Applied Radiology

CTA protocols enabling contrast-dose reductions

CTA techniques are used to visualize vascular anatomy and were initially developed in the 1990s using single-slice and early multislice CT scanners. However, due to technical limitations, the coverage was limited to smaller vascular regions. ²³ Current wide-coverage scanners, using state-of-the-art spiral acquisition techniques, make it possible to consistently acquire high-quality scans of the entire vascular anatomy - from the Circle of Willis (COW), through the carotids and aorta, to the lower extremities-within seconds. The faster rotation speeds and larger detector coverage of these new scanners make it even more important to optimize contrast-injection parameters to obtain maximum enhancement of the vascular structures of interest, while simultaneously minimizing the contrast load delivered to the patient.

Contrast-injection protocol

A wide variety of CT protocols, including CTA exams, require contrast injection. Head and neck, thoracic, abdominal, and peripheral runoff CTA studies are among the most common. The timing of the contrast bolus for CTA scans is typically determined using either a test injection or automated bolus-tracking software. The test injection method involves the administration of a small bolus of contrast to estimate the time to peak enhancement in a region of interest. The results of the test injection are used to set injection parameters for the main spiral scan. The bolus tracking method uses software to automatically analyze contrast enhancement at an anatomic location specified in the particular examination protocol and to automatically begin the CT acquisition at a preset time after the enhancement at that location reaches a predefined threshold.

The protocols and techniques for performing CTA scans vary by institution and clinical indication. On a 64-channel scanner, a typical head, thorax, and abdomen CTA examination requires the administration of approximately 100 mL of contrast. The injection rate is 3 to 4 mL/sec, depending on the patient and the protocol. Additionally, a 30 to 50-mL saline chaser bolus administered at 3 to 4 mL/sec may be used to obtain a tighter bolus. Table 1 shows a typical abdominal CTA protocol. ²⁴

The faster rotation time and wider coverage per rotation of new CT scanners enable the contrast volume used during a typical CTA study to be reduced to 50 to 70 mL per patient, with an injection rate of 4 to 5 mL/sec and a 30 to 40 mL saline chaser injected at 4 to 5 mL/sec (Table 1). The CT angiograms depicted in Figures 1 and 2 were acquired using 70 mL of contrast.

A comparison of the protocols presented in Table 1 reveals a contrast volume reduction of approximately 30 mL per patient, per procedure. This contrast savings can lead to substantial economic benefit and potential reduction in risk of CIN.

Economic analysis of contrast-volume reduction

In addition to the aforementioned clinical benefits of reduced contrast utilization, the potential annualized institutional and national cost of performing contrast-enhanced CTA examinations using a CT scanner capable of achieving faster rotation times combined with wider coverage (CT256) can be compared to performing contrast-enhanced CTA exams with a 64-channel system (CT64). In this comparison, institutional and national benchmark data were used to estimate potential contrast-volume savings and associated cost benefits. At the institutional level, activity-based analyses were used to identify the volume of contrast used along with associated actual contrast cost.

Institutional data was collected from scan histories for CT64 systems from 3 radiology departments located in 2 distinct geographic regions: Methodist Hospital (Indianapolis, IN), MetroHealth Medical Center (Cleveland, OH), and the Oregon Health and Science University (Portland, OR).

Institutional analysis was based on the actual number of scans performed over a prior 12-month period and the associated contrast volumes. Contrast cost estimates were based on a bulk delivery assumption of an average $0.40 per mL. On average, these institutions administered 100 to 125 mL of contrast per patient when scanned on the CT64. Similar patient examinations performed on the CT256 used 70 mL of contrast, on average. For simplicity, this analysis assumes a more conservative per-patient average contrast volume savings of 30 mL. Multiplying 30 mL by the $0.40 manifests a potential savings of $12 per patient. Table 2 presents a summary of the institutional data and the associated cost savings derived by the use of a CT256 scanner.

National analysis was performed using values extrapolated from an industry survey. ¹ The "average number of procedures per system" and the "number of procedures requiring contrast" were derived from this survey data. The average per procedure contrast volume and per patient contrast cost savings found through institutional analysis was then applied (Table 3).

Both institutional and national analyses indicate the potential to realize cost savings through contrast volume reductions if the CT256 is used instead of CT64 for CTA. Such savings will result in a reduction in cost-of-ownership and a positive impact on the department's annual operating budget. It is estimated that 50% of CT providers use bulk contrast at the national level, so contrast savings could have a significant positive impact on total cost-of-care. Extrapolating the estimated $5400 per system contrast cost savings to just 200 CT256 systems would yield >$1 million annual savings-a significant amount given the growing numbers of imaging procedures and the current national debate surrounding healthcare costs and coverage.

Further, the lower limits of contrast volume that enable diagnostic quality scans to be produced are still under investigation. A limited number of procedures performed in the Midwest, however, suggest that diagnostic image quality and vessel visualization can be achieved with greatly reduced contrast volumes (S.D. Teague, MD, unpublished data, September 2008). While this evidence is still anecdotal, it is of interest to postulate the significant potential benefit to a department's annual operating budget if this lower limit were to be achieved more regularly with diagnostic results.

Conclusion

Diagnostic image quality in CT angiography can be achieved using lower effective contrast volumes with the latest CT technology that optimizes speed, power, coverage, and dose management. Operating cost benefits are realized through a reduction in per-patient contrast volumes, while patients benefit from a reduced risk of contrast-induced nephropathy and lower lifetime medical radiation exposure.