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Step & Shoot Cardiac: Low-dose cardiac imaging

White Paper
Philips CT Clinical Science Philips Healthcare USA

Introduction

Computed tomography (CT) has seen rapid clinical acceptance as a viable modality for non-invasive cardiac imaging. With recent increases in detector coverage and concomitant improvements in spatial and temporal resolution, multi-detector CT (MDCT) scanners have gained widespread adoption in the clinical community for coronary artery disease detection, planning of percutaneous coronary interventions (PCI), post-coronary artery bypass graft (CABG) assessment, chest pain work-up, and pre-ablation planning for atrial fibrillation.

 

For such cardiac CT angiography (CTA) applications, retrospectively electrocardiogram (ECG)-gated spiral scanning modes are commonly used. In this scanning mode, the table moves continuously through the gantry while x-ray generation occurs. The ECG signal is stored simultaneously with the scan, or projection data (see Figure 1(a)). A spiral overlap is employed to ensure consistency in retrospective reconstruction of image volumes. Using the ECG-derived heart rate and overlapping projection data, a user can retrospectively reconstruct a desired physiological phase(s) of the cardiac cycle1,2. This scanning mode has been widely adopted, not only for coronary imaging, but also for obtaining adjunct information, such as ventricular function3,4.

 

Since this spiral retrospective technique uses a technique with overlapping data, the risks and benefits of the applied radiation dose relative to clinical need should be considered. If the primary clinical indication for the cardiac CTA exam is coronary artery assessment, several approaches exist to mitigate the risks of radiation exposure. For instance, the x-ray tube output can be significantly reduced during the systolic portion of the cardiac cycle via prospective ECG-controlled modulation of the x-ray tube. In other words, the x-ray tube output is maximized during the quiet phase(s) of the cardiac cycle (i.e., diastasis) and reduced by as much as 80% during the rest of the cardiac cycle (see Figure 1(b)).

 

Philips Healthcare now introduces Step & Shoot Cardiac imaging-a new scanning mode that enables an industry-leading reduction in radiation dose while maintaining or exceeding image quality (as compares to retrospective technique) at a desired physiologic phase of the cardiac cycle. Step & Shoot Cardiac is a prospective, ECG-gated axial scanning mode in which x-rays are turned on only during the physiologic phase of interest (see Figure 1(c)).

Figure 1(a) Retrospectively-gated spiral scans with no dose saving strategy employedFigure 1(b) Retrospectively-gated spiral scans employing ECG-triggered dose modulation to reduce radiation doseFigure 1(c) Prospectively-gated Step & Shoot scans
Figure 1(a)
Figure 1(b)
Figure 1(c)
Retrospectively-gated spiral scans with no dose saving strategy employed
Retrospectively-gated spiral scans employing ECG-triggered dose modulation to reduce radiation dose
Prospectively-gated Step & Shoot scans

Until recent breakthrough developments in hardware and software technology, this approach was not feasible for contrast-enhanced coronary scans. The coverage of scanners before 64-channels (i.e. < 40 mm coverage) previously meant long breath hold times for the patients. As detector coverage becomes wider, dedicated 3-D reconstruction algorithms are required to reduce the cone-beam artifacts in thin-slice images. For cardiac scans, this technique also necessitates precise synchronization of the x-ray with the patient's cardiac cycle, accurate prediction of the arrival of the next cardiac cycle, accurate identification of the quiet physiological phase within of the cardiac cycle and dedicated thin-slice cone-beam reconstruction algorithms.  


This is now a reality with the advantages of Philips Brilliance CT 64-channel MDCT: Using the full detector coverage of 40 mm enables Philips to cover the cardiac anatomy in three to five steps (see Figure 2), depending on patient size and clinical indication. The accurate synchronization of the x-ray with ECG ensures continuity in tracking the same physiological phase of the heart from one step to next, while turning on x-rays only when needed. The near-elimination of interscan delay minimizes the cycle time in between steps. The heart rate is monitored continuously to detect and reject arrhythmic beats, while employing a proprietary Beat-to-Beat algorithm to identify and track the quiet physiological phase. Finally, thin-slice images are reconstructed using dedicated advanced Cone Beam reconstruction algorithms that minimize artifacts.  


All of these factors are critical in making optimal coronary imaging possible.

  • A coverage of 40 mm using a 64 x 0.625 mm detector collimation

  • Accurate synchronization of the x-ray with Philips' Rate-Responsive technology

  • Real-time monitoring of the ECG signal irregularities and Arrhythmia Rejection via advanced algorithms

  • Precise prospective identification of the quiet physiological phase with the Beat-to-Beat algorithm1,2

  • Proprietary 3-D cone-beam reconstruction algorithms for thin-slice imaging  


All of these advantages are enablers of robust prospectively-gated cardiac CTA scans, providing an effective radiation dose of 2 - 5 mSv* - nearly that of annual background radiation exposure.

