Value of Low-Dose Computed Tomography for
Examination after Extracorporeal Shock Wave
Lithotripsy for Urolithiasis
Takashi Hatano1*, Kentaro Chikaraishi1, Hiroyuki Inaba1, Katsuhisa Endo1,
Mayumi Tamari2and Shin Egawa3
1 Department of Urology, JR Tokyo General Hospital, Tokyo, Japan
2 Research Center for Medical Science Core Research Facilities for Basic Science, Jikei University School of Medicine,
3 Department of Urology, Jikei University School of Medicine, Tokyo, Japan
Takashi Hatano, M.D., Ph.D. Department of Urology, JR Tokyo General Hospital, 2-1-3 Yoyogi Shibuya-ku, Tokyo, 151-8528, Japan, Tel: +81-3-3320-2200; Fax: +81-3-3370-8501; E-mail:
Received: January 3, 2017; Accepted: April 4, 2017; Published: April 15, 2017
Hatano.T, et.al. (2017) Value of Low-Dose Computed Tomography for Examination after Extracorporeal Shock Wave Lithotripsy for Urolithiasis. J Urol Nephrol Open Access 3(1):1-5. DOI: 10.15226/2473-6430/3/1/00126
Objectives: To evaluate the usefulness of low-dose computed
tomography after Extracorporeal Shock Wave Lithotripsy (ESWL) for
Methods: We investigated 103 subjects with urolithiasis who
were treated with ESWL. In each case, preoperative plain abdominal
films of the Kidney, Ureter and Bladder (KUB) and Standard Computed
Tomography (SDCT) and postoperative KUB and Low-Dose Computed
Tomography (LDCT) were performed. We compared the Dose Length
Product (DLP) and Effective Dose (ED) of SDCT and LDCT. We
evaluated any residual calculus by performing postoperative KUB
and LDCT, calculating their respective rates of complete fracture, and
comparing the performance of both examinations.
Results: The mean DLP and ED were 409 mGy and 6.1 mSv with
SDCT, and 103 mGy and 1.5 mSv with LDCT; the dose reduction rate
was nearly 75% in both DLP and ED (P < 0.001). The mean ED of
KUB was 1.3 mSv, which was similar in dose to LDCT. The accuracy
of LDCT was 100%, with no false positives or false negatives. On the
other hand, the positive predictive rate of KUB was 90.4%, whereas
the negative predictive rate was 75.6%, and the accuracy was 78.6%
(P < 0.001).
Conclusion: The exposure dose of LDCT was reduced by 75%
compared with that of SDCT. Our findings suggest that LDCT is likely
to be a useful method to assess urinary calculi following ESWL.
Key words: Dose Length Product; Effective Dose; Extracorporeal
Shock Wave Lithotripsy; Low-Dose Computed Tomography;
Abbreviations and Acronyms
BMI = Body Mass Index; CT = Computed Tomography;
Ctdivol = Volume Computed Tomography Dose Index; DLP = Dose
Length Product; ED = Effective Dose; ESWL = Extracorporeal
Shock Wave Lithotripsy; KUB = Plain Abdominal Films of the
Kidney, Ureter and Bladder; LDCT = Low-Dose Computed
Tomography; NI = Noise Index; SDCT = Standard Computed
Tomography; US = Ultrasound
After ESWL for urolithiasis, KUB, US and CT are
widely used for the evaluation of residual calculi . KUB is a
conventional imaging technique, but several conditions, such as
bowel gas and overlapping of fragments and pelvic bones, often
decrease the sensitivity . In addition, KUB cannot evaluate
the degree of hydronephrosis. US is effective for assessment
of nephrolithiasis and hydronephrosis . However, it is not
appropriate for evaluation of the localization or size of ureteral
lithiasis. In addition, the diagnostic accuracy of US images is
remarkably reduced in obese patients. CT is a well-established
technique for the study of the improvement of hydronephrosis
and small calculi, but it cannot be performed frequently because
of the high doses of radiation exposure [4,5].
LDCT is a method that has been developed to reduce
the exposure dose associated with examination, and is mainly
performed for lung cancer screening [6,7]. Recently, the benefits
of LDCT during the diagnosis and follow-up of urolithiasis have
been suggested .To improve our understanding the benefits of
LDCT scanning for urolithiasis after ESWL, we performed LDCT
and assessed the exposure dose and usefulness for examination
Patients and study design: We investigated 103 cases in which
ESWL was performed for urolithiasis in our hospital. In each
case, preoperative KUB and SDCT and postoperative KUB and
LDCT were performed, and exposure dose measurements and
treatment effects were evaluated. For treatment effects, we
defined a complete fracture as one in which the spall fragments
of the treated calculus were less than 3 mm in size, others
being incomplete fractures . We evaluated any residual
calculi by performing postoperative KUB and LDCT, calculating
their respective rates of complete fracture, and comparing the
accuracy of both examinations. In LDCT, evaluation was also
performed to determine whether there was any improvement
in urinary tract obstruction after treatment. We conducted SDCT
once every other follow-up examination to verify the diagnostic
accuracy of LDCT and KUB. Radiolucent calculi or the patient
with BMI more than 30 were excluded. This study was approved
by the institutional review board of JR Tokyo General Hospital
Imaging conditions: The CT apparatus used was an Optima
CT660 (GE Healthcare, Tokyo, Japan). Imaging conditions were:
clear evaluation of the size, localization and configuration
of calculi was possible, and diagnosis was possible for the
surrounding organs. Settings of the CT were as follows: scanning
time: 0.7 seconds/rotation; tube voltage: 120 KV; tube current:
50 mA; slice thickness: 5 mm; reconstruction clearance: 5 mm. All
images were independently judged by one radiological diagnosis
specialist and one urologist.
