Diagnostic performance and radiation dose of lower extremity CT angiography using a 128-slice dual source CT at 80 kVp and high pitch

Abstract

Background

Multi-detector computed tomography (MDCT) angiography is now used for the diagnosing patients with peripheral arterial disease. The dose of radiation is related to variable factors, such as tube current, tube voltage, and helical pitch.

Purpose

To assess the diagnostic performance and radiation dose of lower extremity CT angiography (CTA) using a 128-slice dual source CT at 80 kVp and high pitch in patients with critical limb ischemia (CLI).

Material and Methods

Twenty-eight patients (mean, 64.1 years; range, 39–80 years) with CLI were enrolled in this retrospective study and underwent CTA using a 128-slice dual source CT at 80 kVp and high pitch and subsequent intra-arterial digital subtraction angiography (DSA), which was used as a reference standard for assessing diagnostic performance.

Results

For arterial segments with significant disease (>50% stenosis), overall sensitivity, specificity, and accuracy of lower extremity CTA were 94.8% (95% CI, 91.7–98.0%), 91.5% (95% CI, 87.7–95.2%), and 93.1% (95% CI, 90.6–95.6%), respectively, and its positive and negative predictive values were 91.0% (95% CI, 87.1–95.0%), and 95.1% (95% CI, 92.1–98.1%), respectively. Mean radiation dose delivered to lower extremities was 266.6 mGy.cm.

Conclusion

Lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch was found to have good diagnostic performance for the assessment of patients with CLI using an extremely low radiation dose.

Keywords

Diagnosis radiography multi-detector computed tomography lower extremity arteries

Introduction

Peripheral arterial disease (PAD) is a serious health condition that adversely affects quality of life and increases the risk of mortality (1). Estimates of the prevalence of PAD in the general population vary widely in the range of 3–10% (2,3), and its risk factors include male gender, advanced age, a history of smoking, and concomitant diseases, such as diabetes and hypertension (4,5).

In diagnosing PAD and decision-making for management, computed tomography angiography (CTA) and magnetic resonance angiography (MRA) are more helpful than digital subtraction angiography (DSA) in some respects. CTA and MRA have the advantages of three-dimensional (3D) reconstruction and multiplanar ability, but MRA is slower and more expensive than CTA, and CTA is disadvantaged by the use of ionizing radiation (6).

For these reasons, multi-detector computed tomography (MDCT) angiography is widely used for diagnosis and treatment decision-making in patients with PAD. MDCT is safe, non-invasive, and has been shown to have good diagnostic accuracy. However, radiation exposure during medical imaging examinations can cause serious health problems, including cancer. Recently, radiation dose reduction has attracted considerable attention, and the use of lower X-ray tube voltages and higher pitch during MDCT angiography has enabled dose reductions. Furthermore, several studies have concluded that peak kilovoltage (kVp) reduction, from 120 to 100 kVp, can result in dose reductions greater than 30% (7). However, few reports have presented the diagnostic accuracy of lower extremity CTA using low kVp combined with high pitch. Accordingly, the aim of the present study was to evaluate the diagnostic performance and radiation dose of lower extremity CTA using a 128-slice dual source computed tomography (CT) at 80 kVp and high pitch.

Material and Methods

Study subjects

Our local institutional review board approved this retrospective study and waived the requirement for informed consent. All patients that underwent lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch and subsequently underwent digital subtraction angiography (DSA) of the lower extremities for the investigation of critical limb ischemia (CLI) between January 2013 and January 2014 were considered for enrollment. Patients with stents in lower limb arteries and patients that had previously undergone lower extremity vascular surgery or knee prosthesis placement, were excluded because of beam hardening artifacts. Finally, 28 patients (20 men, 8 women; mean age, 64.1 years; age range, 39–80 years; body mass index, 22.8 kg/m²) with CLI who received lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch and DSA within 1 week of CTA were included. All patients were evaluated using the Fontaine and Rutherford classifications (stage III 35.7% and stage IV 64.3% by the Fontaine classification and stage IV 35.7%, stage V 21.4%, and stage VI 42.9% by the Rutherford classification). The characteristics of the study population are detailed in Table 1.

Table 1.

Patient demographics.

