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MR Venography IX Ruehm 6-06-2005 19:20 Pagina 331 IX MR Venography Stefan G. Ruehm Introduction niques are rather time consuming and of limited use in the presence of inordinately slow flow or The ability of MR imaging to depict flow, in com- tortuous venous anatomy, the use of contrast-en- bination with the inherent soft tissue contrast, has hanced 3D MR venography has been proposed to led to the rapid clinical implementation of this overcome these limitations. modality for vascular imaging. Slower flow and more homogeneous flow profiles make MR venog- raphy technically less demanding than MR arteri- Time-of-Flight MR Venography ography. Since venous pathology usually tends to be more extensive, high resolution MR imaging is Time-of-Flight (TOF) MR angiography is based on not required for routine MR venography to the a GRE sequence with rapid succession of alpha same extent as it is needed for imaging of the arte- pulses and short repetition times (TR). Thus the rial system. Conventional time-of-flight (TOF) and signal of stationary tissue is suppressed, whereas phase contrast (PC) MR techniques, which do not flowing spins in the vessel are consistently re- require the use of a paramagnetic contrast agent, freshed. Two-dimensional (2D) or three-dimen- have therefore evolved as reliable and clinically ac- sional (3D) TOF images with bright intravascular cepted methods for assessment of the venous sys- signal can be obtained (Fig. 1) [2-4]. For vessels tem. However, these techniques do have limitations coursing within the acquired section (“in-plane in that they are susceptible to pulsatility, in-plane flow”), the inflow effect becomes less effective. In- saturation effects, and spin dephasing when lami- travascular signal may be reduced to the level of nar flow is disturbed. Furthermore, lengthy acqui- surrounding stationary spins, prohibiting differen- sition times coupled with the technique’s inability tiation of flowing blood from stationary tissues. to reliably display small deep veins in the calf or Potential difficulties in TOF MR venography may superficial and perforating veins running horizon- therefore arise in situations where longer vessel tal to the imaging plane have restricted the routine sections lie within the imaged section. clinical application of conventional MR techniques Since vessels appear bright on TOF MR venog- [1]. To overcome these limitations, the use of con- raphy independently of flow direction, differentia- trast-enhanced three-dimensional (3D) contrast- tion of arteries from veins can be difficult. Flow in enhanced MR venography has been suggested and a particular direction can, however be saturated by is now used with increasing frequency in many in- using spatial flow presaturation bands (Fig. 2). stitutions. Spins being washed into the section from the pre- saturated area do not carry any magnetization, re- sulting in a lack of inflow enhancement [5]. These Techniques for MR Venography saturation bands can thus be used to obtain selec- tive TOF arteriograms or venograms. Both TOF and PC MR venography sequences have Commonly, two types of 2D TOF sequences are been employed for morphological evaluation of used for MR venography. The first type (spoiled the vascular system. Although they have limited sequences) relies on the inflow of blood alone to applicability for assessment of the arterial system, create vascular signal. FLASH (fast low angle shot) they remain valuable for assessment of the portal [6] and spoiled GRASS (gradient-recalled acquisi- and systemic venous systems. Since these tech- tion in a steady state) sequences belong to this cat- IX Ruehm 6-06-2005 19:20 Pagina 332 332 Magnetic Resonance Angiography a b 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 a b 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*- msec (on a 1.5 T magnet) has been proposed so that weighting is associated resulting in additional the signal from water and fat are out of phase to en- brightness of the blood vessels. However, station- able the signal from fat to be reduced. ary tissues containing fluid, such as bowel or blad- Selection of the appropriate flip angle is impor- der, may also be bright. FISP (fast imaging se- tant. Too large a flip angle may lead to saturation of quence with partial refocusing) sequences are in- the venous signal, whereas too small a flip angle cluded in this category, as are GRASS sequences as results in noisy images. The best flip angle depends well. When imaging is performed to study deep on whether the image slice is oriented perpendicu- vein thrombosis (DVT) spoiled sequences are usu- lar or parallel to the axis of the vessel. For longitu- ally employed. dinal