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Original Research |
1 Department of Radiology, University Hospital Geneva, 24 rue
Micheli-du-Crest-14, Geneva 1211, Switzerland.
2 Department of Internal Medicine, University Hospital Geneva, Geneva,
Switzerland.
3 Urology Clinic, University Hospital Geneva, Geneva, Switzerland.
Received June 16, 2006;
accepted after revision September 12, 2006.
Address correspondence to P.-A. Poletti
(pierre-alexandre.poletti{at}hcuge.ch).
Abstract
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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MATERIALS AND METHODS. One hundred twenty-five patients (87 men, 38 women; mean age, 45 years) who were admitted with suspected renal colic underwent both abdominal low-dose CT (30 mAs) and standard-dose CT (180 mAs). Low-dose CT and standard-dose CT were independently reviewed, in a delayed fashion, by two radiologists for the characterization of renal and ureteral calculi (location, size) and for indirect signs of renal colic (renal enlargement, pyeloureteral dilatation, periureteral or renal stranding). Results reported for low-dose CT, with regard to the patients' body mass indexes (BMIs), were compared with those obtained with standard-dose CT (reference standard). The presence of non-urinary tract-related disorders was also assessed. Informed consent was obtained from all patients.
RESULTS. In patients with a BMI < 30, low-dose CT achieved 96% sensitivity and 100% specificity for the detection of indirect signs of renal colic and a sensitivity of 95% and a specificity of 97% for detecting ureteral calculi. In patients with a BMI < 30, low-dose CT was 86% sensitive for detecting ureteral calculi < 3 mm and 100% sensitive for detecting calculi > 3 mm. Low-dose CT was 100% sensitive and specific for depicting non-urinary tract-related disorders (n =6).
CONCLUSION. Low-dose CT achieves sensitivities and specificities close to those of standard-dose CT in assessing the diagnosis of renal colic, depicting ureteral calculi > 3 mm in patients with a BMI < 30, and correctly identifying alternative diagnoses.
Keywords: abdominal imaging • CT • low-dose CT • radiation dose • urinary tract
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Several authors have reported that low-dose CT protocols, with substantial
reduction of tube charge current or an increase in the pitch, may be used in
the screening of patients with suspected renal colic
[11-15].
More recently, clinical
[16-18]
and ex vivo porcine phantom
[19] studies have evaluated a
low-dose MDCT protocol using low tube charge current (
30 mAs), which
delivers a dose of radiation close to that delivered by abdominal radiography.
Such low-dose CT protocols resulted in a substantial reduction (50%) of
radiation dose when compared with initial management with abdominal
radiography and sonography, by reducing the need for further standard-dose CT
[18]. Furthermore, low-dose CT
might achieve sensitivities and specificities close to those reported for
standard-dose CT in detecting ureteral stones
[16,
17]. To our knowledge, no
study has specifically compared low-dose CT protocols ( 30 mAs) with those
of standard-dose CT as the sole reference standard for the depiction of direct
and indirect signs of renal colic and for determination of calculi size in a
clinical setting.
Some specific concerns have also been raised about using low-dose CT in overweight patients and regarding its capacity to depict alternate diagnoses [20]. This may explain why low-dose CT protocols using 30 mAs (or less) are not yet universally endorsed for the initial investigation of patients admitted with suspected renal colic [20].
The aim of our analysis is to compare low-dose CT using 30 mAs with standard-dose unenhanced CT for the detection of renal and ureteral calculi of various sizes and for the visualization of indirect signs of renal colic, according to the patient morphotype.
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30
(obese) [21].
Technical Imaging Parameters
Standard-dose CT and low-dose CT were performed from the lung bases to the
pelvis using a 4-MDCT scanner (MX 8000, Philips Medical Systems).
Standard-dose CT was obtained with 5-mm contiguous sections, a table speed of
5-mm/s (pitch = 1), 120 kV, and 180 mAs. Low-dose CT was performed with the
following parameters: reconstruction slice thickness, 5.0 mm; pitch, 1.25;
gantry rotation speed, 0.5 second; tube potential, 120 kV; tube charge per
gantry rotation, 30 mAs (75 mA x 0.5 s / 1.25 = 30 mAs). The average
scanned length, for both standard-dose and low-dose CT, was 40 ± 5 cm
for men and 35 ± 5 cm for women.
