Image quality and radiation dose of CT venography with double dose reduction using model based iterative reconstruction: comparison with conventional CT venography using filtered back projection

Abstract

Background

Computed tomography venography (CTV) at low kVp using model-based iterative reconstruction (MBIR) can enhance vascular enhancement with noise reduction.

Purpose

To evaluate image qualities and radiation doses of CTV at 80 kVp using MBIR and a small iodine contrast media (CM) dose and to compare these with those of CTV performed using a conventional protocol.

Material and Methods

Sixty-five patients (mean age = 58.1 ± 7.2 years) that underwent CTV for the evaluation of deep vein thrombosis (DVT) and varicose veins were enrolled in this study. Patients were divided into two groups: Group A (35 patients, 80 kVp, MBIR, automatic tube current modulation, CM = 270 mg/mL, 100 mL) and Group B (30 patients, 100 kVp, filtered back projection [FBP], 120 fixed mA, CM = 370 mg/mL, 120 mL). Objective and subjective image qualities of inferior vena cava (IVC), femoral vein (FV), and popliteal vein (PV) were assessed and radiation doses were recorded.

Results

Mean vascular enhancement in group A was significantly lower than in group B (P < 0.01). Noise in group A was significantly lower than in group B except for PV and contrast-to-noise ratio were not significantly different in the two groups (P > 0.05). In addition, radiation dose in group A was significantly lower than in group B (P < 0.001). Subjective image quality comparison revealed group A was statistically inferior to group B except for subjective image noise.

Conclusion

CTV at 80 kVp using MBIR with small iodine contrast dose provided acceptable image quality at a lower radiation dose than conventional CTV using FBP.

Keywords

Diagnosis radiography multidetector computed tomography lower extremity veins

Introduction

Computed tomography venography (CTV) has been shown to enable accurate diagnosis and evaluation of deep vein thrombosis (DVT) (1). Because of concerns associated with ionizing radiation exposure, several dose reduction methods have been developed by CT vendors (2). Lower tube voltages, e.g. 80 kVp, with model-based iterative reconstruction (MBIR), have been widely applied to body and chest CT in order to preserve image quality and reduce radiation dose (3 –5). According to recent literature, CTV at 80 kVp with MBIR at a noise index level of 21 provides acceptable image quality for the evaluation of DVT and other pathologies, and is superior to filtered back projection (FBP) (6). Furthermore, using low tube voltage protocols for lower extremity CTV was reported to provide significantly higher venous attenuation than conventional high voltage protocols (7,8). Degree of venous enhancement on CTV images is an important requirement for the diagnosis of DVT (1). Low tube voltage CTV protocols provide promising alternatives for increasing venous attenuation without increasing the amount of intravascular iodine (1). Iodinated contrast media (CM) is essential to answer many clinical questions, and thus, the use of CM has increased over past decades (9). However, administration of iodinated CM has various side effects including contrast-induced nephropathy (CIN) and this remains a major concern. Nevertheless, lowering the iodine dose as much as possible can reduce CIN (10). Thus, the purpose of this study was to evaluate image qualities and radiation doses of CTV at 80 kVp using MBIR and a small volume (100 mL) of low concentration (270 mg/mL) CM and to compare those with those obtained by CTV at 100 kVp using FBP and a large CM volume (120 mL) of high concentration CM (370 mg/mL).

Material and Methods

Patient selection

This retrospective study was approved by our institutional review board, which waived the requirement for informed consent. From October 2015 to April 2016, 65 patients (mean age = 58.1 ± 6.2 years, mean body weight = 65.1 ± 3.1 kg) that underwent CTV for the evaluation of DVT and varicose veins were enrolled. Patients were divided into two groups: Group A (35 patients [mean BMI = 24.5 kg/m²], 80 kVp, MBIR, automatic tube current modulation [CT noise index = 21], contrast medium = 270 mg/mL, 100 mL) and Group B (30patients [mean BMI = 24.2 kg/m²], 100 kVp, FBP, 120 fixed mA, contrast medium = 370 mg/mL, 120 mL). Two radiologists unaware of imaging protocols, assessed objective (vascular enhancement, noise, contrast-to-noise ratio [CNR]) and subjective (image quality, noise, confidence of detecting DVT) image qualities of the inferior vena cava (IVC), femoral vein (FV), and popliteal vein (PV) and recorded radiation doses.

