Establishment of diagnostic reference levels in low-dose renal computed tomography

Abstract

Background

Increased radiation doses from computed tomography (CT) examinations is well known with proven risks of inducing cancers for effective dose >100 mSv (according to some studies >50 mSvs).

Purpose

To establish the diagnostic reference level (DRL) for low-dose renal CT examinations in the evaluation of renal stones.

Material and Methods

Patient demographics, CT parameters, and dosimetric indices (CTDI_vol and dose length product [DLP]) were collected from 12 tertiary hospitals that routinely perform renal CT in the detection and evaluation of renal stones over a period of 12 weeks. Data obtained from 1418 average-sized patients in each category were recorded. The median values of dosimetric indices for each site were calculated. The DRL values were defined as the 75th percentile of the distribution of the median values of CTDI_vol and DLP.

Results

There were no significant differences between patient demographics. Mean kVp and mAs for protocols were 121.67 ± 11.56 and 226.91 ± 78.44, respectively. The CTDI_vol values were in the range of 2–36.2 mGy, while the DLP values were in the range of 43–1942 mGy.cm. The DRL for the CTDI_vol was 16.15 mGy and for the DLP 851.77 mGy.cm. The local median values of CDTI_vol and DLP are higher than DRL in two hospitals.

Conclusion

Comparison of local median values of CDTI_vol and DLP with DRL suggests the needs of an optimization strategy in some hospitals.

Keywords

Diagnostic reference levels low dose renal computed tomography radiation

Introduction

Imaging renal colic has evolved over the last decade with controversy existing over the process of diagnosing obstructing kidney stones with strong evidence that supports the use of both non-contrast computed tomography (CT) and ultrasound (1). However, societies advocate that physicians should “avoid ordering CT of the abdomen and pelvis in young and otherwise healthy individuals (age <50 years) when presenting to the emergency department (ED) with colic symptoms” (2).

CT has become the leading imaging modality in imaging patients who present with renal colic (3 –6). It has superior submillimeter spatial resolution and fast imaging turnaround time without the need of intravenous iodinated contrast material administration (4,7). Nevertheless, it also excludes other potential clinically similar conditions that are associated with renal colic symptoms (8 –11). Renal CT accurately quantifies the attenuation of renal stones (12), determines if there is the presence of pyelonephritis—with and without obstruction (6)—and presence of hydronephrosis, and finally helps in predicting spontaneous stone passage (13).

The sensitivity and specificity of standard-dose renal CT was in the range of 90%–98% and 88%–100%, respectively, while sensitivity and specificity of LDCT was in the range of 72%–99% and 86%–100%, respectively, with an equal diagnostic accuracy at 95% (14). Therefore, employing LDCT is seen as advantageous in the accuracy of stone detection at a significantly reduced dose of radiation.

Renal CT is potentially limited by the given radiation dose to the patient, mainly because urinary stone disease relapse mostly affects a younger age group . Katz et al. stated that 4% of the patients exposed to CT for suspected renal colic have had three CT examinations as a minimum during their treatment for the same indication, with cumulated effective doses in the range of 20–154 mSv (15). In view of the ALARA principle (As Low As Reasonably Achievable) and the potential risks of radiation-induced cancer, even using low doses of X-rays (16,17), dose reduction for suspected renal colic is therefore vital. In this manner, several studies have reported that it is likely to detect renal colic with low-dose CT (LDCT).

LDCT is extensively used in screening programs (e.g. for lung cancer (18,19)) and colonography (20,21). In addition, many LDCT protocols were proposed in clinical studies such as sinusitis evaluation (22,23), urinary stones detection (8,24), pyelonephritis (25) acute appendicitis evaluation, (26) and colon diverticulitis (27).

Radiation dose may be decreased by 75%–90% in line with standard acquisition doses, with no modifications on the diagnostic performance when being employed in screening programs (7). Several multidetector CT parameters can be modified to reduce radiation dose, including detector configuration, tube current, kVp, reconstruction algorithm, patient positioning, scan range, reconstructed slice thickness, and pitch (7,28 –32). Finally, a recent study revealed that the majority of imaging centers are employing routine abdominal CT doses instead of LDCT protocols to diagnose renal colic (33).

