Digital Angiography
What is Digital Angiography
Digital Angiography is a diagnostic procedure which produces X-ray pictures of blood vessels. It is performed by the insertion of a small flexible catheter into an artery (Angiogram) or vein (Venogram). The usual point of entry is via the groin vessels. Catheters are flexible, small bore hollow tubes about the diameter of coat hanger wire. The radiologist first inserts a small needle into the blood vessel through which a guide wire is inserted. The catheter is threaded over the guide wire and into the lumen of the blood vessel. By monitoring the catheter on a T.V. screen, the radiologist can carefully guide the catheter tip to the region of interest. Once in place, X-ray contrast is injected through the catheter often via a pressure injector. The contrast fills the lumen of the blood vessel which then becomes visible on X-ray images. The pictures enable the diagnosis of narrowings, occlusions, abnormal dilatations or abnormal communications of blood vessels. The QDI radiologists performing angiograms have a special interest in the technique which ensures the highest standards are maintained.
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The procedure
After positioning the patient on the table, the puncture site in the groin is shaved, cleansed with an antiseptic solution and sterile drapes applied. Local anaesthetic is injected into the tissues at the puncture site prior to introduction of the catheter. Once the catheter is manipulated into position, contrast is injected usually by a pressure injector which automatically regulates the volume and rate of the injection. When the contrast is injected, some patients notice a temporary hot flushed feeling or a metallic smell or taste. A series of digital images are acquired with each injection of contrast. These images are available for review immediately after acquisition. Several 'runs' are often required. At the completion of the procedure, the catheter is withdrawn and firm pressure applied to the puncture site for several minutes to allow sealing of the artery. After an angiogram rest in bed is required for four hours usually in a recovery area or occasionally in a hospital ward. Movement is restricted during this time to allow the seal at the puncture site to strengthen. Vital signs such as pulse rate and blood pressure are monitored and the puncture site and pulses in the feet checked regularly. A responsible person should be available to drive the patient home. Following an angiogram patients are required to rest for a few days and avoid strenuous activities.
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History and Physics of Angiography
Originally angiograms were obtained using conventional cut film. Rapid filming sequences were possible due to a device known as a film changer. In the 1980s, with the development of more powerful computers and suitable hardware, the images could be acquired digitally. The image the radiologist sees on the T.V. screen during the procedure is derived from a device called an Image Intensifier. This consists of a vacuum tube device that is placed near the patient. The X-rays form an image on the face of the device. This image is amplified many fold by a photomultiplier tube. Subsequently the image is viewed by a video camera. The output of the video camera is digitized and fed into a computer system. The advantage over the conventional technique is faster image acquisition rates, instantaneous availability of the images and a lower radiation dose to the patient. The digital images can be manipulated in various ways. A common technique is to acquire an image of the patient prior to the injection of contrast. This image, known as the subtraction mask, is digitally subtracted from the later images in the sequence which contain contrast. The result is an image where the underlying body structures virtually disappear leaving only a picture of the injected contrast. |
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Safety issues
As with any procedure there are possible complications however these tend to be minor in most instances. In angiography the main complications are related to injury or blockage of an artery either at the puncture site or in the region of interest. Also bleeding may occur at the puncture site and occasionally a focal dilatation called a false aneurysm may form where the catheter entered the artery. Safety concerns regarding Contrast and X-rays in general are discussed at our Safety Issues webpage.
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The Digital Angiography Room
An angiography suite includes an X-ray tube and an Image Intensifier assembled on a freely maneuverable mounting called a 'C' arm. A special table with an automated moving table top is used for positioning the patient. The X-ray tube is placed behind the region of interest and the Image Intensifier moved as closely as possible to the patient prior to the 'run'. Television monitors are usually mounted on the ceiling for ease of viewing. The computer console and hardware are sited nearby in a control room which has a direct line of vision with the angio room The angiography team includes the Radiologist, a specially trained nurse and two or more radiographers.
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Preparation
All patients will be fully informed of the details of the procedure prior to signing a consent form. The attending nurse contacts patients the night before the examination to discuss the preparation and to ask relevant questions. Blood tests may be required to check for normal clotting times. A light diet and fluids are permitted on the morning of the procedure. Patients taking blood thinning medication or diabetic medication require specific preparation instructions. A history of allergies is obtained.
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