CT protocols

All patients underwent scanning with a 64-detector CT (Discovery 750 HD, GE Healthcare, Waukesha, WI, USA). CTV was performed in the craniocaudal direction from the T12 level to the tips of toes during a single inspiratory breath-hold. Detailed acquisition parameters were as follows: 80 kVp (group A) or 100 kVp (group B); automatic tube current modulation (ATCM); noise index = 21; section thickness = 3 mm; and reconstruction increment = 2 mm. Contrast-enhanced CT images were obtained after intravenous injection of 100 mL of non-ionic, iso-osmolar iodinated CM (Visipaque 270®, GE Healthcare, Oslo, Norway; group A) or 120 mL of non-ionic, hypo-osmolar iodinated CM (Ultravist 370®, Bayer Pharma AG, Berlin, Germany; group B) at a rate of 3 mL/s using an automated pump through an antecubital vein, and following this with 25 mL of 0.9% saline solution at the same flow rate. CTV was performed approximately 4 min after completing the IV CM injection, as other studies have reported a 4-min delay is most appropriate for CTV (6,7).

Image analysis

Contrast-enhanced CT images were transferred to our picture archiving and communications system (PACS) (Maroview®; Marotech, Seoul, Republic of Korea) and image analysis was performed using transverse images at a window level of 40 Hounsfield units (HU) and a window width of 400 HU. All images were independently reviewed for image quality and noise by two radiologists (with 2 and 18 years of CTV experience, respectively) unaware of imaging parameters. For objective analysis, vascular enhancement was quantitatively evaluated using IVC attenuation values at the left renal vein, right FV at the level of the femoral head, and right PV at the level of the popliteal fossa. When a patient was found to have DVT of a right- or left-sided vein, measurements were performed on the contralateral unaffected vein. Mean attenuation values of veins were measured using regions of interest (ROIs). The readers independently placed ROIs on vessels that included more than two-thirds of vessel diameters. Vascular enhancement was measured in HU and standard deviations were calculated in same slices using identical ROIs. Image noise level was defined as one SD. The same observers also measured the CNR of each vein, by placing a 90–110 mm² circular ROI in homogeneous subcutaneous fat at the mid-level of the medial thigh and in adductor muscle. CNRs were calculated as using, CNR = (V_HU– M_HU) / F_SD (where V_HU and M_HU are the attenuation values of each vein and adductor muscle, and F_SD is the noise of subcutaneous fat (11). Values obtained by the two readers were averaged for the analysis (Fig. 1). For subjective analysis, four images of each reconstructed series (IVC at the level of the left renal vein, right common iliac vein at the level of L5 vertebra, right FV at the level of the femoral head, and right PV at the level of the knee joint) were saved as DICOM (digital imaging and communications in medicine) files and stored in image folders in random order using Microsoft Office Excel 2007 (Microsoft, Redmond, WA, USA). The two radiologists independently scored overall image qualities (12 –15) using 3 - and 5-point scales for image quality and image noise (Table 1). Mean values in individual folders were subjected to statistical analysis (Fig. 2). Objective and subjective evaluations were performed by the two radiologists at least four weeks apart.

Fig. 1.

CT images were obtained of the inferior vena cava (a, b), femoral vein (c, d), and popliteal vein (e, f) for objective analysis. Mean vascular enhancement was higher for 100 kVp CTV using FBP and a large CM volume of highlight concentration CM (b, d, f) than for 80 kVp CTV using MBIR and a small CM volume of low concentration CM (a, c, e). However, image noise (SD) was lower for 80 kVp CTV using MBIR (a, c, e) than 100 kVp CTV using FBP (b, d, f). CNR was no different between the two reconstruction techniques. FBP, filtered back-projection; MBIR, model-based iterative reconstruction.

Table 1.

Scales used for the subjective scoring of image quality characteristics.

Scale and score	Description
Image quality
1	Unacceptable, no diagnosis possible
2	Poor, inadequate for diagnosis of the presence or absence of a clot
3	Fair, enhancement sufficient for diagnosis
4	Good, optimal enhancement allowing confident diagnosis of the presence or absence of a clot
5	Excellent, optimal enhancement superior to a score of 4 allowing for confident diagnosis of the presence or absence of a clot
Image noise
1	Optimal, none perceivable
2	Moderate, but sufficient for diagnosis
3	Unacceptable, no diagnosis possible

Fig. 2.

