Imaging Protocol

Sample protocols for both 2D TOF and 3D CE-MRA are shown below (Tables 1,2). These assume unilateral pedal only imaging, and can be modified accordingly if planned as part of a peripheral study. Some general considerations follow. First, in order to accurately characterize a stenosis, true spatial resolution (in all three planes) must be such that the vessel is spanned by three pixels - i.e. resolution equals (vessel diameter)/3 [33,34]. For pedal arteries with diameters of 2-3 mm (or even smaller), an absolute minimum resolution is 1 mm isotropic. This is not typically achievable with 2D TOF. Sec

Table 1. Suggested parameters for 2D Time-of-Flight MRA of the pedal arteries

2D TOF MRA

Coil

Head, knee, or phased array

Patient Positioning

Supine - feet first

CM dose/flow rate

N/A - must do pre contrast

Saturation band

Inferior

Bolus Timing

N/A

Breath-hold/free breathing

Free breathing

Sequences

2D TOF GRE

Sequence orientation

Oblique axial - angle with dorsalis pedis(23)

TR, TE, flip angle

20 ms, ~7 ms (flow comp), 60

Matrix

256 x 256, phase direction AP

FOV, # slices, st

260 x 260 mm2, 150-200, 1.5-2.0 mm (contiguous)

Voxel size

1.0 x 1.0 x 1.5-2.0 mm3

Acquisition time

6-8 minutes

Landmarks for slab position

Use scouts to cover anatomy

Image subtraction

N/A

Evaluation of images

Source, MIP, MPR

Table 2. Suggested parameters for 3D Contrast-Enhanced MRA of the pedal arteries

3D CE-MRA

Coil

Head, knee, or phased array

Patient Positioning

Supine - feet first

CM dose/flow rate

20 ml, 1.5 - 2.0 cc/sec. Flush 25 ml NS

Saturation band

N/A

Bolus Timing

Axial timing bolus ankle - 2cc contrast

Breath-hold free breathing

Free breathing

Sequences

3D GRE, centric

Sequence orientation

Sagittal - phase direction AP

TR, TE, flip angle

<6.0 ms, < 2.0 ms, 35-45

Matrix

320 x 320

FOV, # slices (true/recon), st (true/recon)

280 x 182 x 65 mm3, 54/108, 1.2/0.6 mm

Voxel size(true/recon)

0.9 x 0.9 x 1.2/0.55 x 0.55 x 0.6 mm3

Acquisition time

~ 60 sec

Landmarks for slab position

Use TOF MIP's to show vascular anatomy

Image subtraction

Yes

Evaluation of images

MIP, MPR

Table 2. Suggested parameters for 3D Contrast-Enhanced MRA of the pedal arteries ond, mask subtraction is mandatory for CE-MRA, as small pedal vessels quickly become obscured by bright signal from the fatty bone marrow. This means a pre-contrast "mask" must be obtained, and the patient must hold the foot still from that point until the contrast examination. Finally, venous contamination is particularly problematic with CE-MRA, especially in diabetic patients with ischemic soft tissue disease, where there is rapid AV shunting. Avoiding venous contamination requires precise timing with respect to contrast administration (timing bolus), or a time resolved acquisition [21, 24,35]. Because 2D TOF MRA can saturate venous inflow using an inferior saturation band, venous enhancement is much less problematic, and TOF images can often be used to help interpret a CE-MRA otherwise clouded by venous enhancement (Fig. 4).

