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COLLEGE OF ENGINEERING GUINDY, ANNA UNIVERSITY, CHENNAI-25

Digital SubtractionAngiography

This report is a part the seminar of Medical Imaging and Radiotherapy

Prepared by:

S. Dinesh

Reg. No. 2011224001

ME / Medical Electronics

II semester

5/2/2012

Digital subtraction angiography refers to techniques which subtract two images that are

obtained before and after contrast media is administered to the patient for the purpose of

studying blood vessels


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CONTENTS

1. Introduction2. Block diagram3. Digital image processor4. Image memory and integration5. Image subtraction6. Contrast medium7. Indications & contradictions8. References


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INTRODUCTIONDigital subtraction angiography refers to techniques which subtract two images that

are obtained before and after contrast media is administered to the patient for the

purpose of studying blood vessels (angiography).

Digital subtraction angiography (DSA) is a new, and rapidly developing, technology in

the field of diagnostic radiology. DSA is one of several computer-assisted radiologic

tools for diagnosing conditions associated with the internal structure of blood vessels.

The technique usually involves injecting contrast medium into the veins and

measuring over time the changing concentration of contrast medium passing through

the vascular structures of interest. Through the use of a computer, the images before

the contrast injection are subtracted from those after injection to give a numerical

representation of the arterial structure under study. This relatively noninvasive

technique can be performed on an outpatient basis with very low risk of morbidity

compared to conventional and (invasive) techniques such as arteriography. DSA has

been shown to have important clinical uses in diagnostic studies of the carotid, renal,

intracranial, and peripheral arteries, the aorta and in pulmonary studies. There are

reasonable expectations that this procedure will develop to the point where it willhave wide applicability in the diagnosis of coronary artery disease in the next several

years.

DSA systems work in the manner depicted in figure as follows: a contrast medium is

injected intravenously; X-ray detection of the contrast medium produces 1 to 30

exposures per second (before and after the injection of contrast medium); and arterial

images are converted from analog to digital form and transmitted to a computer-

storage complex. The digitalized image information makes it possible to subtract

the pre-contrast images from those obtained after contrast injection so as to visualize

arterial structures without direct arterial puncture and injection. The data can be

recalled for viewing on a video screen, and successive images created through

subtraction techniques which allow the contrast of the arterial structures to be

visualized for the detection of abnormalities. The purpose of the subtraction process

used in DSA is to eliminate (or factor out) the bone and soft tissue images that would

otherwise be superimposed on the artery under study . The serial images show


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changes in the contrast appearance over time (temporal subtraction) and at varying X-

ray intensities (energy subtraction). Most DSA examinations require 25to 45 minutes

to perform , if there are no technical complications (e.g., difficulties with

catheterization), and can be performed on an outpatient basis. This is a considerable

advantage in safety and cost over most standard arteriographic examinations, whichrequire at least overnight observation of the patient in the hospital to detect post-

procedure arterial obstruction or hemorrhage. However, a small number of the latter

have been safely performed on an ambulatory basis in recent years.

FIG: Digital Subtraction Angiography

According to this picture, you could find the status of blood vessel whether it is in

normal or abnormal situation. For example, you could easily find the positions where

it becomes narrow, where it has a tumor, where the blood is obstructed, where it is

deformity or malformation and so on.


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FIG: DSA SETUP
http://en.wikipedia.org/wiki/File:Herzkatheterlabor_modern.jpeghttp://en.wikipedia.org/wiki/File:Herzkatheterlabor_modern.jpeg


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FIG: CT STROKE DSA IMAGE

BLOCK DIAGRAM:

The heart of this system is a digital image processing system which acquires images

from a video camera and provides timing signals to both the x-ray generator and the


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image acquisition system to control the flow of data from the x-ray source into the

image processor.

1. The image acquisition process begins when the timing signals, delivered to the x-ray

generator under computer control, initiates the production of x-rays which are

transmitted through the patient and received by the image intensifier.

2. An aperture, placed between the image intensifier and the video camera, controls

the amount of light delivered to the camera. This manages the signal-to-noise ratio of

the acquired image

3. A video camera receives the light image from the image intensifier and converts it

to a electronic video signal which is delivered to the image processor in analog form.

4. The image processor digitizes the image, stores it in memory, and makes it availablein digital form for subtracttion with another image set acquired at a different time or

at a different energy.

5. A common algorithm using digital radiographic systems is temporal subtraction . In

this technique, dynam-ic images of the patient are acquired at a rate of 1 exposure per

second or more. A contrast agent is injected into the patient either intravenously or

intra-arterially.

6. A second set of dynamic images is acquired after the contrast agent flows into the

area being imaged. The unopacified images (no contrast) are subtracted from the

opacified images with the subtraction process isolating the signal (which is present

only in the opacification image), removing the static anatomical structures that are

common to both the opacified and unopacified images. The elimination of

background structures makes the arteries visible in the subtraction image even when

they are not visible or barely visible before subtraction.


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DIGITAL IMAGE PROCESSOR:

FIG: DIGITAL IMAGE PROCESSING SYSTEM

The block diagram for a typical digital image processor is shown in Figure above. Thedigital Processor has basic functions illustrated in this diagram including(1) Acquiring and digitizing the video images,(2) Storing the digital images in memory,(3) Performing arithmetic operations (subtraction, addition and constantmultiplication) on the image data,(4) Displaying the digital images on video monitors, and(5) Storing the image data on magnetic media or an optical disk.

