Radiology Digital Films

Digital film changes from conventional.

Digital is easier to take retakes, no chemical developments, film replaced by reusable image detector.

How does digital x-ray work?

Detector receives X-rays → Intensifying phosphor screen emits light → digital plate transforms light into electrical image and then pixilated image on computer.

Limits of conventional film

Small mistakes can ruin image. The image cannot be adjusted once made. Time, radiation, stress, and cost associated with retakes.

Advantages of digital film

kVp has little effect on contrast of image. Techniques charts do not vary greatly for body part radiographed or patient thickness. Image can be altered for contrast and can be zoomed. Possible to see soft tissue and bone detail in single image.

Disadvantages to digital radiography

Cost (expensive equipment), Training and learning curve. - gross exposure and image enhancement can cause photos to not be correct.

3 types of digital radiography

Computed radiography (CR), charge-coupled device (CCD), Flat Panel Detectors (DR).

Digital image characteristics: RESOLUTION (Pixel number determines)

Related to the size of the pixel – larger number = smaller in size, the sharper the image

Digital image characteristics: IMAGE NOISE

Unwanted variations of gray shades. Is inversely related to image contrast.

Digital image characteristics: Quantum noise

Random distribution of electrons striking image receptor. Higher kVp minimizes but reduces safety factors.

What kind of screen does Computed Radiography have?

Phosphostimulable phosphor (PSP) detector screen.

What is the difference between a conventional screen and a PSP screen in CR?

PSP screen STORES x-ray energy (a conventional screen does not) then the cassette is set in the CR reader which “develops” it and sends it to computer then is wiped clean. Thought of as filmless cassette.

What is important to not forget about when getting CR image?

Don’t forget to put it in processor! by hour 8 25% of the image is gone.

How is the x-ray energy on the CR plate turned into a digital image?

The image reader uses a focused laser beam to release the electrons. The release of light is proportional to the amount of exposure. The released light is changed from electrical signal to digital signal.

How is the CR plate erased?

It is exposed to a flash of white light to “reset” and erase the last image.

What is the CR plate (IP) plate prone to?

To “ghosting”, where the image is not fully erased. The plate needs to be replaced at this point.

Is a cassette, image processor needed for DR?

No

Two types of DR systems

Direct and indirect detectors

What is Detective Quantum Efficiency (DQE)

Describes the sensitivity and accuracy of the detector system. Expressed as % of x-ray energy converted into the image.

What % of DR is from x-rays and from computer filling in?

20% is filled in 80% is the x-rays converted into the image.

DR Direct Detector- how it works

Uses thin film transistor (TFT) to detect and display image - contains Selenium, TFT collects electrons to create the digital image.

two types of indirect detectors

TFT with a scintillator. Scintillator fluoresces. Charged Couple Device (CCD) with a Scintillator. Both the scintillator fluoresces when exposed to x-rays, the light is used to create digital image.

Charged Couple Device (CCD)

Same tech as most digital cameras. Has a rare earth screen coupled to pixelated light detector.

How large is the biggest CCD chip?

8 by 8 cm.

Flat Panel Detector (DR) How does it work?

Silicon flat panel detects light and digitizes it to be sent to computer. Self contained unit is used on horses.

What kind of intensifying screen does a Flat Panel Detector have? (DR)

Scintillator as an intensifying screen.

How big is a full sized Flat Panel Detector?

17 by 14 inch.

Image processing digital.

Computer software converts signal to viewable radiograph, tries to optimize image and minimize artifacts. Can rescale image to correct most exposure errors. Uses “look-up table” to scale contrast. Windowing controls range of density. Labeled with patient info. Quality of image effected by Monitor used. Medical grade monitor is best.

What is the Picture Archiving and Communications System (PACS)

The processes, procedures, and technology, required for creating, distributing and archiving digital images. Includes the imaging device and any associated readers, viewers, the computer (and server) as well as workstations that store and retrieve the images.

What is Digital Imaging and Communications in Medicine. (DICOM)

Used in Vet med and human. Each image is embedded with extensive, specific, and detailed info. Permanent “stamp” in the file. DICOM ensures that images can be transferred and read by any DICOM workstation software.

Viewing Digital Images

Ability of viewer to diagnose and view an image depends heavily on monitor quality. Medical grade, gray-scaled computer is best, but expensive.

Digital Artifact: Uberschwinger artifact.

Radiolucent halo around metallic orthopedic implants that can mimic implant infection and loosening. Occurs when density of adjacent objects is significantly different.

Radiation safety in DR

X-ray tube and radiation is same as film based and CR radiography. Purposefully increasing exposure just to be safe is irresponsible.