Digital subtraction angiography (DSA) is a type of fluoroscopy

Fluoroscopy

Fluoroscopy is an imaging technique commonly used by physicians to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope. In its simplest form, a fluoroscope consists of an X-ray source and fluorescent screen between which a patient is placed...

 technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment. Images are produced using contrast medium by subtracting a 'pre-contrast image' or the mask from later images, once the contrast medium has been introduced into a structure. Hence the term 'digital subtraction angiography'.

Applications

DSA is primarily used to image blood vessels. It is useful in the diagnosis and treatment of:

• Arterial and venous occlusions, including carotid artery stenosis
  Carotid artery stenosis
  Carotid stenosis is a narrowing or constriction of the inner surface of the carotid artery, usually caused by atherosclerosis .-Signs and symptoms:...
  
  , pulmonary embolism
  Pulmonary embolism
  Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
  
  s and acute limb ischaemia
  Acute limb ischaemia
  Acute limb ischaemia occurs when there is a lack of blood flow to a limb. It is usually due to either an embolism or thrombosis of an artery in those with underlying peripheral vascular disease. It is estimated to occur at a rate of 14 per 100,000 per year in the United States...
  
  .
• Arterial stenosis, which is particularly useful for potential renal donors in detecting renal artery stenosis
  Renal artery stenosis
  Renal artery stenosis is the narrowing of the renal artery, most often caused by atherosclerosis or fibromuscular dysplasia. This narrowing of the renal artery can impede blood flow to the target kidney...
  
  .
• Cerebral aneurysms and arterio-venous malformations (AVM).

DSA and Fluoroscopy

In traditional angiography images are acquired by exposing an area of interest with time-controlled x-rays while injecting contrast medium into the blood vessels. The image obtained would also include all overlying structure besides the blood vessels in this area. This is useful for determining anatomical position and variations but unhelpful for visualising blood vessels accurately.

In order to remove these distracting structures to see the vessels better, first a mask image is acquired. The mask image is simply an image of the same area before the contrast is administered. The radiological equipment used to capture this is usually an image intensifier

Image intensifier

An image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low light conditions such as at night, to facilitate visual imaging of low-light processes such as fluorescence of materials to X-rays or gamma rays, or for...

, which will then keep producing images of the same area at a set rate (1 - 6 frames per second), taking all subsequent images away from the original 'mask' image. The radiologist controls how much contrast media is injected and for how long. Smaller structures require less contrast to fill the vessel than others. Images produced appear with a very pale grey background, which produces a high contrast to the blood vessels, which appear a very dark grey.

The images are all produced in real time by the computer, as the contrast is injected into the blood vessels.

The future

DSA is being used less and less routinely in imaging departments. It is being taken over by computed tomography angiography

Computed tomography angiography

Computed tomography angiography is a computed tomography technique used to visualize arterial and venous vessels throughout the body. This ranges from arteries serving the brain to those bringing blood to the lungs, kidneys, arms and legs.-Technique:...

 (CTA), which can produce 3D images through a test which is less invasive and stressful for the patient.

See also

• Intravenous digital subtraction angiography
  Intravenous digital subtraction angiography
  Intravenous digital subtraction angiography is a form of angiography which was first developed in the late 1970s.IV-DSA uses a computer technique which compares an x-ray image of a region of the body before and after radiopaque iodine based dye has been injected intravenously into the body...
  
  
• Angiography
• Contrast medium
  Contrast medium
  A medical contrast medium is a substance used to enhance the contrast of structures or fluids within the body in medical imaging...
  
  
• Image intensifier
  Image intensifier
  An image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low light conditions such as at night, to facilitate visual imaging of low-light processes such as fluorescence of materials to X-rays or gamma rays, or for...
  
  
• Computed Tomography Angiography (CTA)
  Computed tomography angiography
  Computed tomography angiography is a computed tomography technique used to visualize arterial and venous vessels throughout the body. This ranges from arteries serving the brain to those bringing blood to the lungs, kidneys, arms and legs.-Technique:...
  
  
• Peripheral artery occlusive disease
  Peripheral artery occlusive disease
  Peripheral vascular disease , commonly referred to as peripheral arterial disease or peripheral artery occlusive disease , refers to the obstruction of large arteries not within the coronary, aortic arch vasculature, or brain. PVD can result from atherosclerosis, inflammatory processes leading to...