Effective Dose Calculation
Low-dose CT—According to data provided by the manufacturer,
the dose delivered by low-dose CT was estimated using a normalized weighted CT
dose index (nCTDIw) in air of 0.070 mGy/mAs at 120 kV.
Radiation exposure was then calculated as follows:
 |
where DLP is the dose-length product, CTDIvol
is the volume CT dose index
[22], and L is the
scan length. For women, the CTDIvol was equal to
 |
The effective dose (E) was obtained by applying the following
relationship:
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Ewomen = DLPair x f [23] (where f is a specific conversion factor)
Ewomen = 192.5 x 0.0110 = 2.1 ± 0.3 mSv.
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The same calculation was applied to men using a 40-cm scan length instead
of 35 cm:
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Standard-dose CT—Using 180 mAs instead of 30 mAs, the same
calculation as used for low-dose CT was performed to determine the effective
dose delivered by standard-dose CT:
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Data Collection and Analysis
Standard-dose CT images were immediately interpreted on the PACS by the
resident and attending radiologists on duty, and a written report was
transmitted to the referring physician (standard procedure in our
institution). Low-dose CT images were stored in the PACS but were not
interpreted by the radiologists on call. At the end of the study, low-dose CT
scans were interpreted independently by two board-certified attending
radiologists with 9 and 4 years' experience in abdominal CT who were blinded
to standard-dose CT findings, patient names, and demographics. Low-dose CT
images were analyzed in a random order by both radiologists, using the same
workstation and the same visualization software (Cedara I-softview, version
6.1, Cedara software).
The following information was recorded for each low-dose CT examination and
reported on a standardized form: First, the number, size (largest diameter on
the axial plane), and location of calculi in the urinary tract. Renal and
ureteral calculi were sorted into three categories according to their size:
0-2.9, 3-4.9, and 5 mm. Location was reported as renal or ureteral.
Second, the presence of a rim sign, defined as a halo of soft-tissue
attenuation around the circumference of an ureteral stone. This sign has been
considered useful for differentiating ureteral calculi from phleboliths
[24-27].
Third, any indirect signs of ureteral obstruction (renal enlargement,
pyeloureteral dilatation, or periureteral or perirenal stranding). Fourth, the
presence of non-urinary tract-related disorders that could explain the
patient's symptoms. Disparities between reviewers were solved by consensus
after review of the low-dose CT images and were recorded on a separate
form.
Once all low-dose CT examinations were inter-preted, standard-dose CT images were analyzed a second time, in a random order, in consensus, by the same two reviewers, using standardized forms similar to those used for MDCT. If a patient also underwent contrast-enhanced CT, only the unenhanced CT images were interpreted. An independent nurse was especially employed to help organize this data collection and analysis part.
Statistical Analysis
Statistical analyses were performed using statistical software SPSS,
version 11.5 (SPSS). The two-tailed Fisher's exact test was used for group
comparison, and the Student's t test was used for means comparisons.
Interobserver agreement between the two radiologists for the depiction of
renal and ureteral calculi of any size and for the depiction of calculi of at
least 5 mm was analyzed for both groups using Cohen kappa statistics. An
excellent interobserver agreement was defined as a kappa value of 0.81 or
more. A p value of less than 0.05 was indicative of a statistically
significant difference between two different sample populations. Data obtained
for patients with a BMI 30 were analyzed separately.
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30. When compared with standard-dose CT, low-dose CT was 97% (98/101) sensitive and 96% (23/24) specific for identifying at least one direct or indirect sign suggestive of renal colic. Twenty-six (21%) of 125 patients were considered to have no evidence of renal colic on low-dose CT. A direct or indirect sign of renal colic was detected on CT in three (12%) of these 26 patients. No statistically significant difference was seen between patients with different BMIs for the identification of a direct or indirect sign of renal colic.
Indirect Signs of Renal Colic
An indirect sign of renal colic (pyeloureteral dilatation, renal
enlargement, or perirenal or periureteral stranding) (Fig.