International efforts that have been conducted to improve medical imaging radiation safety differ significantly among countries (34); therefore, differences in CT dose are found (35 –38). For instance, the European Union (EU) implemented legislation with obligatory directives regarding exposure to medical CT radiation where the member states are forced to promote and establish the diagnostic reference level (DRL) for CT (39). The USA employed educational efforts by professional societies, such as the American College of Radiology, and recommendations from government agencies requiring institutions to assess their CT doses (40). Still, no organization in the USA is tasked with reporting, monitoring, or collecting doses of CT radiation, where no national US legislation sets standards for CT dose (41). In the USA, states (California and Texas) have related legislation to CT scanning. In Japan, a consortium group has recommended national DRL; however, these have not been adopted widely (42). Moreover, different image quality is required for different clinical indications of the same anatomical area (e.g. evaluation of kidney stones can be performed using lower radiation doses in comparison with those used in the evaluation of general radiology), because detection of high-contrast structures is less affected by high image noise than low-contrast structures (24). To our knowledge, this is the first study that reported DRL low-dose renal examinations.

The aim of the present study was to establish a DRL for renal CT examinations, considering the lack of standard LDCT dose for renal examinations in the Middle East and Jordan.

Material and Methods

The institutional review board approved this nationwide survey before data collection was performed. In 2019, a total of 12 Jordanian tertiary hospitals that routinely perform renal CT for patients presenting with renal colic to the ED department was included in the study. Data from 1418 renal CT scans were gathered over the three-month period from August to October. The renal CT scans were performed using the protocol and scan sequence details listed in Table 1 below on several scanners from four manufacturers (Philips, Toshiba, GE, and Siemens).

Table 1.

Information about the protocols and scanners in the participating hospitals.

Hospital	Hospital type	No. of scans	Manufacturer	Capacity (no. of slices)	Protocol
1	Public	34	Philips	32	From xiphoid process to symphysis pubis
2	Private	31	Philips	32	From diaphragm to symphysis pubis
3	Private	49	Siemens	64	From diaphragm to symphysis pubis
4	Private	48	Philips	64	From diaphragm to symphysis pubis
5	Private	51	GE	64	From kidney to symphysis pubis
6	Public	76	Toshiba	16	From diaphragm to ischium
7	Public	32	GE	128	From diaphragm to symphysis pubis
8	Public	222	Philips	64	From diaphragm to symphysis pubis
9	University	413	Philips	64	From upper border of the kidney to lower symphysis pubis
10	Public	365	Toshiba	64	From diaphragm to symphysis pubis
11	Public	64	Philips	128	From kidney to symphysis pubis
12	Public	33	GE	16	From diaphragm to symphysis pubis

Routine quality control is conducted for scanners in Jordan (43). Automated tube current modulation and iterative reconstruction were used in all systems except hospitals 6 and 12. The DRL quantities (CTDI_vol and dose length product [DLP]) for the scans were provided in different numbers of renal CT scans in each hospital as shown in Table 1.

Patient demographics

There were 1418 participants (589 women [41.5%], 829 men [58.5%]; age range = 18–94 years; interquartile range [IQR] = 24). The standardization of weight for the sampled population is in line with international recommendations; the survey included only those patients who weighed 60–80 kg (Table 2).

Table 2.

Participants’ demographics.

Characteristics		Number (percentage)
Sex	Male	829 (58.5)
Sex	Female	589 (41.5)
Characteristics
Age (years)	24 (18–94)
Weight (kg)	68.4 (60–80)

Values are given as n (%) or median (interquartile range).

Data collection

A clinical coordinator with CT experience was appointed in each hospital to distribute and receive the questionnaires. The questionnaires have been previously designed and validated (40). The first part will record details on standard protocols used for specific examinations including information on exposure factors, procedures, and radiation dose (Table 3). Each survey was filled in a private room and no patient identification was recorded as it was an anonymous survey. The survey focused on patients who were referred from the emergency department requiring a renal CT for suspected renal stones.

Table 3.

Characteristics of the renal CT scans.

Characteristics	IQR (range)
FOV	153 (35–1414)
No. of slices	312.25 (34–1569)
Slice thickness	1 (0.5–10)
Factors
kVp	122 ± 11.56
mAs	222 ± 78.44

Values are given as median (IQR) or mean ± SD.

FOV, field of view; IQR, interquartile range.

Radiation dose data

On CT scanners, CTDI_vol and DLP are provided for every sequence and examination. The patient dose data, CTDI_vol and DLP values, were extracted from the Picture Archiving and Communication System (PACS). A summary of the two relevant dose parameters is given below.

Weighted CT dose index (CTDI_w)

The CTDI_w is a fundamental dosimetric index used in CT. It can be measured using standard cylindrical PMMA phantoms (head and body CTDI phantoms) and a pencil chamber.

