Mr Angiography: Can We Image Arteries Without Injecting Contrast?
Radiology involves ionizing and non ionizing radiation to diagnose pathologies non- invasively. Ionizing radiation are X-rays, Gamma rays, alpha particles and beta particles. Non ionizing radiation is utilized in ultrasound (sound waves) and MRI (magnet). Ultrasound and MRI have the advantage of using non ionizing radiation and MRI has the best soft tissue resolution. MRI has been the modality of choice for the diagnosis and staging of tumours, metastases and cancer. MRI is safe for the embryo and foetus, and it is now possible to diagnose pathologies in brain, kidneys and other organs before birth.
MRI works on the basis of magnet. Patient is magnetized using strong magnets and then the slices are selected using gradient coils (inside the MR bore). Short burst of Radio Frequency pulse is applied to change the direction of magnetized protons. The moment RF energy is over, these protons start to relax and while coming back to the main magnetic field, they lose energy absorbed from the RF pulse. This lost energy is detected by RF coils (the equipments fixed over the part of patient being scanned) and is used to reconstruct image.
So far we have heard about imaging of organs without and with contrast. MR Scanner is capable of much more than mere organ imaging. Utilizing few techniques and a series of Radio Frequency pulse sequences, MRI can also image and display the arteries of body. A combination of different magnetic fields along with Radiofrequency pulses can reproduce images of arteries in different ways. It is painless procedure and does not require any incision of skin.
MR Angiography can be performed in three techniques: Time of Flight, Phase Contrast and Contrast Enhanced MR Angiography. It works on the basis of signal received from moving blood. The surrounding organs do not emit signal and thus are not seen in image. The patient is magnetized and a series of short burst of Radio Frequency pulses is given to the selected slice. The series is planned in such a way that only blood experiences it both times while surrounding organs experience it only once. Ultimately signal is emitted by moving blood. In Phase Contrast Angiography, brightness of blood in image relies on the velocity of moving blood. The faster the flow, the brighter would be the image. On the other hand, no movement, no signal and thus not seen in the image.
MR Angiography can also be performed following injection of contrast agent, known as Gadolinium. MR contrast agents act as strongly magnetic substances and increase the MR signal from blood. MR contrast agents are less likely to cause allergic reactions than the contrast used in X-ray and CT scan.
MR Angiography is very useful in diagnosis of many diseases including aneurysm (broadening of artery) (Fig 1), stenosis (abnormal narrowing of artery)and occlusion (blockage) of arteries. Atherosclerosis is a disease which occurs due to plaque is caused by accumulation of cholesterol in the artery results in hardening of artery. Artery loses its elasticity and can cause stroke or heart attack. Atherosclerosis (fig 2) can be diagnosed in MRA and treated in time. Arteriovenous malformation is another serious condition in which the blood in arteries is directly returned to veins without circulation in cells. It must be detected and diagnosed in time. Thrombus (blood clots) are very dangerous can result in blocked arteries. MR Angiography also detects diseases in kidney arteries and helps prepare for a kidney transplant.
Veins can also be visualized in MR Angiography, but since the diseases involving veins are less common and veins are superficial, they can be diagnosed in ultrasound, MR (atherosclerosis) Venography (MRI of veins) is not so frequently required.
MR Angiography takes 20 minutes to 45 minutes, depending upon the technique used. It does not require any special preparation if contrast is not required. However, for contrast enhanced MR Angiography, patient needs to be fasting for 4 to 6 hours. The patients with cardiac pacemaker (artificial heart) and with any other metallic implants are not allowed for MR Angiography. Nowadays, implants made up of Titanium (a metal which is not magnetic) are available and patients with Titanium implants can be scanned. Body piercing and hearing devices are also contra indicated for MR Angiography.