Adequacy of Low Dose Computed Tomography in Patients Presenting with Acute Urinary Colic

Abstract

Background and Purpose:

Noncontrast abdominal/pelvic CT is the current imaging standard for patients who present with acute urinary colic. Conventional CT, however, exposes the patient to significant amounts of ionizing radiation, which is cumulative when additional CTs are used to monitor stone migration, outcomes, etc. We sought to maintain diagnostic adequacy while decreasing our patients' radiation exposure from CT by using a reduced tube current, an abbreviated scanning area, and the use of coronal reformatted images.

Patients and Methods:

Between March 3, 2011 and October 31, 2011, 101 consecutive adult patients with suspected urinary colic were evaluated with a “low” dose CT. If the suspected calculus(i) was not seen, the patient underwent immediate conventional CT imaging customized to their body habitus. Radiation exposure for each patient was calculated using an established formula of dose length product and scan length. The effective total radiation dose was measured in millisieverts (mSv).

Results:

Overall, 84 patients had an upper tract calculus(i) consistent with the clinical suspicion. Of these, 76 (90%) were adequately imaged with low dose and 8 (10%) with conventional noncontrast CTs. The mean effective radiation dose in the 76 low dose stone-positive CTs was 2.14 mSV (median 2.10 mSv). This was almost seven-fold lower than the mean conventional stone-positive CT dose of 14.5 mSv (median 13.1 mSv).

Conclusions:

Low dose noncontrast CT provided adequate imaging to guide optimal urologic management in the majority of our patients. This modality offered a significantly lower ionizing radiation dose and should be considered in patients who present with acute urinary colic.

Introduction

T he prevalence and incidence of upper urinary tract (UUT) (kidney and ureter) calculi is increasing.^1,2 This is most likely attributable to an enlarging, aging population and possibly to some evolving environmental factors such as changes in diet as suggested by Goldfarb and associates.³ Attendant with the increased number of calculi are increased numbers of imaging modalities used to assess patients with suspected upper tract stone disease. Since the initial report in 1995 by Smith and collealgues⁴ that described the high sensitivity of CT in detecting and detailing upper tract calculi compared with intravenous urography, noncontrast CT has become the examination of choice.

Noncontrast CT imaging is rapid and explicit in describing stone location, stone size, stone number, and stone fragility (Hounsfield units, heterogeneity) in patients with upper tract calculi. Conventional CT exposes the patient to significant amounts of ionizing radiation. When additional CTs are used to monitor recurrence, stone migration, spontaneous stone passage, and outcome, the radiation dose is cumulative. This is a particular concern in young persons who undergo repeated CT examinations for recurrent stone disease (50% within 5–10 years; 75% within 20 years) and who are at risk for greater lifetime radiation exposure because of their anticipated life expectancy.^5,6 The reported effective radiation doses with noncontrast abdomen/pelvis CT range from 2.8 to 13.1 millisieverts (mSv) and 4.5 to 18 mSv, for men and women, respectively, with means around 10 mSv for each area (abdomen or pelvis) imaged.⁷

The increased frequency of diagnostic imaging using ionizing radiation has generated concern for the associated lifetime attributable risk of radiation-induced diseases, including malignancies.⁸ In a recent report, 72 million CTs were performed in the United States in 2007. Nearly one-third of these were used to image the abdomen and pelvis.⁹ Although it is difficult to accurately quantify the potential cancer risk associated with CT, it is projected that approximately 29,000 future radiation-induced malignancies will be attributed to radiation exposure from CTs performed in 2007 alone.⁹

Numerous techniques have been described to reduce the radiation exposure to patients undergoing evaluation and treatment of suspected and known UUT calculi—e.g., in endoscopic stone procedures, adherence to the ALARA (As Low As Reasonably Achievable) principles.¹⁰ In CT scanning, various maneuvers can be used to minimize radiation doses. These include the use of multidetector CT, high resolution coronal reformatting, abbreviated scanning ranges (upper renal pole to bladder), “noise” index elevation, reduction in tube potential (kilovolt peak), tube current (milliamperes [mA]), etc.^11,12

For optimal management of UUT calculi, urologists need an adequate radiographic assessment of the presence or absence of calculi, stone characteristics (including size, location, and density) and their effects on the patient. We sought to determine the imaging adequacy of low ionizing radiation dose CT by using a reduced tube current, an abbreviated scanning area (upper renal poles through the bladder), and coronal reformatted images to image the UUT.

