Digital Imaging and Digital Subtraction (DSA)

components of a digital imaging system

*television image chain *image intensifier *electronic video signal *television camera *digital processing unit

progressive mode

- every line is rad in order, involves TV camera scanning being paused while while an exposure is made to eliminate motion

Signal-to-noise ratio (SNR)

-A method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image -Digital requires about 1000:1 SNR

television camera

-Plumbicon, CCD, or Saticon -forms an image by electronically scanning a target semiconductor

analog-to-digital convertor (ADC)

-component that converts the video images from the TV chain into digital form

light diaphragm or filtering device

-controls the amount of light from the image intensifier that reaches the television camera tube -w/o the use of *apertures*, either not enough light or too much light (saturation) is allowed on to reach the television camera

temporal subtraction

-digital subtraction of post-contrast angiography image from pre-contrast images over a time sequence -most commonly used method of DSA -allows soft tissue subtraction with energy subtraction and bone subtraction with temporal subtraction

pulsed digital fluoroscopy xray beams

-done so that the dose rate is lower than that for continuous analog fluoroscopy -allow 1-6 images per second -have less noise but require high mA

image reregistration or remasking

-helps to eliminate motion artifacts helps correct misregistration when the mask and the series images are not aligned

benefits of DSA

-image manipulation (window level and width) -instantaneous viewing of images -sensitivity to low contrast images -smaller amounts of contrast injected -low cost -spot filming: femoral bifurcation, carotid bifurcation, or oblique views of cerebral aneurysms

slow mode

-inadequate for real-time DSA -limited to 7.5 frames per second

television image chain

-one of the most important links in the digital system

digital subtraction angiography (DSA)

-process of digital radiographic imaging of the blood vessels that "subtracts" or removes structures not being studied -operates at levels of an average of 10 to 20 mA -image is processed as the run is performed, so that the subtraction image can be viewed and the exposure terminated once the area of interest is viewed

image intensifier (II)

-proportional to the amount remnant x-ray photons -II input phosphor is coated with x-ray sensitive phosphor (cesium iodide (CsI))

interlaced mode

-reads information in two separate fields and then, combines or interlaces the information -avoids perception of flicker (detected by human eye below 50 pulses per second) -unnecessary exposure because some images are discarded

Road-mapping

-small amount of contrast injected for means of tracking or tracing -vascular path is remembered by the system and and acts as a guide

Dual energy subtraction

-two separate exposures be made, one right after another, but at different energies (kVp and mAs) -includes k-edge subtraction, time-interval differencing (TID), and temporal filtering

digital fluoroscopy (DF)

-uses a digital image processor -allows the last image to continue to be displayed on the monitor each time the fluoro switch is released

Window width controls

Displayed image contrast

Window level controls

Image manipulation parameter that changes image brightness (density) on the display monitor, usually through the use of a mouse

edge enhancement

a process by which the visual system makes edges as visible as possible, allows for better visualization of small details

Computed Radiology (CR)

an image plate is exposed, then scanned with a laser, resulting in analog to digital transformation

frame integration

averages together several frames and diminishes the effects of electronic and quantum noise

K-edge subtraction

based on k-shell absorption of the photoelectric effect

energy subtraction

based on kVp of the x-ray beam and the attenuation differences between the patient and the iodine in the contrast medium -reduced motion artifact

low lag, low noise

characteristics needed in the TV chain of a DSA system include __________ and _________.

television scan modes

component of the actual television monitor

image intensifier

electronic vacuum tube that converts the remnant beam to light, then to electrons, then back to light, increasing the light intensity in the process. It consists of five basic parts: the input phosphor, photocathode, electrostatic focusing lenses, accelerating anode, and output phosphor

Ghosting

excessive lag produces

quantum mottle

fluctuation in xray photon intensity cproducing noise on the image

Ghosting artifact

image viewed contains information belonging to the previous images

electronic video signal

produced at the output phosphor in proportion to the amount of light, but exceedingly intensified

landmarking

small amount of the mask image is added back to the subtracted image. Used to assist surgeons in preparation of invasive procedures

time-interval differencing

subtracted image is separated by some fived interval of time

low contrast

subtraction studies are usually performed on __________ studies, it is important that noise be kept to a minimum.

pixel shifting

technique use if mask image is unacceptable because of patient motion between images

DSA

the image from the image intensifier is converted from analog data into digital information and processed by the computer for display and storage

digital processing unit

the video signal produced is sent to the __________________.