For the timing bolus, we suggest a single axial 1 cm thick dynamic 2D slice through the ankle using 2 cc of contrast agent injected via power injector at the anticipated contrast flow rate (Fig. 5a). Both this timing dose as well as the diagnostic dose should be flushed with at least 25 cc of saline. Acquisition should be at least 1 image every 2 sec (for a total duration of approximately 120 sec), with simultaneous initiation of timing dose injection and scanning to make timekeeping simple. Both superior and inferior saturation bands should be used. The resultant data can either be

Fig. 4. Sagittal TOF-MRA (a) and CE-MRA (b) MIP's of the right foot in a patient with foot ulcers. Severe venous enhancement on the CE-MRA (b) makes interpretation extremely difficult, however the TOF study (a) clearly defines patent posterior tibial and lateral plantar arteries, and an abrupt occlusion of the mid dorsalis pedis artery. Using this TOF information, the anatomy can be better defined on the sagittal subvolume CE-MRA MIP (c), where the arrow marks the dorsalis pedis occlusion quantitatively plotted from ROI data (as shown in Fig. 5b), or can be examined visually to find the peak. Proper interpretation and application of the timing bolus data is critical to success, and requires an at least basic understanding of fc-space acquisition order. The key is to allow contrast to fill the arteries as long as possible before acquiring the center of fc-space, with the limiting factor of course being onset of venous enhancement. Experience has demonstrated that venous enhancement (when it occurs) is readily seen on the timing bolus images (Fig. 5a) [36]. Hence using the timing bolus data, an arteriovenous transit time or "AV window" can be determined. The optimum position for the center of k-space (i.e. start time for an elliptical centric sequence) is as far into this window as possible without obtaining overwhelming venous enhancement, thus allowing for maximal arterial opacification. Computer modeling, corroborated by data from peripheral MRA studies, strongly suggests that venous enhancement will be acceptable (50% or less signal intensity relative to artery) provided the center of k-space occurs at least 3-4 seconds before onset of venous enhancement [36]. This window varies depending on vessel size and pulse sequence parameters, and the 3-4 sec given here is for the parameters in Table 2 and a 2.5 mm vein. Thus if venous is seen relatively rapidly on the timing bolus, we recommend timing a centric acquisition such that acqui-

Plantar Vascular Mra

Fig. 4. Sagittal TOF-MRA (a) and CE-MRA (b) MIP's of the right foot in a patient with foot ulcers. Severe venous enhancement on the CE-MRA (b) makes interpretation extremely difficult, however the TOF study (a) clearly defines patent posterior tibial and lateral plantar arteries, and an abrupt occlusion of the mid dorsalis pedis artery. Using this TOF information, the anatomy can be better defined on the sagittal subvolume CE-MRA MIP (c), where the arrow marks the dorsalis pedis occlusion

Sc10ft T1FFE f.H

Sc10ft T1FFE f.H

Sc10f1 T1FFE /M

Fig. 5. Four single slices from the dynamic timing series with ROI's around the posterior tibial artery and an anterior vein (a). The signal intensity for artery and vein is plotted (b). Arterio-venous separation should be possible, as the time from arterial peak to onset of venous (dashed lines in (b)) is 7-8 sec. Resultant pedal sagittal and coronal MIP's are seen in (c) and (d). The dorsalis pedis is occluded proximally (arrows) and continues via collaterals. The lateral plantar artery is mildly diseased, and there is no complete plantar arch. Some mild venous contamination in seen

Fig. 5. Four single slices from the dynamic timing series with ROI's around the posterior tibial artery and an anterior vein (a). The signal intensity for artery and vein is plotted (b). Arterio-venous separation should be possible, as the time from arterial peak to onset of venous (dashed lines in (b)) is 7-8 sec. Resultant pedal sagittal and coronal MIP's are seen in (c) and (d). The dorsalis pedis is occluded proximally (arrows) and continues via collaterals. The lateral plantar artery is mildly diseased, and there is no complete plantar arch. Some mild venous contamination in seen sition begins approximately 3-4 sec before venous onset. If on the other hand, no venous is seen or venous delay is more than 9-10 sec beyond the arterial peak, we recommend acquisition begin approximately 6 sec after arterial peak (allows for improved small vessel filling without missing the bolus). For the example in Figure 5b, we would begin acquisition of the CE-MRA sequence at approximately 31 sec, which is approximately 5 sec after the arterial peak.

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