dark pixels have high values and white pixels low values.
What is windowing? (3)
- The 256 grey scale is spread over a narrow range of pixel values (the window) and it is centred around a level.
- The 4096 levels of pixel intensity can only be displayed over 256 shades of grey on a computer monitor.
- If all units were displayed on a single image it would have very poor contrast (ability to distinguish different regions of the image).
What advantages does digital radiography have in terms of PACSs and what does it stand for? (6)
- Picture Archiving and Communication Systems
- Images can be instantly available in any location, not just the hospital
- Images can be viewed simultaneously at different locations
- Images cannot get lost
- Film stores are eliminated
- Imaging can be integrated with other electronic records
How is PACSs particularly useful in terms of carious modalities? (1)
- Image data from the various modalities (x-ray, ultrasound, CT, MRI etc.) are all in the same format that can be recognized and used by the system – the current standard is DICOM (Digital Imaging and Communications in Medicine).
What is PACSs also integrated with? (1)
- PACS is also integrated with RIS (Radiology Information System), which stores data regarding previous imaging investigations.
Describe the high quality Monitors in Digital Radiography (4)
- Images are accessed via monitors connected to standard PC’s.
- Reporting monitors (used by Radiologists) are high-quality and incorporate a wide range of software tools for image manipulation.
- Monitors must be calibrated to the DICOM standard, so that the greyscale rendering of the image is optimised to the performance of the human eye.
- This can be checked using a standard test image developed by the Society of Motion Picture and Television Engineers.
Describe the lower quality Monitors in Digital Radiography (2)
- Elsewhere, in clinics etc. there are review workstations with lower specification monitors and limited imaging software.
- The images displayed on these monitors is not as good as those on Reporting workstations, and caution is needed when interpreting the images.