Parameters	n
Mean age, years (range)	64.1 (39–80)
Sex	20 men/8 women (71.4%/28.6%)
Mean height, cm (range)	164 (150–176)
Mean weight, kg (range)	61.5 (41–94)
Mean BMI, kg/m² (range)	22.8 (16.9–31.8)

The 128-slice dual source MDCT angiography protocol

CTA was performed using a 128-slice dual source CT system (Definition FLASH®, Siemens Medical Solutions, Forchheim, Germany) in the supine position in a craniocaudal direction from 2 cm above the renal artery origin to the toes during a single inspiratory breath-hold. All patients were administered 120 mL of the non-ionic, iso-osmolar contrast agent ioversol (Iversense® 320; Accuzen, Seoul, Republic of Korea) at a flow rate of 4 mL/s followed by 30 mL of normal saline at the same rate through a 20 G cannula placed at the antecubital fossa. Faster acquisition speeds mean that a scanner can “outrun” the contrast bolus, and thus, delayed CTA acquisition was preprogrammed into the scanning protocol. The scanning parameters used were as follows; low kVp (80 kVp); high pitch (3.0); effective tube current, 160 mAs with dose modulation (Care Dose®, Siemens Medical Solutions); collimation, 20 × 1.2 mm; rotation time, 0.33 s; increment, 1 mm; and reconstruction section thickness, 3 mm.

DSA protocol

DSA was performed using a flat-detector angiographic system (AXIOM Artis Zee®, Siemens Medical Solutions) at 3 frames/s. Twenty-five of the 28 patients underwent unilateral angiography from the level of the common femoral artery to the toes, and three underwent bilateral angiography. A range of 10–20 mL of iodixanol (320 mg iodine/mL; Visipaque® 320, GE Healthcare, Cork, Ireland) at a flow rate of 3 mL/s was injected. Twenty-seven patients underwent angiography using an antegrade CFA approach and one using a retrograde approach.

Image and data analysis

All images were reviewed on picture archiving and communication system workstation (Marosis®, Infinitt, Seoul, Republic of Korea). CTA images including axial images with 3 mm thickness, maximum intensity projection (MIP) images, and volume rendered images (VR) were independently evaluated by two radiologists (with 16 and 2 years of vascular CTA experience); both were unaware of the results of DSA image analyses, which were used as reference standards, performed by two experienced interventional radiologists (with 19 and 15 years of experience) who were unaware of CTA results.

The analysis of images from CTA and DSA was performed independently and each reader was blinded to the results of analysis of images from the other modality and of analysis by the other reader. The lower extremity arterial tree was divided as follows: common femoral arteries, superficial femoral arteries (upper, middle, lower), popliteal arteries, tibioperoneal trunk, anterior tibial arteries (upper, middle, lower), posterior tibial arteries (upper, middle, lower), and peroneal arteries (upper, lower). These arterial segments were also divided into two subgroups, that is, into above-the-knee segments (common femoral artery – popliteal artery) and below-the-knee segments (infrapopliteal segments). All 14 segments were evaluated by CTA and DSA in the 31 lower extremities of the 28 patients. The following 5-point visual grading was used: grade 1, no stenosis; grade 2, <50% stenosis; grade 3, 50–75%; grade 4, 75–99%; and grade 5, occlusion. Significant stenosis was considered as grade 3 or higher (50% stenosis to occlusion).

Evaluation of radiation dose

Dose-length product (DLP) was used as CTA radiation dose descriptor. DLP values were provided by the scanner system.

Statistical analysis

The statistical analysis was performed using commercially available software (SPSS II, version 11.0.1; SPSS Inc., Chicago, IL, USA). CTA and DSA results were entered into Excel (Excel 2003 SP2, Microsoft Corporation, Redmond, WA, USA). Image analysis results were evaluated for inter-observer agreement using linear weighted kappa (κ_w) statistics. Disagreements between readers regarding arterial segment grades were resolved by consensus. On the basis of image analysis by consensus, sensitivities, specificities, diagnostic accuracies, positive predictive values (PPV), and negative predictive values (NPV) for significant stenotic lesions (50% stenosis – occlusion) were determined using image analysis results, and the above-mentioned DSA results were viewed as reference standards.

Results

A total of 434 segments of 31 lower extremities of the 28 patients were evaluated by CTA and DSA. Among the patients undergoing delayed CTA acquisition, no case of CTA overrun was detected. DSA showed 52 segments without stenosis, 71 segments with mild stenosis (<50%), 19 segments with moderate stenosis (50–75%), five segments with severe stenosis (75–99%), and eight occlusions in the above-the-knee subgroup. CTA correctly diagnosed all 32 significant lesions (≥50%) detected by DSA. Conversely, three of the 123 non-significant lesions by DSA were deemed significant by CTA (Table 2). In the above-the-knee subgroup, the sensitivity, specificity, and accuracy of lower extremity CTA were 100.0% (95% CI, 100.0–100.0%), 97.6% (95% CI, 94.8–100.0%), and 98.1% (95% CI, 95.6–100.0%), respectively, and its PPV and NPV were 91.4% (95% CI, 82.2–100.0%) and 100.0% (95% CI, 100.0–100.0%), respectively (Table 3).

