Vascular Application Tips

Contrast Bolus Timing and Scan Delay
(Sample case)

Contributor: Jingbo Zhang, M.D. and Manmeen Kaur, M.D.

Introduction

Three dimensional, contrast-enhanced magnetic resonance angiography (3D CE-MRA) is increasingly being used for the diagnosis of vascular pathology. This technique requires timing of the intravenous contrast so that images are acquired while the contrast injection is in the vessel of interest and coincides with the acquisition of central k-space. Usually, this is during the arterial phase of the bolus in order to image the ar teries without the confounding effects of venous enhancement. Several strategies have been developed for optimizing bolus timing including: “best guess” technique based on patient history, fluoroscopic triggering, automatic triggering (SmartPrep or CARE Bolus), and the use of a test bolus in a timing run.

The “best guess” technique is the simplest method and involves estimating the contrast travel time from the site of injection to the vascular structure of interest. This technique takes into account the site of contrast injection, age of the patient, cardiac output, and vascular anatomy. The general guidelines for travel time from the antecubital vein to the abdominal aorta are as follows (add 3-4 extra seconds is the IV line is in the hand):

• 15 Seconds for a young, healthy patient.
  • A young hypertensive or athletic patient would require a few seconds less.
• A healthy, elderly patient would require approximately 20-25 seconds.

• Patients with cardiac disease or aortic aneurysm would be in the 25-35 second range.

• Patients with severe cardiac failure in conjunction with aortic pathology might require up to 40 to 50 seconds.

The fluoroscopic triggering technique uses 2D Sagittal gradient refocused images that are obtained rapidly (< 1 sec/image) through the vascular structure of interest. In this fashion, images are generated in near real-time and are updated at > 1 image/sec. The operator can then watch the contrast bolus arrive and then switch to the centric 3D MRA sequence when the desired enhancement is detected.

The automatic triggering techniques are commercially available bolus detection software termed SmartPrep by General Electric Medical Systems and C.A.R.E (Combined Application Reduce Exposure) by Siemens. With SmartPrep a region of interest is placed on the vessel of interest, e.g. the aorta, which will be sampled at 20 msec intervals for contrast tracking. The scan operator also sets a threshold value for the signal intensity as the trigger for the initiation for image acquisition. The software tracker detects the baseline signal intensity levels within the vessel of interest. Contrast is injected, and, as the baseline signal intensity in the region of interest increases to above a defined threshold value (approximately 20-30% above the base level), the 3D MRA gradient-echo sequence is automatically triggered to acquire the contrast-enhanced MR images. The trigger threshold will detect the leading edge of the contrast bolus. This often gives the patient time to take a deep breath and suspend respiration before the actual scan starts. The CARE bolus technique is similar. Two-dimensional (2D) gradient-echo images are obtained at the rate of about one or two images per second. These images are reconstructed in real-time and displayed on the console as they are being obtained. When the contrast is seen arriving in the vessel of interest, the patient is given the breath-hold instruction and the main 3D MRA sequence is triggered. Automatic bolus detection techniques use a centrically reordered k-space sequence because the center of k-space contributing to the contrast information on the final image should coincide with the arterial peak of the contrast. The major disadvantage to these techniques is that they are very sensitive to motion. If patient motion occurs when the center of the k-space is being acquired, the entire data set will be degraded with motion artifacts.

Our institution uses a test bolus in a timing run to optimize contrast bolus timing and to determine scan/imaging delay. The three main drawbacks to the test-bolus technique are:

• Setting up, performing, and analyzing the test bolus lengthens the overall examination time.

• The test bolus rapidly redistributes into the interstitial space, thereby, increasing the background signal. However, with small test doses these effects are negligible.

• There is no absolute guarantee that the imaging bolus will behave identically to the test bolus. This is because a patient's variables, such as venous return and cardiac output, can vary from moment to moment.

Timing Run and Scan Delay Technique

With this technique, 1 to 2 mL of gadolinium contrast is injected at the same rate as planned for the actual injection, followed by a 10 to 15 mL saline flush. Multiple single-slice fast gradient echo images (e.g. turboFLASH) of the vascular region of interest are acquired as rapidly at fixed time intervals (1-2 images per second) for approximately 40 seconds to 1 minute. The images should be oriented along the vessel of interest, i.e. sagittal or coronal for the aorta, or alternatively, be relatively thick (> 1cm) with a superior saturation band or a blood-nulling inversion pre-pulse. This will help to minimize the time-of-flight (TOF) effects and, thereby, maximize contrast conspicuity.

A timing run should be done for all studies that need arterial phase imaging, including liver, pancreas, kidney and all MRAs. With regard to the timing run:

• Before the study, check/advance the injector to get rid of air bubbles.
• A typical timing run uses 1 ml gadolinium plus 20 ml saline flush at 2 ml/sec. The injection rate should match that of the full gadolinium bolus.
• Simultaneously start the power injector (no delay) and the timing run sequence.
• Evaluate the timing run images in order to obtain the time to peak arterial contrast enhancement (i.e. contrast travel time) in the vessel of interest, most often the aorta, but the femoral or carotid arteries can also be used. The contrast travel time can be determined by visual inspection or by using a region-of-interest (ROI) analysis to produce a time/intensity curve. In this manner, image acquisition can be timed such that the presence of the contrast bolus coincides exactly with the sampling of the central of k-space.

Next, the appropriate scan delay is calculated for the actual CE-MRA acquisition, so that the presence of the contrast bolus peak coincides with the sampling of the center of k-space. In order to obtain an angiographic phase use the standard timing formula:

Scan delay = time-to-peak + ½(injection time) – time-to-center of k-space

• For symmetric k-space filling, the time-to-center of k-space is ½(scan time).
  

  Example: time-to-peak = 18 sec
  Injection time = 10 sec (20 ml at 2/ml/sec)
  Scan time = 20 sec (symmetric k-space filling)
  Scan delay = 18 + 5 – 10 = 13 sec

• Enter a 13 second scan delay into the power injector and use the remaining 19cc of Gd and 20 ml saline at 2/sec.
• Other phases may be obtained by using different formulas, as described in specific protocols.

The full MRA study is then performed by using the knowledge of the time delay between injection and time to peak arterial contrast enhancement (i.e. contrast travel time) in the vessel of interest.

References:

1. Prince MR, Grist TM, and JF Debatin. 3D Contrast MR Angiography . New York : Springer, c 1997. pp. 3-29.
2. Maki JH, Knopp MV, and M Prince. Contrast-Enhanced MR Angiography. Applied Radiology Online . 2003; 32(4): 182-210.
3. Montgomery ML, and RS Case. Magnetic Resonance Imaging of the Vascular System: A Practical Approach for the Radiologist. Topics in Magnetic Resonance Imaging. 2003; 14(5): 376-385.