CT images were obtained of the inferior vena cava (a, b), femoral vein (c, d), and popliteal vein (e, f) for subjective analysis. Mean image noise scores were lower in the low CM dose MBIR group (a, c, e; mean = 1.00) than in the conventional FBP group (b, d, f; mean = 1.2). However, mean image quality scores and reader confidence scores for detecting DVT were no different in the two groups. FBP, filtered back-projection; MBIR, model-based iterative reconstruction.

Evaluation of radiation dose

The dose-length products (DLPs), measures of CT radiation doses, were provided by the scanner system. Total scan ranges were also recorded.

Statistical analysis

Statistical analyses were performed using the SPSS ver. 21.0.0 (SPSS Inc., Chicago, IL, USA). The Chi-squared test was used to compare patient age and sex in groups A and B, and the Student’s t-test was used to compare group BMIs and objective and subjective data. Statistical significance was accepted for P values < 0.05. Cohen’s Kappa coefficients (CKCs) were used to determine inter-observer agreements for objective and subjective assessments; CKC values in the range of 0.00–0.20 were defined as slight agreement, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.8 as substantial, and 0.81–1.00 as almost perfect agreement.

Results

Overall patient characteristics are shown in Table 2. Age, sex, and BMI were not significantly different in groups A and B. Of the 65 patients, 16 had DVT (group A = six patients, group B = ten patients), and 11 had varicose veins (group A = six patients, group B = five patients). The IVC, FV, and PV had significantly higher attenuation values in group B (P < 0.001). Mean vascular enhancements of all veins in group A were over 125 HU. Mean attenuation values of the IVC, FV, and PV in both groups are provided in Table 3. Notably, image noise was significantly lower in group A, except for PV. Image noises of the IVC, FV, and PV in the two groups are summarized in Table 3.

Table 2.

Patient backgrounds and radiation dose.

Characteristics	Group 1 (n = 35)	Group 2 (n = 30)	P value
Age (mean ± SD, years)	58.9 ± 13.1	58.1 ± 16.9	0.715
Sex (F:M)	17:18	15:15	0.89
BMI (mean ± SD, kg/m²)	24.5 ± 3.6	24.2 ± 4.1	0.111
DLP (mean ± SD, mGycm)	342.92 ± 20.05	457.4 ± 45.12	<0.001

Table 3.

Result of the objective analysis.

Characteristics	Level	Group 1 (BMI ≥ 25 kg/m²)	(BMI < 25 kg/m²)	Group 2 (BMI ≥ 25 kg/m²)	(BMI < 25 kg/m²)	P value
Vascular	IVC	127.8 ± 19.3	127.9 ± 20.7	151.4 ± 18.8	152.4 ± 19.8	< 0.001
enhancement	FV	127.8 ± 21.2	127.9 ± 22.9	146.4 ± 15.3	147.4 ± 15.9	< 0.001
(HU)	PV	129.2 ± 30.4	129.8 ± 31.5	152.4 ± 15.3	154.4 ± 16.3	< 0.001
Image noise	IVC	12.1 ± 4.11	12.4 ± 4.31	31.9 ± 1.4	32.5 ± 1.9	< 0.001
(HU)	FV	12.3 ± 2.6	12.6 ± 2.7	26.4 ± 2.1	26.9 ± 2.7	< 0.001
	PV	11.9 ± 3.6	12.1 ± 3.8	13.1 ± 2.5	13.8 ± 3.0	> 0.05
CNR	IVC	5.3 ± 2.8	5.5 ± 3.0	4.5 ± 1.7	4.8 ± 2.1	> 0.05
	FV	5.2 ± 2.8	5.3 ± 2.9	4.2 ± 1.7	4.4 ± 1.9	> 0.05
	PV	5.4 ± 3.3	5.5 ± 3.7	4.6 ± 1.9	4.7 ± 2.2	> 0.05

Data are mean ± standard deviation.

IVC, inferior vena cava; FV, femoral vein; PV, popliteal vein.

Mean group CNRs were not statistically different (P > 0.05). Mean CNRs at IVC, FV, and PV in the two groups are detailed in Table 3.

Subjective noise was significantly lower in group A (1.20 versus 1.76, P < 0.01); however, subjective image quality and confidence at detecting DVT were not significantly different in the two groups (P > 0.05) (Table 4).

Table 4.

Result of the subjective analysis.