The image processor also contains a microprocessor or system controller that controlsthe basic operations of the image processor, the x-ray generator, and othercomponents, coordinating and controlling the operation of the digital imaging system.

1. We will assume that an analog video image has been acquired by the x-ray system,

image intensifier, and video camera.


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2. The analog signal is delivered to the image processor that provides some

preprocessing to adjust the amplitude and level of the video signal to satisfy the input

specifications of the analog-to-digital converter. Many images are acquired in a 512 x

512-pixel matrix although some systems use a 1024 x 1024 matrix. The image matrix

controls the sampling rate of the analog to digital converter.

3. Following digitization, the image data are logarithmically transformed, meaning that

the pixel values are replaced by their logarithm. The logarithmic transformation is

required to remove stationary anatomical structure during image subtraction.

IMAGE MEMORY AND INTEGRATION

After logarithmic transformation and digitization of the incoming video signal, theimage is stored in one memory of the image processor. Each pixel in the digital image

is represented by a digital number having a minimum of 10 bits corresponding to the

digit-ization range of the image-processors analog-todigital converter.

Often more than one image is added ("integrated") to reduce noise and improve the

SNR of the image. This averaging is provided by a feedback loop in which the

incoming image is added to the contents of the previ-ously stored image on a pixel-

by-pixel basis.

IMAGE SUBTRACTION

In digital subtraction angiography, two images are acquired. The first is the "mask"

image, which is obtained before contrast media is injected into the patient. The

second is the "opacification" image, which follows injection of the contrast media and

is obtained when the contrast bolus reaches the artery to be imaged.

The mask and opacification images can be modeled mathematically by assuming that

the patient has a thickness xt and a linear attenuation coefficient of t. Before contrast

media is injected into the patient, the photon fluence delivered to the image intensifieris:

Im = I0 e(t.xt)


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CONTRAST MEDIUM:

Contrast medium are selected based on their:

1. OSMOLALITY

2. VISCOSITY

3. IONICITY

Viscosity:

Resistance of a liquid to shear forces

Osmolality:

Osmolality is a count of the number of particles in a fluid sample. The unit for

counting is the mole which is equal to 6.02 x 1023 particles (Avogadro's Number).

Ionicity:

The ionic character of a solid is defined by ionocity..

Because of their chemical properties, contrast media are usually thicker (viscosity) and

have greater osmolality (more molecules per kilogram of water) than blood, plasma, or

cerebrospinal fluid. Viscosity and osmolality play a part in the development of

contrast reactions.

Ionicity is the characteristic of a molecule to break up into a positively charged cation

and a negatively charged anion, resulting in more molecules per kilogram of water and

thus increasing osmolality. Nonionic agents do not have this property and hence are

less osmolar.

Iodinated contrast media are among the most commonly used injectable in radiology

today. Modern iodinated contrast agents can be used almost anywhere in the body.

Most often they are used intravenously but can be administered intra-arterially and

intra-abdominally.


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They are usually safe, and adverse effects are generally mild and self-limiting.

Nonetheless, severe or life-threatening reactions can occur.

Commonly Used Iodinated Contrast

Medium:

Name Type Iodinecontent

(mg/mL)

Osmolality

Ionic

Diatrizoate(Hypaque 50; GE

Healthcare)

Monomer 300 1,550 (high)

Metrizoate

Isopaque (Conray

370; Nycomed

A/S)

Monomer 370 2,100 (high)

Ioxaglate(Hexabrix;

Mallinckrodt, Inc.)

Dimer 320 580 (low)

NonionicIopamidol

(Isovist-370;

Bracco

Diagnostics Inc.)

Monomer 370 796 (low)

Iohexol

(Omnipaque 350;

GE Healthcare)

Monomer 350 884 (low)


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Iodixanol

(Visipaque 320;

GE Healthcare)

Dimer 320 290 (iso)

INDICATIONS:

-Unstable angina or chest pain

-Heart attack

-Before a bypass surgery

-Abnormal treadmill test results

-Disease of heart valve

-To monitor rejection of heart failure

CONTRAINDICATIONS:

-Allergy to contrast medium

-Hypertension(uncontrolled blood pressure)

-Problems with blood coagulation

-Severe anemia

-Fever

-Uncontrolled rhythm(arrhythmias)


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REFERENCES:

1. Wikipedia2. https://www.beaumonthospitals.com/diagnostic-procedures-

cerebral-angiography

3. Physics of medical x-ray imaging4. American College of Radiology. Manual on Contrast Media5. http://www.atlantabrainandspine.com/subject.php?pn=cerebral-

angiogram-064

6. Photos Courtesy: Google search engine
https://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiographyhttps://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiographyhttp://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064http://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064http://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064http://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064http://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064http://www.atlantabrainandspine.com/subject.php?pn=cerebral-angiogram-064https://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiographyhttps://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiographyhttps://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiographyhttps://www.beaumonthospitals.com/diagnostic-procedures-cerebral-angiography

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