Digital subtraction angiography (DSA) does this whole process of image acquisition and subtraction in a computerised system hance the name 'digital'. It offers near-instanteous view of the blood vessels and is the current state of the art technique.
Other techniques of angiography
1. CT angiography
2. MR angiography
The above two techniques use CT and MRI respectively to study the blood vessels.
Their techniques have evolved significantly and for most routine purposes they are eminenetly sufficient for diagnosis and planning further management.
Why DSA is needed?
1. Best resolution: important to look for very fine blood vessels
2. Hemodynamic information: means the ability to provide information regarding blood flow amount, flow etc.
3. When CTA or MRA are inconclusive.
DSA will be a must in evaluation of many brain and spine vascular diseases like AVM, dural AVF, vasculitis, dissection etc.

How to prepare fo the test?
On the day of your procedure, you may be asked not to eat and drink for 4 to 6 hours – this will depend on the type and time of your procedure. You may also be asked not to take some of your medication either for several days before or on the day of your procedure.
You will need to be admitted to hospital 3 hours before the time of your procedure. Before the procedure is carried out the staff on the ward will take a medical history, take blood tests and prepare you for your visit. This includes putting a small needle into a blood vessel, a groin shave and asking you to put on a gown and a pair of paper underpants.
What is the procedure?
Before this procedure is performed, you will remove all clothing and jewelry and will put on a hospital gown. You will be asked to lie on a narrow table. Your pulse and blood pressure will be taken regularly and there will be equipment around you. Everything will be explained as each part of the procedure takes place.
You will be awake during the procedure, you may be given a sedative to help you relax during the procedure.
A local anesthetic will be given where an incision is made prior to the placement of the catheter. In cerebral angiography, a catheter (long, thin, flexible tube) is inserted into an artery in the arm or leg. Using the catheter, we inject a special 'dye' into the blood vessels that lead to the brain.
During the injection of the contract dye, you may feel a warm feeling. A regular x-ray will be used to take pictures of the blood vessels in the brain (cerebral DSA), neck (carotid DSA) or spine (spinal DSA). A nurse, technologist and a radiologist will stay with you while the procedure is being performed.
You will need to keep very still, so that we can produce clear and crisp images. You may also be asked to hold your breath many times.
The length of the procedure is dependent upon the ease of the procedure as well as the requirements of the disease you have.The DSA procedure normally takes between 30 minutes to 1.5 hours.
After the procedure is done you will be monitored carefully to check for any complications. You should normally be allowed home later the same day or the following day if you have been admitted specifically for this test.
Complications:
You may feel some discomfort or pain at the site of puncture. Other rare complications are.
1. Allergic reaction to  contrast
2. renal failure
3. Blood collection ( hematoma) at the puncture site in the groin, gangrene
4. Stroke