Step & Shoot acquisition
Step & Shoot acquisition

Performance validation

The Step & Shoot Cardiac scanning mode first underwent validation tests on phantom models in-house to ensure quality. Performance measures, such as slice-sensitivity profile (SSP), resolution (both in-plane and longitudinal), low contrast detectability (LCD) and image noise were compared to those obtained using a spiral retrospective scanning mode with identical scan (e.g. rotation time, collimation) and reconstruction (e.g. kernel, slice thickness) parameters for heart rates up to 65 beats per minute (bpm). Additionally, effective radiation dose was measured for both scanning modes and compared. Comparable performance was achieved without any sacrifice to the image quality using the Step & Shoot Cardiac mode while providing a significant reduction in effective radiation dose (Figure 3).

Step & Shoot performance measures
Step & Shoot performance measures

Preliminary clinical experience

Once the in-house validation tests were complete, the Step & Shoot Cardiac acquisition mode was tested in a clinical setting. In a preliminary study of 32 patients, diagnostic image quality was obtained in 96.9% [95% CI: 83.8-99.9%] of the patients. The mean (± standard deviation) patient heart rate during acquisition was 60 ± 7.2 bpm (Range: 36-88). Intravenous (IV) Beta-blockers were administered in 96% of patients to stabilize heart rate. A vasodilator (nitroglycerine) was also given to the patients to dilate the arteries. The number of steps needed to cover an average craniocaudal anatomy of interest of 15.7± 1.9cm (Range: 13-22 cm) was 4.9 ± 0.6 (Range: 3-7). The mean acquisition time was 9.4 ± 2.0 seconds (Range: 6-14), with an average x-ray on time of 2.1 ± 0.3 (Range: 1.7-3.0) seconds. The mean effective radiation dose was 3.3 ± 0.7 mSv (Range: 2.1 - 4.8). Shown in Figure 4 are reconstructions from some of these preliminary clinical scans.

Step & Shoot Cardiac image Step & Shoot Cardiac image
Step & Shoot Cardiac image
Step & Shoot Cardiac image

Summary

Step & Shoot Cardiac offers an attractive, low-dose alternative for cardiac imaging on Philips Brilliance CT 64-channel scanner by using technological advancements like Arrhythmia Rejection and proprietary thin-slice axial reconstruction algorithms. Preliminary studies have shown that, with appropriate patient selection and preparation (i.e. with stable heart rates = 65 bpm), Step & Shoot Cardiac can provide excellent image quality for a variety of clinical indications while drastically reducing the effective radiation dose.

 

* using a conversion coefficient k=0.017 as defined by the European Guidelines for Quality Criteria

References

  1. Heuscher DJ, Chandra S. Multi-phase cardiac imager. US patent number 6,510,337. 2003.
  2. Vembar M, Garcia MJ, Heuscher DJ, Haberl R, Matthews D, Boehme GE, Greenberg NL. A dynamic approach to identifying desired physiological phases for cardiac imaging using multislice spiral CT. Med Phys 2003; 30:1683-1693.
  3. Belge B, Coche E, Pasquet A, Vanoverschelde J-LJ, Gerber BL. Accurate estimate of global and regional cardiac function by retrospectively gated multi-detector row computed tomography: Comparison with cine magnetic resonance imaging. Euro Radiol 2006; 16: 1424-1433.
  4. Lessick J, Mutlak D, Rispler S, Ghersin E, Dragu R, Litmanovich D, Engel A, Reisner SA, Agmon Y. Comparison of multidetector computed tomography versus echocardiography for assessing regional left ventricular function. Am J Cardiol 2005; 96: 1011-1015.

 

Acknowledgements

  • Dr. Jaydip Datta & Brian Snipes (Wellstar Imaging Center)
  • Dr. Charles White, Dr. Jean Jeudy & tech (U of Maryland)
  • Dr. Srikant Sola, Dr. Scott Flamm, Dr. Milind Desai & Stacie Kuzmiak (Cleveland Clinic Foundation)
  • Dr. Samuel Wann & Shelly Deleeuw (Wisconsin Heart)


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White Paper
Brilliance 64-channel
Brilliance v2.0, Cardiac, Cardiac CTA, chest CTA, coronary artery disease, pediatric cardiac, prospective, retrospective, Step & Shoot, Vascular
 

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