flow an angle of 20°to 25° is regarded as ap- Blood brightness can be increased by using propriate, whereas an angle of 45°should be cho- longer repetition times (TR). Increasing the TR re- sen for imaging in the transverse plane. sults in an increased number of relaxed spins enter- Image slices need not be contiguous if only a ing the imaging plane. This is accomplished, howev- survey of the venous system is desired. However, er, at the cost of a longer acquisition time. The echo thin contiguous or overlapping slices are required time (TE) should be short although the exact value if Maximum Intensity Projection (MIP) images is not defined and is of minor importance.A TE of 8 need to be calculated. IX Ruehm 6-06-2005 19:20 Pagina 333 IX • MR Venography 333 Phase-Contrast MR Venography traction requires the acquisition of pre- and post- contrast data. Image subtraction works less well in Phase-contrast (PC) MR venography is based on the chest and abdomen due to potential spatial the observation that spins moving through a mag- misregistration artifacts caused by respiratory netic field gradient acquire a different phase motion. The advantage of the indirect approach is (phase shift) as compared with static spins. For PC that there is no requirement for direct cannulation imaging two interleaved views are acquired over of the vein in the affected extremity. successive TRs. There is only one difference be- For the direct approach diluted paramagnetic tween the two views: the second view has an added contrast agent is continuously injected upstream bipolar gradient along one direction [7]. This on the side of the affected extremity. This approach bipolar gradient only affects moving spins which permits a full display of the deep and superficial acquire a different phase based on their specific venous system in a manner similar to that achieved flow characteristics. The difference between the with conventional venography. Compared to the in- phase data of the two successively acquired images direct approach the direct injection technique re- is thus limited to phase shifts from moving spins.A sults in superior CNR values although considerably true velocity map is therefore acquired. The meas- less contrast agent is required. To avoid T2-short- ured phase difference in individual pixels with ening effects the dilution factor should be in the flow is directly related to the flow velocity along range of 1:10-20.The 3D data set should be collect- the direction of the first moment change, which is ed during contrast administration. Repeated acqui- referred to as the “velocity-encoded direction”, sitions can be performed, e.g. with and without and which may be along the x-, y-, or z-axis. By placement of a tourniquet or with the extremity in convention, flow is bright if it flows from right to different positions to evaluate for functional ob- left (x-plane), anterior to posterior (y-plane) and struction of veins [12, 13], e.g. in patients with sus- superior to inferior (z-plane). Flow in the opposite pected thoracic outlet syndrome. direction is depicted as black. The technique is The use of a surface coil to increase signal-to- unique in the sense that it is a direct velocity map, noise ratio (SNR) and resolution is advantageous. in which the voxel intensity values are proportion- For data collection, a 3D data set with very short TR al to the actual flow velocity in a particular flow di- and TE values and a flip angle of 30-40°should be rection. The flow sensitivity can be adjusted. The used. Imaging should start following the injection “velocity encoding value” (VENC) helps to deter- of the first 50-60 ml of diluted contrast agent. To al- mine the largest measurable velocity. The appro- low for continuous contrast agent infusion, a tubing priate VENC value should be chosen to exceed the set is helpful which permits the simultaneous at- maximum expected velocity by about 25%. For tachment of two 60 ml syringes. Contrast agent in- some time PC imaging was the preferred MR tech- jection should continue during data collection with nique for assessing the portal venous system [8] sequential k-space filling. This enables central k- since it permitted direct quantitative characteriza- lines to be acquired in the middle of the data acqui- tion of flow dynamics over time [9]. sition and helps avoid artifacts arising from chang- ing gadolinium concentration. In addition, this ap- proach allows more time for filling of venous collat- 3D Contrast-Enhanced MR erals in the presence of a venous occlusion. Venography Techniques Direct Thrombus Imaging Contrast-enhanced MR venography can be per- formed using an indirect or direct approach.
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