14-05 Maximum-Intensity Projection

With both time-of-flight and phase-contrast methods we now have isolated vas­cu­lar structures by acquiring individual two- or three-di­men­sio­nal images.

However, an angiogram reflects entire vas­cu­lar structures in a two- or pseudo three-dimensional overview. In conventional x-ray and digital subtraction an­gio­gra­phy, this is created by sub­trac­tion of a mask from the contrast-enhanced an­gio­gra­phy images.

In MR angiography, a special com­pu­ter al­go­rithm is used for this pur­po­se – the maximum-intensity pro­jec­tion (MIP). Vascular structures on MR an­gio­gra­phy images show bright signal intensity. The MIP algorithm allows the se­lec­tion of bright pixels in all parallel 2D slices or in the 3D slab or vo­l­ume and their projection into one image. In 2D imaging, this is easily un­der­stan­dable as depicted in Fi­gu­re 14-15. The projection method is similar to a shadowgram, with the exception that only high-signal-intensity pi­xels are projected into the final ima­ge. The corresponding pixels in each original image finally form the projection angiogram. Figure 14-15: The MIP operation. In this case, six slices have been acquired.

The figure shows how flow-related structures give the highest signal intensity (the big white circle on the final reconstructed image); however, there are other structures visible with intermediate signal intensity (small gray circles on the ori­gi­nal images). Only the highest valued pixel is represented on the final image. Thus, other structures will disappear.

One disadvantage of MIP is that in routine clinical angiography bright non- vas­cu­lar tissues (e.g., fatty tissues) can represent highest signal intensity on the original pictures and thus be depicted on the angiogram. Such tissues can only be discriminated from vascular structures by their anatomy.

In 3D imaging, this algorithm can be used to create images in any projection wanted. On screen, one can visualize a rotating 3D angiogram.

MIP is a relatively simple and useful technique for processing angiographic MR data. It has some drawbacks such as the lack of discrimination between ar­te­ries and veins and high-signal-intensity non-vascular structures such as fat. New methods are being developed to overcome these problems, among them vessel tracking and volume rendering.

For black blood angiography, the contrary of MIP is done: those pixels with the lowest signal intensity are chosen, and with minimum-intensity projection (mIP) a black blood angiogram is created.

14-06 Reduction of Saturation Effects

Saturation effects are the gradual loss of T1 signal intensity by repeated ex­ci­ta­tion pulses. Too short TR leads to progressive loss of Mz; the same happens when increasing the flip angle. Saturation effects associated with thick slabs can be partly overcome with a technique called multiple overlapping thin slabs ac­qui­si­tion (MOTSA). Instead of one thick slab, several smaller slabs are acquired. MOTSA creates the Venetian blind artifact (Figure 14-16).

Figure 14-16: MOTSA (multiple overlapping thin ac­qui­si­tion). Instead of one thick slab, multiple thinner slabs are acquired. MOTSA re­du­ces overall saturation effects, but in­tro­du­ces the Venetian blind artifact.

Such artifacts do not exist when one uses tilted optimized non-saturation ex­ci­ta­tion, TONE. TONE applies ramped flip angles to the different slices of the slab. Increasing the flip angle counteracts saturation effects, in this case of slow flowing blood in deeper slices. Combined with magnetization transfer contrast (MTC), which suppresses background signal from brain parenchyma, TONE boosts the visibility of small vessels (Figure 14-17).

Figure 14-17: Brain angiogram using TONE and MTC.