TwinTree Insert

14-03 Magnetic Resonance Angiography

arly on in MR imaging, blood vessels were visible. However, they could not be marked, singled out and made stand out as they could in con­trast-en­hanc­ed x-ray an­giography.

Figure 14-09 shows an example of high-signal blood in an examination of the pelvis. This behavior has been exploited for cardiac studies (Figure 14-10) and is fun­da­­men­tal to magnetic resonance angiography.

Figure 14-09:
In gradient-echo images, flowing blood appears bright. Note: These images are plain GRE ima­ges; they are not (yet) MR angiograms. Parameters: B₀ = 0.5 T; TR = 400 ms, TE = 28 ms, FA = 20°.

Figure 14-10 shows that in the spin-echo image, flowing blood within the heart cham­bers and the ascending aorta is black, whereas in the gradient-echo image, flow­­ing blood appears bright.

Figure 14-10:
Effects of blood flow upon signal intensity in a (a) spin-echo and a (b) gradient-echo pulse sequence.

MR angiography (MRA) is a further de­velopment of flow-related MR me­thods. In con­trast to x-ray angiography, MRA does not require the use of contrast agents, ra­ther the blood itself is used as an intrinsic con­trast agent.

However, there are general problems with all MRA techniques which are diffic­ult to overcome. They include flow voids or re­gions of low signal, where tur­bu­lent flow pre­vails, and difficulties in depicting smaller vessels.

spaceholder redTwo groups of techniques are used for di­rectly imaging flow in arteries, veins, and CSF-containing spaces:

spaceholder darkbluetime-of-flight (TOF) angiography, and
spaceholder darkbluephase-contrast (PC) angiography.

Although both techniques are fundamen­tally different, they are markedly af­fect­ed by normal and abnormal blood-flow pat­terns. TOF and PC are both bright blood me­th­ods, i.e., the blood appears bright on im­ages. Both are available in two-di­men­si­o­nal or three-dimensional versions, and PC also as a cine-technique. The choice of me­thods depends on flow ve­lo­ci­ty, imaging time available, and a number of other con­di­tions. Both techniques have advantages and dis­advantages, which are sum­ma­ri­zed in Table 14-02.

Black blood techniques are derived from TOF methods. They depict flowing blood dark and are preferably used in regions with high turbulence, e.g., in the exact as­sess­­ment of stenotic lesions.

14-03-01 Time-of-Flight Angiography

Time-of-flight techniques were first de­scribed in 1959 by Jerome R. Singer [⇒ Sin­ger 1959]. These tech­­ni­ques are also known as inflow or wash-in/wash-out techniques.

They take advantage of the contrast be­tween inflowing fully magnetized blood and the saturated surrounding tissue (Figure 14-06). In this case, the flowing blood is bright, whereas the surrounding tissue is dark [⇒ Hausmann 1994]. However, this holds for a single-slice experiment only. If we move to multiple slices, flow effects be­­come more complex — one has to consider the flow pattern of blood in vessels, as de­scrib­ed earlier Figure 14-02.

Flow signal intensity and image contrast also depend on some of the principal ex­trin­­sic con­trast parameters of MR imaging. In multiple-slice images, flow sig­nal in­ten­si­ty is influenced by the position of the particular slice and the di­rec­tion of flow relative to slice excitation. At the time of inflow, one slice of the flowing blood is ex­ci­ted. This excited blood continues traveling in laminar flow and leads to dif­fe­rent signal patterns in the neighboring slices.

If flow direction and slice excitation direction are the same, the flow is called con­cur­rent. Flow enhance­ment changes according to the direction of flow and se­quen­ce of slice excitation (Fig­ure 14-11).

Figure 14-11:
Flow-related contrast enhancement in mul­tip­le slice SE images. The dark gray de­picting the spins in slice 1 illustrates the fate of these spins while flowing through the imag­ed region. Because parabolic la­mi­­nar flow possesses a higher velocity in the cen­ter, dough­nut shaped patterns de­velop. This kind of flow enhancement is not pa­tho­lo­gi­cal.