Patients and Methods

Patient population

Between March 3, 2011 and October 31, 2011, 101 consecutive adult patients with suspected UUT calculi-induced colic were assessed with a noncontrast low dose abdomen and pelvis CT. Institutional Review Board approval was obtained. The low dose CT was prospectively reviewed, for the presence or absence of stones, by 1 of 11 randomly assigned board certified radiologists. If a suspected symptomatic calculus was identified, targeted views (additional focused images) were performed for confirmation through the area of interest. In patients with a clinical diagnosis of renal colic in whom a calculus was not identified on the low dose CT, immediate additional conventional dose images were obtained using Auto-mA technology customized to the patient's body habitus. Auto-mA allows the scanner to optimize image quality by automatically adjusting the amount of dose needed on a specific patient's body part at the lowest radiation dose. Each patient's body mass index (BMI) was calculated at the time of the urologic visit.

Technical imaging considerations

All studies were performed without contrast on a standard software 64 slice CT scanner. The imaging protocol for the CT scan of the abdomen and pelvis was from the superior poles of the kidneys to the pelvic floor at 5-mm increments helically.

The tube charge per gantry rotation used during the low dose scan was as follows: Tube current: 85 mA; time slice: 0.5 seconds; pitch (degree of overlap between adjacent CT slices): 1.375. The total mAs was calculated using the formula, mA×sec/pitch, as follows: 85×0.5/1.375=30.9 mAs. After the low dose images were acquired, a new set of axial (2.5 mm thickness with 1.25-mm spacing) and coronal (2.5 mm thickness with 2.5-mm spacing) images for reconstruction purposes were created. Because the urinary system is coronally oriented, the detection of urinary stones is improved with coronal reformatted images. This can aid in the differentiation of urinary stones from calcifications such as phleboliths, calcified vascular plaques, and renal parenchymal calcifications.¹² Thinner (1–3 mm) reconstructed sections offer improved detection and characterization of urinary calculi—particularly small stones, because of the reduction in partial volume averaging effect.¹³

If a stone was not detected during the low dose CT, additional images immediately followed using the body habitus auto-mA technology. The Auto-mA factors used were as follows: Tube current: 150 mA to 700 mA range; time slice: 0.6 seconds; pitch: 1.375; noise index: 10. The total mAs were calculated as follows: Tube current (range 150–700)×0.6/1.375=mAs.

Effective dose calculation

The radiation dose delivered by low dose CT, according to the manufacturer, was formulated using a normalized weighted CT dose index (_nCTDI_w) in air of 0.070 milligray (mGy)/mAs at 120 kV. Radiation exposure for each patient was calculated as follows: DLP (mGy-cm)=CTDI_vol)×L where DLP is the dose length product, CTDI_vol is the volume CT dose index, and L is the scan length. The effective dose delivered by CT was calculated using the 64 slice CT dose report that uses DLP (mGy-cm)×k factor of 0.0171. The effective radiation dose was measured in mSv, which takes into account the sensitivity of the exposed organs and uses the absorbed radiation dose measured in mGy and the scan length.

Data collection and analysis

Reference standard: We used only the presence or absence of an offending stone on the low dose or conventional CTs as our reference standard for the detection of a calculus(i). All CTs were immediately interpreted detailing the presence, number, size (largest diameter), density (Hounsfield units), and location of the upper urinary tract calculus(i). These data and the patient's BMI are detailed in Table 1. Secondary signs of upper tract obstruction (perinephric stranding, collecting system dilatation, etc.) were noted, but without a CT evident calculus(i) were not considered as our reference standard. In scans inconsistent with the clinical diagnosis of a symptomatic UUT calculus(i), notation was made of any CT evident disease entity mimicking the patient's symptoms of urinary colic.

