DIGITAL SUBTRACTION ANGIOGRAPHY DR.SHRIKANT PAWARA UNDER THE GUIDANCE OF- DR.TEJAS SIR AND DR.RATHI SIR

DSA It is a digital imaging technique for better visualization of contrast filled vessels.

In this technique pre-contrast image is digitally subtracted from post-contrast image to provide background suppression.

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Processes involved in DSA : I Preprocessing : Digitalization : Analog to digital conversion. II Processing :- Subtraction Techniques III Post-processing :- Manipulations to enhance visibility IV Information Extraction

Why Digitalization For all the image processing and manipulations done in DSA, computer is must. Because computer can handle data only in bit format, conversion is necessary for subtraction techniques to be applied by computer. Also Easy storage and retrieval is possible

Basic operation :- Central control computer is incharge of all the other components of the image processor Suppose we tell the computer to run a series of a certain number of exposures and to store each image of the series in the digital form for later use. Computer uses first image as a mask Display each subsequent images in subtracted form. We set the exposure techniques on generator and instruct the computer to begin. It sets up the digitizer to convert entire TV frame into digital image and starts the digitizer at appropriate time. Meanwhile computer has instructed what to do with first digitized image which is simply to place the image in appropriate form and to store it in a particular block of image memory. The computer tells the digital disc where that particular block is located in image memory, how big the image is, and when to start storing the image.

ANALOG to DIGITAL conversion One of the most important step Analog information is any information represented in continuous rather than discrete fashion. Digital information is any information that is represented in discrete units. Video images are in form of lines with each point on line represented as voltage Digitalization involves conversion of all the points of voltage into pixels of discrete value.

ANALOG DIGITAL

MATRIX SIZE and PIXEL Pixel is smallest element of digital image. The size of a pixel determines the smallest detail visible on the image A digital image is normally composed of a two-dimensional (square) matrix of pixels The matrix size of an image is used to describe the number of pixels in each row and column of the image. .As matrix size decreases, larger pixels are required to maintain a constant field of view. Less resolution. Binary number system :This system is used by computer to store image data in the form of pixel. Uses 0 and 1 to store bits.

MATRIX SIZE and PIXEL

512 x 512 256 x 256 128 x 128 64 x 64 32 x 32 16 x 16

PROCESSING…

TYPES OF SUBTRACTION Mask mode subtraction Time interval difference subtraction Dual energy subtraction Hybrid subtraction

1.Mask mode subtraction Most widely used process of subtraction. In which temporal subtraction is done.

Mask mode subtraction It involves initial acquisition of a frame of region of interest. Then a second image is taken and stored as mask image. This mask image is subtracted from subsequently acquired images and show only contrast filled structures.

Mask mode subtraction

MASK SUBTRACTION

Disadvantage If any movement occurs after acquisition of mask image misregistration occurs in the subtracted images. This can be overcome to some extent by pixel shifting.

2.Time interval difference subtraction… Another mode of temporal subtraction where a consecutive previous frame is subtracted from current frame(e.g. frame 1 from frame 2, frame 2 from frame 3 and so on). This technique is very useful in cardiac imaging where there is rapid motion.

Time interval difference subtraction…

Time interval difference subtraction…

3.Dual energy subtraction Another technique in which the region of interest is exposed to higher kV(120 to 130kV) and lower kV (70kV) at very short interval (about 50ms). Then, the higher kV image is subtracted from lower kV image to produce the image.

Dual energy subtraction

Advantages This technique has the advantage of elimination of motion artifacts. It does not require the acquisition of images before arrival of contrast material as in mask subtraction. Provides selective cancellation.

