Digital Subtraction Angiography

3 subareas of DSA are:

- Image Formation - Roadmapping - Patient Dose

how the time interval difference mode works

- Image acquisition begins 5s after injection at the rate of 15 images per second for 4s. - A total of 60 images is obtained. - Images are identified as frame numbers 1 through 60. - - Each image is stored in a separate memory address as it is acquired.

Why is TID used for cardiac evaluation?

- In real time the images observed convey the flow of contrast medium dynamically. - each TID image shows it to be relatively free of motion artifacts but with less contrast than mask-mode imaging.

process of mask mode

- Injector is fired, and after a delay of 4 to 10s, before the bolus of contrast medium reaches the anatomic site, an initial x-ray pulsed exposure is made. - The image obtained is stored in primary memory and is displayed on video monitor

the two most common temporal subtraction are:

- Mask Mode - Time Interval Difference (TID) Mode

remasking

- Post-processing that selects another mask image to obtain the best subtracted image of the anatomy of interest - If, on subsequent examination, the initial mask image is inadequate because of patient motion or improper technique or for any other reason, later images may be used as the mask image

Time Interval Difference (TID) Mode

- Some examinations call for each subtracted image to be made from a different mask and follow-up frame - Sequentially obtained images are subtracted in a time-interval difference study - Produces subtracted images from progressive masks and following frames.

There are two methods for image formation:

- Temporal Subtraction - Energy Subtraction

Mask Mode

- The patient is positioned under normal fluoroscopic control to ensure that the region of anatomy under investigation is within the field of view. - Mask mode results in successive subtraction images of contrast-filled vessels.

process to how the mask is subtracted from subsequent images

- This mask image is followed by a series of additional images that are stored in adjacent memory locations. While these subsequent images are being acquired, the mask image is subtracted from each and the result stored in primary memory. At the same time, the subtracted image is displayed on video monitor B. - Pre-injection, contrast, subtraction

use of power injector on mask mode

- Venous -deliver 30 to 50mL of contrast material at the rate of approximately 15 to 20mL/s - Arterial -deliver 10 to 25mL of diluted contrast material at 10 to 12mL/s

what happens when a TID of four images (268ms) is selected?

1. first image to appear will be that obtained when frame one is subtracted from frame five. 2. second image will contain the subtraction of frame two from frame six 3. third will contain the subtraction of frame three from frame seven and so forth.

A typical examination may require a total of ___ images in addition to the mask image.

30

each image was obtained from a _____ ms x-ray pulse, which is the time required for one video frame.

33-ms

Hybrid Subtraction

DF system that is capable of combining temporal and energy subtraction. Produces high contrast image

why is image contrast enhanced in hybrid subtraction?

Image contrast is enhanced because of reduced patient motion between subtracted images.

temporal Subtraction

Image obtained at one time is subtracted from an image obtained at a later time

imaging sequence after acquisition of the mask can be controlled in two ways:

Manually or Preprogrammed

roadmapping

Overlaying of two images. A stored image is superimposed upon a current fluoroscopic image, or a current image can be copied for storage and later used in roadmapping (angiograms to see landmarks - not diagnostic)

advantage of digital fluoro

Patient dose is reduced as compared to conventional image intensified fluoroscopy

how to reduce patient dose in DF

Pulse rate can be adjusted, further reducing radiation dose (may result in a more choppy looking video).

what is "subtracted" in DSA images?

Radiopaque structures such as bones are eliminated ("subtracted") digitally from the image, thus allowing for an accurate depiction of the blood vessels.

image Integration

Several video frames (four or eight) may be summed in memory to create each image.

Energy Subtraction

Uses two alternating x-ray beams operating at two different energy levels-subtracted image results from differences in photoelectric absorption in contrast vs. non-contrast status of subject

mag mode in flat panel fluoroscopy

With flat panel technology, no image intensifier is used, so using mag mode is an electronic process rather than a geometric one.

Why is patient dose increased during image integration?

because a greater number of image frames are acquired

What happens when the imaging system is changed from the fluoroscopic mode to the digital fluoroscopic (DF) mode?

increase in x-ray tube current of 20 to 100 times the fluoroscopic mode and the activation of a program of pulse image acquisition

Digital Subtraction Angiography (DSA)

is a fluoroscopic technique used extensively in interventional radiology for visualizing blood vessels.

DF images appears as a continuous dynamic video, but are actually

single frames (pulse-progressive fluoroscopy) pulsed to fill one or more 33-ms video frames

patient dose in mag mode

slight radiation dose increase, unlike with II fluoroscopy where it results in a big increase.

preprogrammed control of image sequence

the computer controls data acquisition in accordance with the demands of the examination. Example: To evaluate carotid flow after a brachial vein injection, the radiologist could inject contrast media and acquire a mask image 2 s after the injection. After another 2-s delay, images are obtained at the rate of two per second for 3 s, one per second for 5 s, and one every other second for 14 s

If the intended mask image is technically inadequate and maximum contrast appears during the 15th image, a better subtraction image may be obtained by

using image number 5 as the mask rather than image number 1

results of image integration

•Video system is relatively slow to respond •Video noise may be high •Improves contrast resolution •Increases the patient radiation dose