Using CTDI_w values obtained with a specific CTDI phantom for a CT scanner model, with specific exposure factors, CTDI_w can be calculated for any sequence, for any model according to the following formula, suggested by IMPACT group:

C T D I_{w} (m G y) = (n C T D I_{w})_{scanner, phantom, k V, N \times T} \times (F_{N \times T})_{scanner} \times C

(1)

Where (

n

CTDI_w)_{scanner, phantom, kV, N×T} is the normalized CTDI_w for a specific scanner at a particular kV, slice number (N) and thickness (T), (F_N×T)_scanner is a coefficient for all other slice numbers and thicknesses and C is mAs value.

Volumetric CT dose index (CTDI_vol)

Since CTDI_w does not consider whether axial slices are contiguous, non-contiguous, or overlapping, a “pitch” correction must be added, which provides a more representative volume CTDI or CTDI_vol (Eq. 2):

C T D I_{v o l} (m G y) = \frac{C T D I_{w}}{Pitch factor}

(2)

Where pitch factor is the distance, the table moves in the z axis divided by the slices number and each slice thickness. CTDI_w is calculated from Eq. 1.

Dose length product

The dose measurements above do not consider the total length of the patient who has been irradiated during each examination sequence. This is calculated using Eq. 3:

D L P (m G y \cdot c m) = C T D I_{v o l} \times Scan length

(3)

Where CTDI_vol is calculated from Eq. 2.

Data analysis

The minimum, maximum, and the first, second, and third quartiles were calculated for CTDI_vol, and DLP using SPSS software version 22.0 (IBM Corp., Armonk, NY, USA). The median values of each site were calculated, and the 75th percentile of the distribution of the median value of CTDI_vol and DLP were used to define DRL.

Results

Table 4 shows the characteristics of the renal CT protocol results in the field of view (FOV) range of 35–1414 (IQR = 153), while the number of slices were in the range of 34–1569 slices (IQR = 312.25). The mean kilovoltage peak (kVp) factor was 122 ± 11.56, while the mean milliampere-seconds (mAs) was 227 ± 78.44.

Table 4.

Parameters recorded from each clinical center for each scanning sequence to calculate DRLs.

Exposure factors	Procedural data	Dose data
Tube voltage (kV)	Anatomical range	CTDI_vol
Tube rotation time (s)	Standard sequence always used?	DLP
Tube current (mA)	Beam collimation
Displayed mAs:	Axial or helical scanning?
mAs/slice	Table increment and pitch
effective mAs	Imaged slice thickness
Auto dose reduction used?	Scan length

DRL, diagnostic reference level.

The results of the statistical evaluations for the 12 hospitals are listed in Table 5. The differences among the calculated quantities of the 12 participating hospitals for the renal CT examinations are reported in Table 6. The DRL for the CTDI_vol was 16.15 mGy and 851.77 mGy.cm for the DLP. Fig. 1 shows the median CTDI_vol and DLP distribution for the 12 surveyed hospitals, with the horizontal line representing the DRL. The median CTDI_vol values were in the range of 8.9–18.58 mGy, while the median DLP values were in the range of 494.10–975.9 mGy.cm. The differences between the median values among the hospitals were not significant except for some cases where some of the values were doubled compared with the other hospitals. For example, the median CTDI_vol value for the hospital (7) was 8.84 mGy compared with 17.63 mGy for the hospital (1). Similar observations were made for the DLP values for the same hospitals. The local median values of CDTI_vol are higher than DRL in two hospitals; considering the local median value of DLP, a third hospital slightly exceeds the DRL.

Figure 1.

(a) Distributions of median CTDI_vol values for renal CT examination. (b) Distributions of median dose length product values for renal CT examination. CT, computed tomography.

Table 5.

DRLs for the surveyed renal CT scans in the 12 hospitals.

DRLs		Value
CTDI_vol	Minimum	8.84
	25th	10.82
	Median	14.07
	75th	16.15
	Maximum	18.85
DLP	Minimum	594.19
	25th	568.05
	Median	728.17
	75th	851.77
	Maximum	975.90

CT, computed tomography; DLP, dose length product; DRL, diagnostic reference level.

Table 6.

Results of the statistical evaluation for the 12 participating hospitals for the renal CT examinations.

