VolumeShuttle™ is a new scanning technique of LightSpeed® VCT XT that delivers a larger acquisition coverage than any other CT systems for perfusion examination. VolumeShuttle™ provides the 80 mm coverage margin needed, to allow for patient variability in the Circle of Willis and from the basal ganglia to lateral ventricles, in one single contrast injection.
CT perfusion (CTP) imaging provides quantitative maps and measurements of vasculature morphology and function that have high diagnostic value.
The technique is gaining acceptance for diagnosing ischemic stroke and determining the appropriate treatment.1.2 It also shows great potential to help doctors evaluate tumor angiogenesis and response to treatment3 by detecting changes in blood flow, blood volume and capillary permeability.
The new scanning technique and analysis software developed by GE Healthcare now stand to expand the diagnostic power of CTP.
Until now, CTP in neurology and oncology has had limited anatomic coverage on advanced CT scanners. Because the procedure requires repeated scans at the same location in short intervals, CTP is traditionally performed with no table movements between scans, for coverage equal to the width of the detector, typically 19 mm to 40 mm.
Now, VolumeShuttle™ scanning on the GE LightSpeed® VCT XT scanner lets clinicians double the anatomic coverage in CTP, without a dose penalty, while producing high-quality images.

VolumeShuttle provides rapid scanning at two table positions during one bolus injection of contrast.This enables clinicians to follow, measure and accurately analyze the contrast wash in and out of blood vessels and tissue. The technique takes advantage of key attributes of the LightSpeed VCT XT scanner:

• A 40-mm-wide detector array with 64 rows of 0.625-mm elements
• Advanced systems for controlling X-ray generation and patient table movements.
• Advanced image-reconstruction algorithms.
• Innovative analysis algorithms and software.

Figure 1: Example of perfusion maps (Courtesy of Dr. Ting-Yim Lee, Robarts Research Institute)

Conventional CTP scanning provides blood flow and morphology information in a slab of anatomy using repetitive scanning at 0.5- or 1.0-second intervals for 30 to 60 seconds. The data is then analyzed to generate detailed maps of regional blood flow (BF), blood volume (BV) and mean transit time (MTT). Figure 1 shows perfusion maps in a stroke patient.
The thickness of the scanned slab in this technique is limited to the width of the detector. The concern is that important information, such as a brain infarct or a highly perfused component of a tumor, may lie outside the scanned volume and so cannot be seen.
One response to the coverage limitation is to perform two consecutive series of scans at contiguous locations, each with its own contrast injection. However, this doubles the contrast load, doubles the effective radiation dose, and lengthens the exam, potentially delaying sometimes-critical treatment decisions.
The VolumeShuttle mode instead scans alternately at two contiguous locations in one exam and with one contrast injection. During an exam, scans can be performed at 3.0-second intervals for 60 seconds – a total of 20 scans. This technique provides all the information needed to calculate perfusion maps and parameters, while doubling the volume covered in conventional CTP scans, reducing the radiation dose, and preserving clinical accuracy.

• Top of the page

Figure 2: Comparison of blood flow maps at different sampling

To be clinically useful and accurate, VolumeShuttle scanning has two key requirements:

1. Advanced control of table movement and X-ray beam timing. The table must move rapidly back and forth between the two scan locations with minimal delays in positioning and in triggering of the X-ray exposure. This ensures enough sampling frequency at each location to enable extraction of blood-flow parameters.

2. Precisely repeatable table position. The table must stop exactly at the same two scanning locations. Otherwise, there will be misregistration of pixels over the course of the exam, and the perfusion maps will be inaccurate – just as they would be if the patient moved.

The LightSpeed VCT XT scanner precisely controls both the X-ray beam and the table. As soon as the X-rays are turned off in one axial scan, the table moves rapidly to the next position. The patient is then in position for the X-rays to be turned on again. The scanner’s cradle drive uses a highly accurate stepper motor drive with two belt reduction mechanisms. It moves the patient 40 mm (1.6 in.) in 0.7 second – smoothly and quietly.
The patient experiences a maximum acceleration of less than 1/30 of gravitational pull (G) and a top speed of less than 0.25 mph (0.4 km/hr), causing negligible discomfort. The table position is repeatable to within 0.25 mm (0.1 in.) under full load of 500 pounds.
The scanner also has a precise, high-speed on-off switch for the X-ray beam, synchronized to table movement and location. It ensures that X-rays are turned on as soon as needed and completely off before the table starts moving – thus minimizing the delay between scans and the radiation dose.
As an example, a typical VolumeShuttle exam with a scan interval of 3 seconds at each location may deliver only two-thirds the effective dose of a single-location conventional scan at the same X-ray kVp and mA. This means a 33 percent dose reduction for the same image quality and double the coverage.

• Top of the page

Figure 3: Sampling at 3-second intervals correlates well with 0.5-second interval sampling

In development of VolumeShuttle scanning, one concern was that the longer 3.0-second sampling intervals at each scan location could affect the accuracy of the perfusion maps and the perfusion parameters calculated from them. To verify the adequacy of VolumeShuttle scanning for clinical needs, GE Healthcare collaborated on a study with a clinical partner.4
The study used data gathered from previous CTP exams of acute stroke patients scanned within seven hours of symptom onset. These exams were performed conventionally with a scan interval of 0.5 seconds, ensuring highly accurate perfusion maps.
Images from these studies were progressively removed to simulate a CTP exam with scans performed at up to 3.0-second intervals. GE CT Perfusion software was used to analyze the original data and the reformatted data, and the resulting perfusion maps were compared.
Figure 2 shows a comparison of the perfusion maps, and Figure 3 shows the good correlation for an exam involving an interval of 3 seconds between scans. The study concluded that scan intervals of up to 3 seconds can be used in CTP scanning while retaining the accuracy of blood flow, blood volume, and mean transit time values – without dose penalty.
A separate study of brain perfusion scanning, conducted independently, verified that the VolumeShuttle technique increased coverage without dose penalty and required only a single contrast injection without loss of visual diagnostic accuracy.5 In particular:

• Top of the page

CTP scanning in the VolumeShuttle mode takes advantage of the latest technology on the GE LightSpeed VCT XT scanner to double the anatomical coverage with one bolus of contrast and without dose penalty. Clinical studies show that in terms of image quality and accuracy of quantitative measurements, VolumeShuttle is an acceptable alternative to conventional scanning.

• Top of the page