1A,
1B,
1C,
1D) was detected in 18 (40%) of
the 45 patients with no direct sign of calculus on low-dose CT and in 16 (39%)
of the 41 patients with no calculus on standard-dose CT.
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The sensitivity and specificity of low-dose CT for detecting direct or
indirect signs of renal colic in patients with a BMI < 30 and 30 are
reported in Table 1.
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Direct Signs of Renal Colic and Detection of Ureteral Calculi
An interobserver agreement between the two senior radiologists of 98.4% and
a kappa value of 0.97 ± 0.02 were obtained for the identification of
ureteral calculi on low-dose CT. An interobserver agreement of 96.1% was
achieved for identifying calculi as being < 5 mm, with a kappa value of
0.87 ± 0.06.
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5 mm.
No calculus smaller than 3 mm was overestimated at low-dose CT. The size of
ureteral calculi was correctly estimated to be between 3 and 4.9 mm on
low-dose CT in 69% (20/29) of calculi, underestimated in 24% (7/29), and
overestimated in 3% (1/29). Of the 29 calculi 5 mm at standard-dose CT,
76% (22/29) were correctly estimated as being 5 mm at low-dose CT.
Twenty-four percent (7/29) of them were underestimated at low-dose CT; in this
group, the smallest size reported at low-dose CT was 4.2 mm (5 mm at
standard-dose CT).
Renal Calculi
An interobserver agreement of 98% and a kappa value of 0.89 ± 0.04
were obtained for identification of renal calculi. At least one renal stone
was reported in 39 (31%) patients on low-dose CT and in 47 (38%) patients on
standard-dose CT.
A total of 96 renal calculi were detected at standard-dose CT, of which 58
(60%) were 0-2.9 mm; 23 (24%), 3-4.9 mm; and 15 (16%), 5 mm. No calculus
< 3 mm at standard-dose CT was overestimated at low-dose CT. The size of
renal calculi was correctly estimated to be between 3 and 4.9 mm on low-dose
CT in 96% (22/23) of calculi, underestimated in 43% (10/23), and overestimated
in 4% (1/23). The size of renal calculi was correctly estimated as 5 mm
at low-dose CT in 93% (14/15) and underestimated in 7% (1/15). An
interobserver agreement of 94.6% was achieved for the identification of a
renal calculus as being < 5 mm, with a kappa value of 0.89 ±
0.07.
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30, are reported in Table
2.
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Alternate Diagnoses
A diagnosis other than renal colic was directly suggested on six low-dose
CT and standard-dose CT examinations: appendicitis (n = 3),
pyelonephritis (n = 1), fecal impaction (n = 1), and
diverticulitis (n = 1) (Fig.
3A,
3B,
3C). Standard-dose CT did not
depict more alternative diagnoses than low-dose CT.
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Until our study, the accuracy of low-dose CT protocols for detecting ureteral stones, using a tube charge current < 50 mAs, was evaluated in only two series to our knowledge. In a study of 142 patients using a tube charge current of 20 mAs, low-dose CT was 97% sensitive and 95% specific in detecting ureteral calculi when compared with clinical and radiologic follow-up [17]. In another study, low-dose CT using a tube charge current of 30 mAs achieved a sensitivity between 89.5% and 94.7% and a specificity between 94.1% and 100% for detecting ureteral calculi [16]. Both series were limited by the absence of a reference standard and by the fact that further imaging was not systematically performed after low-dose CT was performed. Nevertheless, those authors had already suggested that low-dose CT protocols delivering a radiation dose close to that of an abdominal radiograph may be comparable to standard-dose CT for detecting ureteral stones. Our study data substantiate these observations.
To suspect the diagnosis of renal colic without having further information
about the calculus size and location is usually not sufficient to select the
most appropriate therapeutic approach, the need for hospitalization, or the
need for extracorporeal shockwave lithotripsy or other urologic procedures
[3,
28]. The size and location of
the calculus are determinants of the likelihood of spontaneous stone passage
[6,
29,
30]. Characterization of stone
morphology and location are the major advantages of CT over other imaging
techniques (abdominal radiography, sonography) commonly used in the initial
evaluation of patients with suspected renal colic
[1,
3]. A recent study using a
higher tube current (50 mAs) reported the limitation of low-dose CT for
detecting calculi 2 mm
[13]. However, to our
knowledge no prior study has specifically compared the size of calculi
measured on low-dose CT with those measured on standard-dose CT or on direct
stone analysis.