Table 1.

Calculus(I) Location (Site Frequency), Size Range, Density Range, and Body Mass Index Range in Stone Positive Computed Tomography in Patients Presenting with Clinical Urinary Colic N=84

“Low” dose CT				Conventional dose CT
N=76				N=8
Calculus(i) location (frequency)	Size range (mean)	Hounsfield unit range (mean)	BMI range (mean)	Calculus(i) location (frequency)	Size range (mean)	Hounsfield unit range (mean)	BMI range (mean)
Right renal only (8)	2–14 mm (5.25)	132–1483 (786.9)	15.5–36.9 (26.4)	Right renal only (1)	2 mm (2 mm)	450 (450)	21.8 (21.8)
Right ureteral only (13)	2–9 mm (4.69)	150–1282 (792)	18.2–34 (26.3)	Right ureteral only (0)	n/a	n/a
Both right renal and ureteral (5)	2–14 mm (7.6)	600–1481 (1168)	22.6–31.0 (25.0)	Both right renal and ureteral (1)	2–3 mm (2.5)	392–408 (400)	20.9 (20.9)
Left renal only (9)	2–13 mm (3.44)	167–1400 (792)	21.7–28.3 (26.6)	Left renal only (0)	n/a	n/a	n/a
Left ureteral only (10)	2–7 mm (4.1)	330–1410 (778)	22.5–32.6 (26.5)	Left ureteral only (3)	2–4 mm (2.6)	889–1500 (1140)	26.2–26.6 (26.4)
Both left renal and ureteral (5)	2–15 mm (4.4)	400–1387 (840)	22.5–30.2 (25.8)	Both left renal and ureteral (0)	n/a	n/a
Bilateral (26)	1–16 mm (3.88)	200–1487 (909)	19.6–36.0 (27.2)	Bilateral (3)	1–4 mm (2.9)	278–507 (420)	26.6–27.7 (27.2)

CT=computed tomography; BMI=body mass index.

Following the initial radiologist's written findings, three radiologists blinded to the CT report independently reviewed the stone positive low dose CT (n=76), the stone positive conventional dose CT (n=8), as well as the stone negative CTs (n=17).

Statistical analysis

Assuming that the patients tested positive using a low dose CT at a rate of 80%, we estimated that we would need a total of 97 patients to detect this rate with a 95% confidence and a maximum difference of 8% between our sample rate and the true population rate. Statistical analyses were performed using the SPSS statistical software program (SPSS version 20, Chicago, IL) Interreader agreement of low dose scan results was assessed by calculating the Cohen kappa coefficient between each pair of the three readers. The Cohen kappa index is an established index that measures the proportion of agreements that is actually observed between raters, after adjusting for the proportion of agreements that take place by chance.

Results

Between March 3, 2011 and October 31, 2011, 101 consecutive adult patients with suspected acutely symptomatic UUT colic necessitating CT assessment underwent noncontrast low dose abdomen and pelvis CT examinations. There were 70 men and 31 women patients. The median age was 53.4 years (range 21–82).

Eighty-four patient CTs (84%) demonstrated an upper tract calculus(i) consistent with the clinical impression of acute urinary colic. Low dose CT identified a stone consistent with the clinical diagnosis in 76 of the 84 (90.5%) patients. Direct CT evidence of a stone on low dose CT was confirmed by additional targeted images of the identifiable calculus imaged in all cases. The remaining 8 of the 84 (8.5%) patients with clinical urinary colic had the suspected upper tract calculus(i) reported on conventional CT only. The mean added radiation dose to the patients who needed an additional conventional scan because no stone was visualized on the low dose scan was 14.475 mSv (standard error: 1.6788, standard deviation: 4.7457).

