Digital Subtraction Angiography (DSA) is one of the fluoroscopic imaging techniques. This method is used to visualize the blood vessels that are present under dense structures like bones and tissues. Before entering into DSA, let’s have a look at angiography. Conventional angiography is a fluoroscopic type X-ray imaging technique that is used to visualize the blood vessels. A contrast medium was inserted into the blood through a catheter and was imaged using time-controlled X-rays. This was one of the remarkable innovations in the field of radiography as the prior methods were not good for analyzing the blood structures. Hence angiography played a major role in the diagnosis of cardiovascular diseases.
But this came with a problem, the resultant image contained the blood vessels but was surrounded by other structures, hence the technique sooner became unhelpful to visualize the blood vessels accurately. Hence to overcome this, the subtraction angiography was developed. In this process, an image is taken prior to the insertion of the contrast medium (pre-contrast), and another image is taken after the insertion (post-contrast). The final image is the subtraction of these two images. Hence by this operation, the irrelevant structures in the background get removed and we can visualize the blood vessels more accurately.
Radiopaque iodine is injected intravenously and the post image is digitally subtracted from the pre-image. In this way, the tissues and other substances are removed and the artery becomes clearly visible. This method is used to study the vessels in the brain and heart, useful for mapping the blood route in those vital organs.
X-rays are used to get the image as in conventional radiography. Then the digitization takes place where the analog voltage gets converted to pixels of discrete values. Larger the size of the pixels, more the resolution of the resultant image. After this, the process of digital subtraction takes place. There are four methods by which the subtraction takes place.
Mask Mode
Time Interval Difference
Dual Energy
Hybrid
The standard and commonly used method of subtraction is the mask mode, where temporal subtraction of the image with a pre-contrast mask takes place. The problem associated with this is if movement occurs after capturing the mask, it can drastically change the results. This can be overcome using pixel shift, only to an extent.
In the time interval difference method, a sequence of images is taken and the subtraction occurs in between the successive frames with the constant time difference. In the dual-energy method, the image is exposed to a dose of high energy (130 keV) and then to low energy (70keV). The resultant image is obtained by subtracting these two images. The advantage of this method is that it doesn’t require any mask and it can easily eliminate artifacts due to motion. The problem is that this method is expensive and complex since it has to switch between energy states. Hybrid is the combination of mask mode and dual-energy mode.
Processing Methods
• Mask Pixel Shift
• Remasking
• Image Summation
• Stacking
• Landmarking
• Noise smoothening
• Edge enhancement
Hence the digital subtraction angiography eliminates the problem of conventional angiography and this method provides a promising diagnosis for carotid artery stenosis, embolism, limb ischemia, aneurysms, and malformations.
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