In the current series, low-dose CT was equivalent to standard-dose CT for
detecting stones 3 mm in the ureter of patients with a BMI < 30.
However, low-dose CT was limited (83% sensitive) in the detection of stones
< 3 mm. Nevertheless, these calculi rarely require urologic procedures; 5
mm is generally the critical size for urology referral because the likelihood
of spontaneous passage progressively decreases as a calculus exceeds this size
[6,
29-31].
The current analysis reveals that low-dose CT does not miss any calculus
3 mm in patients with a BMI < 30. Despite this excellent sensitivity,
low-dose CT is more limited than standard-dose CT in the determination of the
exact stone size, which is a major drawback of low-dose CT. Therefore,
clinicians should be clearly informed, before determining the optimal
treatment, that the size of calculi on low-dose CT may vary by ± 20%
compared with standard-dose CT results.
This observation does not corroborate the results of a porcine kidney phantom study showing that renal stone detectability and size remained constant on low-dose MDCT, despite a tube current reduction from 170 to 30 mA [19]. It is possible that the detectability and size of the calculi may depend of their chemical content. Indeed, oxalate stones were exclusively used in the porcine kidney phantom, whereas stone composition was unknown in our study, which constitutes a limitation of our data interpretation.
Some authors have reported that patients with a BMI > 31 should not
undergo a low-dose CT examination for the assessment of ureteral calculi
[12]. Despite the limited (not
statistically significant) number of patients with a BMI 30, our data
support their observations, although low-dose CT was highly accurate for the
detection of indirect signs suggestive of renal colic in obese patients.
Our study also analyzed the value of low-dose CT for detecting renal
calculi, which is important to evaluate the potential for recurrence. Our data
show that the sensitivity of low-dose CT for identifying renal calculi 3
mm is close to that obtained for detecting ureteral stones, but it is more
limited in the evaluation of smaller calculi (63% in patients with a BMI <
30). This can be explained by the fact that the renal stroma is more
heterogeneous than the ureter and periureteral space content; small calculi
may therefore be more easily confused with slightly hyperdense pyramids.
In our series, no alternative diagnosis was missed by using low-dose CT when compared with standard-dose CT. This observation is mitigated by the small percentage (4.8%, n = 6) of alternative diagnoses depicted on low-dose CT and standard-dose CT, which prevents our performing a statistically significant comparison between the two techniques and which is therefore a limitation of our study. In addition, the percentage of alternative diagnoses is inferior to the range (9.6-31%) reported in prior series [16, 18, 32, 33]. This observation can be explained by the fact that, in our study design, alternative diagnoses did not correspond to the final diagnoses but only to disorders revealed on low-dose CT and on unenhanced standard-dose CT. Some other diagnoses assessed by further examination (i.e., contrast-enhanced CT) were not considered in this comparative study. In the absence of a true reference standard, it cannot be inferred from our data that the diagnostic workup of renal colic should be strictly limited to low-dose CT. In unclear clinical presentations, various authors recommend performing higher-dose CT or contrast-enhanced CT after negative results on low-dose CT [16, 18, 20]. However, a recent clinical evaluation of low-dose CT (using 30 mAs) at our institution suggested that only a minority of patients (8/27, 30%) with suspected renal colic and normal findings on low-dose CT will require standard-dose CT [18]; most patients are spared the additional dose.
In conclusion, low-dose CT achieves sensitivities and specificities close to those of standard-dose CT for diagnosing renal colic, for correctly identifying alternative diagnoses, and for showing ureteral calculi of at least 3 mm in patients with a BMI < 30. However, the estimation of the calculi size on low-dose CT may vary by ± 20% with regard to standard-dose CT findings.
The results of this comparative study, along with those obtained from prior series [16-18], suggest that a low-dose CT protocol can be used as the first-line imaging tool in the workup of patients with suspected renal colic and a BMI < 30, providing that clinicians and patients are aware of the limitations and advantages of this technique with regard to standard-dose CT.
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