Table 2.

Cross table of hemodynamically significant stenosis (≥50%) as measured using DSA and lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch.

Segment	CTA	DSA
Segment	CTA	≥50% (n)	<50% (n)
Above the knee	≥50%	32	3
	<50%	0	120
Below the knee	≥50%	151	15
	<50%	10	73
Overall	≥50%	183	18
	<50%	10	193

CTA, computed tomography angiography; DSA, digital subtraction angiography.

Table 3.

Diagnostic performance of lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch using DSA findings as the reference standards.

Segment	Sensitivity, % (95% CI)	Specificity, % (95% CI)	Diagnostic accuracy, % (95% CI)	PPV, % (95% CI)	NPV, % (95% CI)
Above the knee	100.0 (100.0–100.0)	97.6 (94.8–100.0)	98.1 (95.6–100.0)	91.4 (82.2–100.0)	100.0 (100.0–100.0)
Below the knee	93.8 (90.1–97.5)	83.0 (75.1–90.8)	90.0 (86.2–93.7)	91.0 (86.6–95.3)	88.0 (81.0–95.0)
Overall	94.8 (91.7–98.0)	91.5 (87.7–95.2)	93.1 (90.6–95.6)	91.0 (87.1–95.0)	95.1 (92.1–98.1)

CTA, computed tomography angiography; DSA, digital subtraction angiography; NPV, negative predictive value; PPV, positive predictive value.

DSA showed 31 segments without stenosis, 57 segments with mild stenosis (<50%), 28 segments with moderate stenosis (50–75%), 32 segments with severe stenosis (75–99%), and 101 occlusions in the below-the-knee subgroup. Three (1.1%) non-diagnostic segments (lower anterior tibial artery, lower posterior tibial artery, and lower peroneal artery) in one patient were excluded from the analysis, because below-the-knee DSA data were not obtained. CTA correctly diagnosed 151 of the 161 significant lesions (≥50%) detected by DSA (Fig. 1). However, 15 of the 88 non-significant lesions by DSA were deemed significant by CTA (Fig. 2). For arterial segments with significant disease in the below-the-knee subgroup, the sensitivity, specificity, and accuracy of lower extremity CTA were 93.8% (95% CI, 90.1–97.5%), 83.0% (95% CI, 75.1–90.8%), and 90.0% (95% CI, 86.2–93.7%), respectively, and its PPV and NPV were 91.0% (95% CI, 86.6–95.3%) and 88.0% (95% CI, 81.0–95.0%), respectively (Table 3).

Fig. 1.

A 64-year-old female diabetic patient. (a) CTA image showing grade 5 (occlusion) of the right anterior tibial artery (arrow head) and grade 2 (<50% stenosis) of the right peroneal artery (arrow). (b) Corresponding DSA image showing grade 5 (occlusion) of the right anterior tibial artery (arrow head) and grade 2 (<50%) of the right peroneal artery (arrow). CTA, computed tomography angiography; DSA, digital subtraction angiography.

Fig. 2.

A 78-year-old female patient. (a) CTA image showing grade 3 (50–75% stenosis) of the right peroneal artery. (b) Corresponding DSA image showing grade 1 (no stenosis) of the right peroneal artery, which was clearly overestimated by CTA. CTA, computed tomography angiography; DSA, digital subtraction angiography.

The overall sensitivity, specificity, and accuracy of lower extremity CTA were 94.8% (95% CI, 91.7–98.0%), 91.5% (95% CI, 87.7–95.2%), and 93.1% (95% CI, 90.6–95.6%), respectively, and its PPV and NPV were 91.0% (95% CI, 87.1–95.0%) and 95.1% (95% CI, 92.1–98.1%) (Table 3). The total mean radiation dose administered during lower extremity CTA was 266.6 mGy.cm (range, 182–310 mGy.cm).

For CTA and DSA evaluations, inter-observer agreement was excellent (κ_w, 0.86; 95% CI, 0.84–0.89; Table 4 and κ_w, 0.89; 95% CI, 0.87–0.92, Table 5, respectively).

Table 4.

Inter-observer agreements of CTA for grading stenosis in all segments.

	Reader 1
Reader 2	Grade 1	2	3	4	5	Total
Grade 1	58	18	0	0	0	76
2	23	92	9	1	0	125
3	0	13	18	10	0	41
4	0	0	5	25	8	38
5	0	0	0	8	116	124
Total	81	123	32	44	124	404

Data are presented as segment numbers.