Characteristics	Readers	Group 1	Group 2	P value
Image quality	R1	4.04 ± 0.71	4.12 ± 0.6	>0.05
	R2	4.08 ± 0.81	4.20 ± 0.83	>0.05
Image noise	R1	1.18 ± 0.47	1.74 ± 0.42	<0.001
	R2	1.22 ± 0.48	1.78 ± 0.39	<0.001
DVT detecting confidence	R1	4.64 ± 0.71	4.76 ± 0.6	>0.05
DVT detecting confidence	R2	4.58 ± 0.81	4.62 ± 0.83	>0.05

Data are mean ± standard deviation.

R1, reader 1; R2, reader 2.

The findings of the inter-observer agreement analysis are provided in Table 5. Inter-observer agreement for vascular enhancement was excellent, for objective image noise was excellent or good, and for subjective image quality, noise, and DVT detecting confidence were good. Mean DLP in group A (342.92 ± 20.05 mGy cm) was significantly lower than in group B (457.4 ± 45.12 mGy cm). Furthermore, radiation exposure, contrast volume, and contrast concentration were 25%, 17%, and 27% lower, respectively, in Group B.

Table 5.

Analysis of the interobserver agreements with the Cohen Kappa coefficient.

	Characteristics	Level	Kappa
		IVC	0.98
	Vascular enhancement	FV	0.96
		PV	0.97
		IVC	0.97
Objective analysis	Image noise	FV	0.71
		PV	0.68
		IVC	0.69
	CNR	FV	0.66
		PV	0.7
Subjective analysis	Image quality		0.64
	Image noise		0.70
	DVT detecting confidence		0.67

IVC, inferior vena cava; FV, femoral vein; PV, popliteal vein; CNR, contrast-to-noise ratio.

Discussion

In this study, CTV at 80 kVp using MBIR and a small iodine contrast dose (volume = 100 mL; concentration = 270 mg/mL) provided image qualities suitable for the evaluation of DVT at lower radiation doses than CTV at 120 kVp using FBP and a high iodine contrast dose (volume = 120 mL; concentration = 370 mg/mL). Reduced tube voltage has the advantage of improving vascular enhancement at lower radiation doses (5,16). On CTV, previously reported mean venous attenuation range was 91–115 HU (17,18) and mean DVT clot attenuation was 51 HU or less (19,20). In the present study, 80 kVp CTV using MBIR and a small volume of less concentrated CM produced slightly greater venous attenuation than previously reported (128.3 ± 23.6 HU) (14). However, lowering tube kVp also lowers total energy flux and increases image noise, which might degrade image quality (21). Recently, MBIR was used to overcome this limitation and several studies have reported significant dose reductions for low dose CT with MBIR of up to 80% for various parts of the body (22 –27). Our finding that low tube voltage CTV with MBIR showed significant improvement in image noise over high tube voltage CTV with FBP is consistent with previous reports. Thus, it appears that low tube voltage CTV using MBIR and a small CM volume of less concentrated CM can detect DVT despite the intravenous administration of 20% less CM volume and 17% less iodine. Furthermore, mean DLP of the low CM dose protocol used was 342.92 ± 20.05 mGy, which was 75% lower than the conventional CTV protocol. To increase degree of venous enhancement, a larger volume of CM or a higher concentration of iodine CM must be used (28), but CM dose cannot be increased indefinitely due to the dose-dependent risk of contrast-induced nephropathy. On the other hand, the present study shows the use of a low tube voltage protocol with MBIR improved venous attenuation without the need to administer additional CM due to the use of a low kVp and noise reduction by MBIR.

Our study has several limitations. First, it is limited by its retrospective design and a relatively small cohort. Accordingly, we suggest a larger-scale prospective study be undertaken to confirm our findings. Second, Visipaque® (iodixanol) and Ultravist® (iopromide) differ chemically, and thus, further studies are needed using the same contrast materials. Third, CM volume reduction and low concentration CM were evaluated simultaneously. Fourth, our study was performed on specific GE Discovery 750 HD and thus we suggest further study be conducted using the most modern CT scanner. Finally, group diagnostic accuracies were not compared.

In conclusion, CTV at 80 kVp using MBIR with a small CM volume (100 mL) and low CM concentration (270 mg/mL) provided acceptable image quality for DVT evaluation at lower radiation doses.

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) received no financial support for the research, authorship, and/or publication of this article.

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