Timeof Flight MR Venography

Time-of-Flight (TOF) MR angiography is based on a GRE sequence with rapid succession of alpha pulses and short repetition times (TR). Thus the signal of stationary tissue is suppressed, whereas flowing spins in the vessel are consistently refreshed. Two-dimensional (2D) or three-dimensional (3D) TOF images with bright intravascular signal can be obtained (Fig. 1) [2-4]. For vessels coursing within the acquired section ("in-plane flow"), the inflow effect becomes less effective. In-travascular signal may be reduced to the level of surrounding stationary spins, prohibiting differentiation of flowing blood from stationary tissues. Potential difficulties in TOF MR venography may therefore arise in situations where longer vessel sections lie within the imaged section.

Since vessels appear bright on TOF MR venog-raphy independently of flow direction, differentiation of arteries from veins can be difficult. Flow in a particular direction can, however be saturated by using spatial flow presaturation bands (Fig. 2). Spins being washed into the section from the pre-saturated area do not carry any magnetization, resulting in a lack of inflow enhancement [5]. These saturation bands can thus be used to obtain selective TOF arteriograms or venograms.

Commonly, two types of 2D TOF sequences are used for MR venography. The first type (spoiled sequences) relies on the inflow of blood alone to create vascular signal. FLASH (fast low angle shot) [6] and spoiled GRASS (gradient-recalled acquisition in a steady state) sequences belong to this cat

Fig. 1a, b. Maximum Intensity Projection (MIP) display of pelvic venous anatomy based on a 2D MR venography protocol with single slice acquisition in the transverse plain. a Regular display of venous anatomy. b Missing visualization of left internal iliac vein (arrow) due to thrombosis

Venogram Pelvis

Fig. 1a, b. Maximum Intensity Projection (MIP) display of pelvic venous anatomy based on a 2D MR venography protocol with single slice acquisition in the transverse plain. a Regular display of venous anatomy. b Missing visualization of left internal iliac vein (arrow) due to thrombosis

Pelvic Venous Anatomy
Fig. 2a, b. MIP display (inferior view) of TOF MR angiography of pelvic vasculature with selective visualization of flow from (a) superior to inferior (arteries) and (b) inferior to superior (veins) using presaturation bands

egory. With the second type of sequence some T2*-weighting is associated resulting in additional brightness of the blood vessels. However, stationary tissues containing fluid, such as bowel or bladder, may also be bright. FISP (fast imaging sequence with partial refocusing) sequences are included in this category, as are GRASS sequences as well. When imaging is performed to study deep vein thrombosis (DVT) spoiled sequences are usually employed.

Blood brightness can be increased by using longer repetition times (TR). Increasing the TR results in an increased number of relaxed spins entering the imaging plane. This is accomplished, however, at the cost of a longer acquisition time. The echo time (TE) should be short although the exact value is not defined and is of minor importance. A TE of 8

msec (on a 1.5 T magnet) has been proposed so that the signal from water and fat are out of phase to enable the signal from fat to be reduced.

Selection of the appropriate flip angle is important. Too large a flip angle may lead to saturation of the venous signal, whereas too small a flip angle results in noisy images. The best flip angle depends on whether the image slice is oriented perpendicular or parallel to the axis of the vessel. For longitudinal flow an angle of 20° to 25° is regarded as appropriate, whereas an angle of 45° should be chosen for imaging in the transverse plane.

Image slices need not be contiguous if only a survey of the venous system is desired. However, thin contiguous or overlapping slices are required if Maximum Intensity Projection (MIP) images need to be calculated.

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  • pasqualina arcuri
    WHERE DOES A SAT BAR BE ON A MR VENOGRAM?
    8 years ago

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