Linear weighted kappa (k_W) = 0.86 (95% CI, 0.84–0.89).

CTA, computed tomography angiography.

Table 5.

Inter-observer agreements of DSA for grading stenosis in all segments.

	Reader 1
Reader 2	Grade 1	2	3	4	5	Total
Grade 1	70	11	0	0	0	81
2	16	103	5	2	0	126
3	0	14	32	1	1	48
4	0	1	5	29	2	37
5	0	1	2	4	105	112
Total	86	130	44	36	108	404

Data are presented as segment numbers.

Linear weighted kappa (k_W) = 0.89 (95% CI, 0.87–0.92).

DSA, digital subtraction angiography.

Discussion

Lower extremity CTA is known to have high diagnostic accuracy and good sensitivity and specificity for PAD, and in most cases, CTA can be used as the only pre-interventional imaging modality to decide on management and treatment planning (8). The present study shows that good diagnostic performance can be achieved by lower extremity CTA using a 128-slice dual source CT and high pitch at a dose of only 80 kVp.

In 1995, Mayo et al. suggested that a three-fold reduction in radiation dose could be achieved by reducing tube current from 400 mA to 140 mA without causing a significant deterioration in subjective image quality or diagnostic utility (9). Subsequently, attempts were made to reduce dosages by reducing tube voltages (10,11), which provides a facile means of reducing radiation doses and offers the benefit of vascular enhancement despite increasing image noise (10). Furthermore, Fanous et al. (11) suggested that reducing the tube voltage from 120 kVp to 100 kVp reduced applied doses by 37% without significant impacting diagnostic image quality.

Kubo et al. (12) defined helical pitch (beam pitch) as table increment (table feed) per gantry rotation divided by X-ray beam width. Raising pitch commonly increases image noise on MDCT (13). Also tube current is higher with raising pitch. Lastly, use of a high pitch protocol can result in data loss caused by wide intergap. To compensate, dual-source tubes were used in this study. Nonetheless, the use on high pitch has some advantages. In particular, high pitch reduces total scan times, and thus, radiation doses (14). The radiation dose could be lower by using a narrow filter compared to a standard filter on condition that other factors were the same. As per the vendor’s manual, narrowing filter and standard filter were matched automatically to the high pitch and low pitch protocol, respectively. As the result of using a narrowing filter, a reduction of radiation dose was approximately 20% compared to a standard filter (15). In other studies, mean radiation dose was found to be reduced by 20% and 45% when pitch was increased from 1 to 3.2 for routine chest and abdominal CT scans (16). Similarly, our results show that dual source, high pitch CTA could be used to reduce applied radiation dose for the diagnosis of PAD.

Heusch et al. concluded that use of high concentration iodine contrast media reduced tube current time, and thus, radiation doses (17). However, in our practice, patient conditions vary, and most of the patients enrolled in the present study had diabetes or chronic kidney disease. Hence, we use low iodine concentration (320 mg iodine/mL) contrast agent for all patients that underwent CTA. Shen et al. suggested that the use of a low iodine concentration could prevent contrast-induced acute kidney injury (CI-AKI) in some patients, as the risk of CI-AKI is positively associated with the amount of iodine delivered (18).

In this study, we used 80 kVp and high pitch (3.0) to reduce the radiation dose delivered by a 128-slice dual source CT at an acceptable diagnostic accuracy. Our results are almost identical to those of an earlier study, in which it was reported lower extremity CTA at 70 kVp using high pitch acquisition and iterative reconstruction enables radiation to be lowered without compromising image quality as compared with a 120 kVp protocol (19). In the present study, we achieved 38.6–60.5% reductions in radiation dose as compared with the 120 kVp protocol (19 –21), and diagnostic accuracy, sensitivity, and specificity were excellent (94.9%, 97.4%, and 92.9%, respectively).

Several limitations of the present study warrant consideration. First, it is inherently limited by its retrospective, single-center design and the small number of patients enrolled. Second, the CTA and DSA images were acquired specifically from patients with critical limb ischemia. Third, we used 120 mL of contrast agent at a flow rate of 4 mL/s in all patients, regardless of BMI. Finally, we used both 80 kVp and high pitch simultaneously and consequently their individual contributions to radiation dose reduction and image quality were not studied. Accordingly, a further larger-scale, prospective, multi-center study is required to confirm the generalizability of our results.

In conclusion, lower extremity CTA using a 128-slice dual source CT at 80 kVp and high pitch was found to provide good diagnostic performance for the assessment of patients with CLI at an extremely low radiation dose.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by a grant from Accuzen Pharmaceutical.

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