The radiologists' explanations as to the reason these stones were not imaged on low dose CT were as follows: Two were missed (each <2 mm), three patients had symptoms on the contralateral side, one patient had an upper tract stone and lower abdominal pain, one stone was initially thought to be a vascular calcification, and one patient had contralateral hydronephrosis. Of the eight missed stones, four were intrarenal (two lower-pole, two midpolar) and four were in the ureter (one proximal, three distal). We used only the presence or absence of an identifiable stone on the low dose (targeted confirmed) or conventional CTs as our reference standard. With this, the sensitivity of low dose CT in detecting UUT calculus(i) in our selected group of consecutive patients presenting with clinical urinary colic was 90.5%.

The urologists were uniformly satisfied with the quality of the low dose images. Urologic management of the 84 patients consisted of conservative therapy (observation, medical expulsive therapy) in 56, ureteroscopy in 14, shockwave lithotripsy in 11, percutaneous nephrolithotomy in 2, and ureteral stent only in 1.

In 17 (17%) patients, an upper tract calculus was not identified on CT. There were no abnormal radiographic findings in seven of these patients. The remaining 10 patients had varied findings, including bladder calculi in 2, adnexal cysts in 2, collecting system dilatation, pyelonephritis, perinephric stranding, bladder tumor, diverticulitis, and pleural effusion in 1 each.

Interreader variability

In addition to the original radiology report, independent blinded radiographic assessment yielded high interobserver agreement (Cohen kappa coefficient) among the three radiologists (R1, R2, R3). This was observed to be as follows: Kappa R1R2=.88 (.03), P<0.001, kappa R1R3=.87 (.03), P<0.001, and kappa R2R3=.87 (.03), P<0.001.

Effective dose results

The dose length product in the low dose CT (median 123; range 90–184) was six to seven fold less than that of the conventional dose CT (median 764; range 579–1273). These reductions occurred primarily because of significant diminutions in the tube charge gantry rotations in low dose CT compared with conventional CT. Similarly, the amount of ionizing radiation expressed as the effective radiation dose in mSv was six to seven fold less with the low dose CT (median 2.1; range 1.53–3.1) than with conventional dose CT patients (median 13.1; range 9.9–21.8). The mean mSv dose in the low dose (2.14±.24) was significantly lower (mean mSv difference 12.3, 95% confidence interval: −13.4 to −11.3, P<0.001) than the conventional dose group (14.5±4.7) (See Table 2).

Table 2.

76 Stone Positive “Low” Dose Computed Tomography (Confirmed by Targeted Scans)

	N	Range	Minimum	Maximum	Mean		Std. deviation
	Statistic	Statistic	Statistic	Statistic	Statistic	Std. Error	Statistic
DLP (“low” dose)	76	94	90	184	125.30	1.601	13.953
DLP targeted	76	997	47	1044	260.34	21.321	185.870
mSv (“low” dose)	76	1.6	1.5	3.1	2.143	.0274	.2386
mSv targeted	76	17.0	.8	17.9	4.452	.3646	3.1784
Total mSv	76	17.2	2.5	19.8	6.594	.3682	3.2101
Valid N (listwise)	76

8 Stone Positive Conventional Computed Tomography (Not Imaged Initially On “Low” Dose)
	N	Range	Minimum	Maximum	Mean		Std. deviation
	Statistic	Statistic	Statistic	Statistic	Statistic	Std. error	Statistic
DLP (“low” dose)	8	41	112	153	128.38	4.615	13.053
DLP Conventional	8	694	579	1273	846.50	98.120	277.524
mSv (“low” dose)	8	.7	1.9	2.6	2.195	.0789	.2232
mSv Conventional	8	11.9	9.9	21.8	14.475	1.6778	4.7457
Total mSv	8	12.4	11.9	24.4	16.670	1.7294	4.8915
Valid N (listwise)	8

Std=standard.

Discussion

CT is the radiographic imaging modality of choice for the initial diagnosis of urolithiasis and is often used for follow-up aside from the difficult to visualize uncommon pure matrix and indinavir stones (Hounsfield units 15–30).^14
–16 Despite the unparalleled accuracy of conventional CT in diagnosing UUT calculi, the amount of ionizing radiation exposure to the patient is significant.

