MR ANGIOGRAPHY

• MR angiography (MRA) is a type of magnetic resonance imaging(MRI) scan that uses a  magnetic field and radio frequency pulses to provide images  of blood vessels inside the body.
• MRA is based on flow information and gives anatomical as well as hemodynamic information.
• There are different techniques based on either the flow effect or on the use of contrast agent.

• CONTRAST ENHANCED MRA (CEMRA)
• NON-CONTRAST MRA (NCMRA)

• In these techniques blood appears black as these are spin-echo sequence based techniques.
• Protons in the flowing blood usually do not receive either 90 or 180 degree pulse. Hence, signal is not produced and flowing blood appears dark.
• Slow flow and clot can produce signal because they receive both 90 degree and 180 degree pulses.
• Blood flow- black
• Slow flow and clot- bright

• In this type of imaging, blood appears bright by the use of gradient-echo sequences.
• In GRE sequences excitation pulse is slice selected, but the rephasing which is done by gradient rather than  180 degree pulse is not limited to the slice of  interest and is applied to whole imaging volume.
• Therefore a flowing proton will receive an  excitation pulse is rephrased regardless of its slice  position and produces a signal.
• Moreover, short TR is used in GRE sequences,
• saturation of protons in the stationary tissues
• Blood flow- bright Slow flow and clot- black

Types of MRA

• Time of flight angiography(TOF- MRA)
• Phase contrast angiography(PC- MRA)

Time of flight MR Angiography (TOF- MRA)

• To produce a signal, a nucleus must receive both excitation and rephasing pulse. Stationary protons always receive both these pulses but flowing protons in excited slice may have exit the slice before the rephasing pulse hits them. This is called as “Time Of Flight Phenomenon”
• Effect of TOF Phenomenon will be different in SE and GRE sequences.
• In SE sequence, TOF will result in signal void.
• In GRE seq. TOF will cause flow enhancement with bright signal from flowing blood (Bright Blood Imging)
• In TOF-MRA vascular contrast (bright) is produced by keeping TR short than T1 of the stationary tissues. This saturates the stationary protons, reducing signal from them.
• For the evaluation of arteries, flowing protons in the veins are saturated or nulled  by applying saturation pulses in the direction of venous flow.
• For example, for carotid MRA, saturation band is applied superior to the imaging volume.

Types of TOF-MRA

TOF-MRA can be of three types :

• •2D time of flight
• •3D time of flight
• •MOTSA(Multiple overlapping thin slab acquisitions)

2D-TOF

• In 2D TOF, acquisition is slice by slice. 2D TOF is sensitive to slow flow and a large area can be covered. Hence it is used for slower velocity vessels like peripheral arteries and for venography.
• Resolution of 2D TOF is lower than that of 3D TOF.

3D-TOF

In 3D TOF, acquisition is from whole volume of the tissue. It gives good resolution with better visualization of smaller vessels. It is usually used for high velocity flow. In 3D TOF, there is a higher risk of saturating protons within the volume.

MOTSA

MOTSA combines advantages of 2D (larger area of coverage) and 3D (high resolution). Imaging volume is divided into multiple thin overlapping slabs during the acquisition. These slabs are then combined to a single volume of data.

Reformation of angiography:

• Data from TOF MRA is reformatted by technique called MIP (maximum intensity projection) to get angiogram.
• In MIP, pixels with maximum intensity are selected while other pixels are suppressed such that only vessels are visualized because they have pixels with maximum intensity.

Phase contrast imaging(PC-MRA)

• PC-MRA uses change/shift  in the phase of TM of the flowing blood to produce image of the flowing blood.
• The initial RF excitation pulse brings all the protons in phase. Gradient of a given strength is then applied to both stationary and flowing protons that causes phase shift in both stationary and flowing protons but at different rates.
• A second gradient pulse of the same amplitude and duration but of opposite polarity is applied.
• In the stationary tissue protons, reversal of the phase shift occurs of exact amount, canceling the effect of original phase shift and resulting into no net phase shift.
• However, since flowing protons have changed their position, the phase shift will not be corrected. This phase shift is directly proportional to the change in location or distance the protons travel between applications of first and second gradients.
• This phase shift is used by PC-MRA to create angiographic image and to measure flow velocity.
• To provide information, velocity encoding gradients (VENC) are applied in one or all three directions.
• Velocity encoding technique compensates for projected flow velocity within the vessel by controlling the amplitude or strength of the bipolar gradient.
• If velocity encoding is selected lower than the velocity within the vessel, aliasing occurs.
• Aliasing results in low signal intensity in the center of the vessel and better delineation of the vessel wall.

Typical values of the VENC are:

• 20–30 cm/s for venous flow
• 40–60 cm/s for higher velocity with some aliasing
• 60–80 cm/s to determine velocity and flow direction.
• PC-MRA also provides information about flow direction.
• If flow is encoded from superior to inferior, flow from the head appears bright whereas flow from feet appears black.
• PC-MRA can be in 2D or 3D acquisition.
• 2D is more commonly used in routine practice because of its acceptable acquisition time of 1–3 minutes.

APPLICATIONS OF MRA

• Stroke – TOF-MRA of carotid arteries & circle of willis.
• Cerebral Venous Sinus Thrombosis- MR Venography.
• Aortic Aneurysm, renal artery stenosis, Aortoarteritis- Body CE-MRA
• Peripheral vascular disease, Atheroscerotic arterial disease- peripheral CE-MRA

Advantages of MR angiography

• MRA does not require catheter unlike catheter angiography thus reduces damage to a vessel.
• Less invasive
• Less  expensive.
• Faster to perform.

Disadvantages of MR angiography

• Overestimation of stenosis.
• Ghost artifacts.