	Hospital	1 (n = 34)	2(n = 31)	3(n = 49)	4(n = 48)	5(n = 51)	6(n = 76)	7(n = 32)	8(n = 222)	9(n = 413)	10(n = 365)	11(n = 64)	12(n = 33)
CTDI_vol	Minimum	6.3	9.7	6.09	7.59	3.85	3.2	2	5.8	10.11	7.9	5.2	8.48
	25th	10.97	12.88	9.13	9.67	12.58	6	7.07	10.2	16.12	16.12	9.05	12.99
	Median	17.63	14.4	11.13	9.875	18.58	8.9	8.84	13.74	16.13	16.13	11.7	16.2
	75th	28.3	23.53	13.12	19.25	24.78	13.9	12.73	18.39	17.2	19.3	16.92	19.09
	Maximum	28.3	28.3	30.34	20.61	24.78	21	21.08	51.7	36.2	28.7	27.1	25.42
DLP	Minimum	298.2	439.5	189.01	400.5	184.3	130	303.25	284.6	43	43	231.9	316.14
	25th	578.05	698.76	395.25	487.84	555.82	317.62	343.65	502.22	670.8	737.4	451.5	633.02
	Median	975.9	778.69	494.19	596.4	947.21	525	494.4	673.65	855	850.7	582.4	822.17
	75th	1301.85	990.73	550.37	987.82	1264.97	775.65	656.76	859	905.2	911.02	851.92	1086.9
	Maximum	1602.2	1595.1	1448.12	1204.2	1302.15	1073.7	1209.14	1535	1942	1942	1305	1397.13

CT, computed tomography; DLP, dose length product; DRL, diagnostic reference level.

Discussion

The results have shown variations among the hospitals regarding the DRL quantities (CTDI_vol and DLP) in non-contrast renal CT in the detection of renal stones. Those variations can be attributed to many factors including protection of commercial interests and recent changes to imaging prescribing regulations (44). Depending on the protocol employed, large variations in LDCT have been reported with effective dose ranges of 0.7–16 mSv (7). In this study, the doses were in the range of 0.7–24 mSv.

Regarding the range of doses recorded, large variations were evident across the surveyed hospitals in reported CTDI_vol and DLP values. This is in line with previous work, which has shown that variations may occur depending on CT scanner design (43,45) and the protocol employed (45–47). The particular make and model of the CT scanner may lead to some variation in doses owing to inherent differences such as filtration, beam geometry, number of detector rows, scattered X-rays, and noise value calculations (48,49). When identical scanners across different sites were examined here, variations of up to 89% were noted, demonstrating that dose differences are not all attributable to the CT scan design and a large scope for optimization exists (50).

One would also rightly expect a certain range of doses if hospitals are correctly varying parameters for each individual patient. However, such large variations between hospitals cannot be accounted for based on differences in patient size alone. The main CT parameters that affect dose are peak tube potential, tube current, automatic tube current modulation use, collimation, scan length, and the use of either spiral or sequential scanning. It is evident from the review of each site that large differences in scan parameters exist for each CT examination and that some CT sites performed identical examinations using significantly less radiation. This is an immediate cause for concern and implies an urgent need for optimization among sites. A number of sites in particular require attention because their median DLP values exceeded the DRL proposed here.

The findings of this study indicate that there is a large potential for dose optimization across Jordanian hospitals and especially in those that consistently exceed the proposed DRL. This can be achieved by mandatory clinical audits within Jordan. This, combined with greater awareness, and adherence to new DRL should improve the quality of care given to patients and ensure that all doses of clinical radiation are kept as low as reasonably achievable.

In conclusion, the results of this survey suggest that a LDCT protocol can be used as the first-line and follow-up imaging tool in the clinical workup of patients with suspected renal colic provided that radiographers are aware of the limitations of dose-reduced fixed-tube-current protocols, especially in obese patients, compared with standard-dose CT. Effective strategies to reduce radiation dose are available but some strategies are not frequently used and thus further education is necessary. The comparable diagnostic image quality may support increased use of dose-saving strategies inadequately selected patients. Further research is necessary to define noise thresholds for the detection of urolithiasis and therefore reduce the collective dose employing automatic tube current modulation.

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.

ORCID iD

Mohammad Rawashdeh

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Establishment of diagnostic reference levels in low-dose renal computed tomography

Abstract

Background

Purpose

Material and Methods

Results

Conclusion

Keywords

Introduction

Material and Methods

Patient demographics

Data collection

Radiation dose data

Weighted CT dose index (CTDIw)

Volumetric CT dose index (CTDIvol)

Dose length product

Data analysis

Results

Discussion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Weighted CT dose index (CTDI_w)

Volumetric CT dose index (CTDI_vol)