The International Commission on Radiologic Protection accepts a recommended radiation dose limit per adult for occupational exposure of no more than 20 mSv per year averaged over a 5-year period with no more than 50 mSv in any year.¹⁷ There are no currently acceptable guidelines for annual patient (medical) radiation exposure. The amount of ionizing radiation exposure from noncontrast abdomen and pelvis CT scans varies considerably given the wide range of variability between CT scanners and settings, but often approximates 20 mSv.⁷

There are numerous ways in which the effective radiation dose exposure from noncontrast abdomen/pelvis CT can be reduced while maintaining adequate image quality for optimal urologic evaluation and management. Urologists wish to know of the presence or absence of a calculus(i), and, if present, the number, size, location, density of the calculus(i), and its effect on the patient's urinary tract.

Our study revealed direct CT evidence of a clinically identifiable calculus(i) in 84 of 101 consecutive patients who presented with suspected acute urinary tract colic. Of the 84 (90.5%) stone positive CTs, 76 were low dose studies and were deemed by the treating urologists to provide adequate information for patient treatment. In addition, there was high interobserver agreement as to the CT findings among the three independent blinded radiologists (Cohen kappa interpretations).

Poletti and coworkers¹⁸ reported on a comparison of CT findings in 125 patients presenting with urinary colic who underwent both low dose and conventional dose CTs. They reported an overall 98% (91/93 patients) sensitivity and 100% (32/32 patients) specificity of low dose CT scans compared with conventional scans when identifying indirect signs of renal colic, and 95% sensitivity (77/81) patients) and 97% specificity (30/31) when identifying those with direct signs of a ureteral calculus and a BMI≤30. The sensitivity was 50% (2/4) and the specificity 89% (8/9) in patients with a BMI≥30. Their low dose CT protocol sensitivity in patients with BMI≤30 and a ureteral stone≥3 mm was 100%; for calculi≤3 mm, the sensitivity for identification was 86%.

Of our eight stone-positive patients visible only on conventional dose CT, two were clearly missed on low dose CT and were each ≤2 mm. None of our eight stone-positive patients whose stone was visible only on conventional dose CT had a BMI≥30, although we would expect there to be less accurate/adequate imaging in low dose CT in patients with a high BMI.

We performed conventional stone protocol abdomen/pelvis CTs only in patients who did not have direct evidence of an upper tract calculus visualized on low dose CT to limit the amount of ionizing radiation exposure to the majority of our patients. We did not consider a scan positive if it demonstrated only indirect signs of renal colic (perinephric stranding, dilated collecting system, etc.) even though these findings have been reported to have had high sensitivity and specificity for the presence of an existing and/or recently passed stone.¹⁹ Accordingly, it is conceivable that the analysis of our low dose CT results may have slightly underreported the number of patients with urinary colic secondary to calculi; however, it is unlikely that such a discovery would have altered clinical management.

By modifying our CT technique without specialized software or labor/time consuming modalities, we were able to significantly reduce the ionizing radiation exposure in the majority of our patients, compared with that of conventional CT. A six to seven fold decrease in radiation dose was achieved by using a low dose technique (tube current reduction to 85 mA), limiting the examination to the coronally oriented urinary tract and using reformatted imges. Compared with conventional (noncontrast abdomen/pelvis CT), the median dose (mSv) reduction for low dose CT was 84%.

We recognize that limitations of the low dose CT include reductions in image resolution and increased time spent in image reformatting. The 100% confirmation of all calculus(i) identified on low dose images by subsequent targeted scans, however, validates its accuracy in adequately imaging UUT stones. Overall, low dose CT was adequate for optimal urologic management in >90% of our CT stone-positive patients who presented with acute UUT colic.

Conclusions

A low dose CT scan provided adequate information for the optimal urologic management in the majority of our patients who presented with acute symptoms of UUT urolithiasis. Low dose CT offered the benefit of significantly minimizing radiation exposure. Physicians may wish to consider use of low dose CT scans in the evaluation and treatment of their patients with acute urinary colic.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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