PROBLEMS OF DUAL ENERGY SUBTRACTION More complex x-ray machine is needed to rapidly switch kvp and mA Due to difference in beam hardening in soft tissue and bone improper subtraction results (Higher kVp x ray beams will have different beam hardening than lower energy Not a problem in mask subtraction because only one x ray beam energy is applied.) But advantage is that minor motion effects can be tolerated. Less exposure time. as both images are taken at same patient position as against mask subtraction technique

4. Hybrid subtraction Is a combination of dual energy and temporal subtraction. Advantage – eliminates effect of patient motion.

Hybrid subtraction

Techniques used in DSA Road map technique. Fluoroscopy fade technique. Dynamic 3d road mapping technique.

1.Road map technique In which static fluroscopic image is subtracted from densely opacified vessel. Here , a short contrast run of vessel is done under fluoroscopy to select the frame with maximum opacification of the vessel as the road map mask. Then subsequent live fluoroscopic image is subtracted from the road map mask for visualisation of the vessel and the catheter/guide wire.

Road map technique….. This technique is very useful in placement of catheters and guide wires in complex and small vessels.

Limitations… Motion after road map acquisition may affect subsequent intervention.

2.Fluroscopy fade technique In which a reference DSA image is overlaid on the real time fluoroscopic image.

3.Dynamic 3D road mapping New development allows projection of 3D reconstructed vessel on live 2D fluoroscopic image.

Peripheral DSA Can be performed with single contrast injection using stepping technique. Two types- 1)Stepping table technique 2)Stepping gantry method

Stepping table technique In this table moves into three stations with x ray tubes and detector remaining fixed.

Stepping gantry method In this method the tube –detector or image intensifire moves keeping the table fixed.

In both methods precontrast images are acquired at different stations and stored as mask image. Subsequently matching post contrast injection images are taken at same position and subtracted from the corresponding mask image to produce clear image of peripheral arteries.

Benefits Significantly reduce use of contrast. Reduce examination time.

Disadvantage Increased chances of movement between precontrast and postcontrast images.

An important advancement in digital fluoroscopy is rotational angiography. In this x ray tube-detector system rotates through visually 90 degree to 180 degree while acquiring continuous images.

POST PROCESSING

Mask pixel shift It is a software modification feature. Used when smaller patient motion occurs after the mask image is acquired. By shifting the pixels of the mask image reregistering of the mask with post contrast mask image is possible- thus obviating motion artifacts This technique may be manual or automatic.

Remask It is a similar feature where another mask image is selected which is temporally closer to the contrast image. This is useful when patient motion occurs prior to contrast image , but after initiation of the acquisition.

Image summation It is property in which two or more frames of DSA acquisition are summed into a single image.

Benefits of image summation This is beneficial when rapid acquisition opacifies part of a vessel in each frame and summation adds up the frames to produce a single image showing the entire vessel.

Stacking Similar technique used in carbon dioxide angiography. Where the fragmented boluses of the gas are added to produce a complete picture of the opacified structure.

Landmarking It is a feature in which lesser intensity of original image is added to the subtracted image. This provides anatomical landmarks in subtracted images, useful in subsequent intervention.

NOISE SMOOTHENING NOISE SMOOTHENING The operation has suppressed small structures and had little effect on large structures. Final image is blurred vision of original. Noise smoothening is attempt to decrease the visual prominence of noise so that low contrast objects of moderate to large size may be better appreciated The techniques operates by reducing the statistical fluctuation in each pixel by averaging the pixel with its closest neighbours. the first pixel is smoothened by averaging nine nearest neighbours. The final image is blurred image of the original.

NOISE SMOOTHENING

EDGE ENHANCEMENT Substracting noise smoothened image form the original image results in edge enhancement

d. Edge enhanced image (a + c) EDGE ENHANCEMENT b. Noise smoothening a. Original image c. a - b d. Edge enhanced image (a + c)

EDGE ENHANCEMENT

Information extraction Extraction of numerical or graphic information from images Quntitative algorithms are available to measure:- Stenosis sizing:- Length, Area Distance Measurements Perfusion- functional Studies Vessel Tracking Relative flow and volumetric measurement

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