This method belongs to a class of MR angiographic techniques known as "time-of-flight." This technique gives rise to 3D information about the vessels in the volume of tissue being imaged with high contrast between the stationary tissue and the flowing blood. The INFLOW method relies on the flow related image enhancement caused by the movement of fresh, unsaturated blood into an already saturated slab of tissue. The INFLOW method has a number of advantages over other angiographic imaging methods. First, image subtraction is not necessary, thereby reducing scan time and computing requirements while speeding data manipulation. Second, high contrast can be obtained virtually independent of flow velocity. Third, the arteries or veins may be selectively imaged by the use of presaturation slabs. Finally, the technique does not require the use of self-shielded gradients. It is less sensitive to motion than the phase contrast methods. Using the INFLOW technique, angiograms may be obtained in only 10-15 min. For example, the data can be processed by sending a batch job or processed interactively with AP500 within 10 min. The choices are available on selecting INFLOW processing under the ANPROC key. Both batch and interactive processing are discussed later in this section.
To achieve the best possible contrast in the final images, the imaging parameters must provide for maximum refreshment of blood in the imaging volume. The threshold minimum velocity (Vt) is given by:
where d is the slice thickness and TR is the repetition time. For a typical 2D INFLOW sequence with 2 mm slices and TR = 50 ms, threshold velocity ( Vt) will be 0.04 m/sec. For velocities greater than Vt, the signal intensity is essentially independent of the flow velocity. Typical velocities range from 1 m/sec for the aorta and 0.8 m/sec in the carotid artery and 0.03 m/sec in small veins.
3D Volume t 3D Volume 2
3D Volume t 3D Volume 2
The phase of the transverse magnetization is made independent of the flow velocity by the use of velocity compensated gradients. However, higher order flow terms may cause signal void in the areas of turbulent flow. The use of short echo times compensates for this. If the stationary tissue is selected as the volume of interest, it may be saturated using a short TR and a large tip angle (see Fig. 3.27). During this pulse sequence, fresh unsaturated blood moves into the imaging slice. This results in good contrast between the unsaturated blood and the stationary tissue.
The INFLOW technique may be used with 2D multiple single slice or 3D acquisition with a flow compensated gradient echo sequence. For 2D multiple-single-slice INFLOW, many thin (2-3 mm) contiguous (or over contiguous) slices are collected in a plane that is orthogonal to the blood flow. The optimum contrast between flowing blood and stationary tissue should be obtained with the shortest TE, a TR of the order of 40-60 msec, and a large tip angle of 45-90°, depending on the anatomy being studied and the flow rate of blood. Presatura-tion of a slab above or below the imaging slice allows selective imaging of the veins or arteries. The single sided, parallel presaturation slab moves with the slice position, ensuring good suppression. The slab thickness is adjusted in the second pass parameters and is typically set to 50 mm. An alternative method for certain imaging protocols employs a presaturation plane that is perpendicular to the imaging slice. An example is the use of a sagittal or coronal slice for imaging the carotid arteries. A perpendicular presaturation slab is necessary to remove the venous flow.
Since the 2D method is a multiple-single-slice technique, the slices are reconstructed as they are collected. They may be viewed while subsequent slices are being collected. This feature allows the operator to monitor the data collection. Later, data collection may be stopped to correct the protocol, if necessary, without waiting for all the data to be collected. It also shortens the study time by reconstructing the slices while the acquisition is still in progress. In our experience for routine transverse slices of the carotids, processing methods consist of projecting the stack of slices in a plane orthogonal axis. Top-down projections or perpendicular projections in the AP direction may be generated with appropriate selection of projections in the select procedure menu (see Fig. 3.28). First projection will be generated when slices are reconstructed. INFLOW image processing uses a maximum intensity projection with the interpolation between the slices. The maximum intensity voxel in a given vector is used for that projection view.
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