During the image-acquisition period, the volume-of-interest receives multiple RF pul­ses saturating the non-moving spins within the volume. Fully magnetized flow­ing spins enter the volume, presenting greater signal intensity than the sta­tio­na­ry tis­sue. Reversing the slice-excitation direc­tion changes this signal pat­tern (coun­ter­cur­­rent flow). In this case, the central signal void is usually less pronounced but still visi­ble.

The most common implementation is to acquire a series of parallel thin slices using a rapid GRE sequence, usually with flow-compensated gradients to mi­ni­mi­ze the de­phasing effects.

Flow-compensation meth­ods include such tech­ni­ques as gradient moment nulling (GMN), motion-artifact suppression technique (MAST), and field even-echo re­phas­ing (FEER). They return spins moving at a constant velocity along a magnetic gra­dient into phase at the same time as sta­tio­na­ry tissue. This enhances blood (or CSF) sig­nal intensity (Figure 14-12).

The blood flowing perpendicular, or with a perpendicular component, to the slices gives a strong signal. The series of slices are viewed on screen as a 3D stack forming a 3D picture of the flow perpendicular to the slice direction or as a single-pro­jec­tion image.

Figure 14-12:
Time-of-flight angiography.

14-03-02 Phase Contrast Angiography

Regular MR images are magnitude (modulus) images, with signal intensity be­ing the basis for image reconstruction. Phase-contrast MR angiography exploits the shift in the phase that occurs when spins move and dephase in the presence of an imaging gra­dient (as discussed in Chapter 6) [⇒ Gedroyc 1994]. For flow perpendicular to the gradient, the motion will cause the spins to ex­pe­ri­en­ce different gradient strengths. To counteract this for­ce, the gradients are balanced for stationary spins and thus have no influence on these spins (cf. Figures 06-09 and 06-10).

The gradients, however, influence moving spins, leading to a net phase shift (Fi­gu­re 14-13 and 14-14). Because of the balanced gradients, stationary spins in the sur­round­ing tissue completely re­phase. The flowing spins will stay out of phase. The phase angle depends on the flow velocity. This method has the advantage that its sen­si­ti­vi­ty can be adjusted to the ve­lo­ci­ty of moving blood or CSF.

Figure 14-13:
Phase-contrast angiography.

Figure 14-14:
Phase contrast. (a) Spin system at time 0; (b) spins are dephasing; (c) after switching of gradient spins are rephasing; (d) stationary spins are rephased.
 The red dephasing arrows represent flow­ing spins; the blue dephasing arrows re­pre­sent stationary spins.

Weak gradients allow the detection of fast-moving flow, whereas strong gradients are more sensitive to slow flow.

Because the ve­lo­ci­ty of blood flow differs in the human body, as seen in Table 14-01, which is a great advantage of the PC method. The size of the phase shift de­pends on the gra­di­ents, the time separating them, and the flow rate. Thus, for known gradient parameters, the flow rate can be calculated. In this way, PC an­gio­gra­phy can produce quantitative velocity-encoded images or, in general, velocity map­ping.

spaceholder redBecause PC techniques depend on encoding of flow in all three spatial di­­rec­­tions, data acquisition takes longer than for TOF methods — after acquisition of a re­fe­ren­ce phase image, up to three images sensitized to flow in the three di­rec­tions have to be collected; to eliminate the stationary background signal, the reference phase image is then subtracted from each of the three sensitized ima­ges.

Similar considerations hold for 2D versus 3D: 2D acquisition is faster, 3D ac­qui­si­tion has better signal-to-noise ratio and spatial resolution.

When acquiring PC-MRA images, one has to know the approximate velocity of the flow­ing blood, the velocity-encoding (VENC) value. This value represents the ma­xi­mum velocity present in the imaging volume.

The faster the spins in the vessel are moving, the greater their phase shift will be. Spins with a higher velocity than the VENC value will cause aliasing artifacts on the final image (wrap around artifact; see Chapter 17).

spaceholder redAn overview of TOF and PC characteristics is given in Table 14-02.

Table 14-02:
Advantages and disadvantages of various TOP and PC imaging techniques used for MRA.