Dose settings: The dose at the time of LDCT imaging was
configured to take into account noise and signal dispersion in the
SDCT images. The Noise Index (NI) of SDCT was configured as
8-10, while that of LDCT was configured at around 25. For this
reason, the Effective Dose (ED) was reduced by approximately
75% (Figure 1).
Figure 1: Comparison of preoperative SDCT and postoperative
The preoperative SDCT diagnosed a right R2 calculus. The NI was 8.4
and the ED was 6.2 mSv (a). The postoperative LDCT evaluated complete
fracture. The NI was 25 and the ED was 1.5 mSv (b). The dose reduction
rate of LDCT was 76%.
Detailed settings of the NI were optimized by the radiologist
and radiology technician by considering the physical status and
localization of the calculi.
Measurement of exposure dose: The exposure dose of the CT
examinations was calculated from the dose reports obtained
after the examinations. The absorbed dose was measured using a
phantom, and defined as CTDIvol, represented in mGy units. The
ED was calculated by deriving the Dose Length Product (DLP)
using the formula CTDIvol×L (length of imaging range: cm) and
multiplying the DLP value by the convention coefficient 0.015,
representing the result in mSv units.
ESWL: The lithotripter used was the LITHOSTAR Multiline
(Siemens, Erlangen, Germany). In each session 3,000 shocks
were delivered at a rate of 60 or 90 shock waves per minute.
The patients were monitored during the procedure by checking
the vital signs heart rate, respiratory rate, blood pressure, and
oxygen saturation (pulse oximetry).
All parametric variables were compared by using paired t-tests.
All analyses were considered statistically significant with a
p-value of < 0.05.
Patient characteristics and radiation dose
Table 1: Patient and stone characteristics
Median age (range)
Median BMI (kg/m2)
Median stone size (mm)
Table 1 shows the patient and stone characteristics. Of
the 103 patients, 79 were males and 24 were females. Twentyone
female patients were of reproductive age. The median stone
size was 9mm; 32 were renal and 71 were ureteral calculi.
The radiation exposure doses with DLP and ED are shown in
(Table 2). ). The mean DLP and ED were 409 mGy and 6.1 mSv,
respectively, with SDCT and 103 mGy and 1.5 mSv with LDCT;
the dose reduction rate was nearly 75% in both DLP and ED (P
< 0.001). The mean ED of KUB was 1.3mSv and almost the same
dose as with LDCT.
Table 2: The mean radiation dose according to the CT
Dose reduction rate (%)
409 ± 137
103 ± 36
6.1 ± 1.6
1.5 ± 0.6
Diagnostic performance of postoperative LDCT and
The rate of complete fracture by ESWL was 79.6% in
our study. The diagnostic accuracy of postoperative LDCT and
KUB is shown in (Table 3).
Table 3: Diagnostic performance of postoperative
LDCT and KUB
Positive predictive rate (%)
Negative predictive rate (%)
*: P< 0.001
The accuracy of LDCT was 100%, with no false positives or false
negatives. On the other hand, for KUB, there were false positives
for 2 of 21 cases that were true positives with LDCT, and false
negatives for 20 of 82 cases that were true negatives with LDCT.
The causes of false negatives included: intestinal gas surrounding
the calculus (9 cases); overlapping of the calculus and bones due
to movement of the calculus (7 cases), and others (4 cases). The
positive predictive rate of KUB was 90.4%, while the negative
predictive rate was 75.6%, and the accuracy was 78.6% (P <
Even if a calculus was clearly found in preoperative
KUB, it may not have been sufficiently diagnosed based on
postoperative KUB due to intestinal gas and movement of the
calculus following surgery (Figure 2). On the other hand, LDCT
was able to accurately diagnose the size, shape, and location of
the calculus regardless of intestinal gas and movement of the
calculus. In addition, LDCT was able to sufficiently evaluate
improvement in hydronephrosis and the existence of hematoma.
In LDCT, there were no images that were impossible to diagnose
due to noise and artifacts in our study.
Figure 2: The preoperative KUB and SDCT diagnosed a left U1 calculus
(a, c). The postoperative KUB evaluated complete fracture (b); however,
the postoperative LDCT showed a residual calculus (d). KUB was insufficient
in this case. The EDs of preoperative SDCT and postoperative LDCT
were 7.0 and 2.0 mSv, respectively. The dose reduction rate of LDCT was
72%.This patient was obese, so the EDs were higher than the mean dose.
CT provides clearer images and enables more specific
diagnostic imaging; however, we should be aware of the radiation
risks of CT and keep radiation exposures as low as possible
while achieving the required image quality and clinical benefit
. Since an earthquake led to a nuclear disaster in Japan in
2011, medical service providers and patients have recently paid
more attention to the radiation dose in medical imaging.
Urinary calculi occur in a wide range of age groups
from juveniles to the elderly. We must pay careful attention to
reproductive cells when conducting diagnostic X ray examinations.
Women aged 12-50 years are of reproductive age. In this
study, 87.5% of female patients were in this group. In the U.S.,
75% of urinary calculi are assumed to recur and require treatment
and follow-up for extended periods [11,12]. Accumulative
exposure may reach high dosages after X-ray examinations for
them. The dosage of radiation exposure during X-ray examinations
in Japanese patients is the highest in the world . The
majority of exposure is caused by CT. Radical improvement in CT
performance has dramatically decreased the exposure dose and
enabled the provision of clear images in recent years, and several
studies have shown the effectiveness of LDCT for diagnosing urinary
calculi [14-16]. ESWL is an operation to shatter the calculus,
not to extract it. Therefore, follow-up assessment using medical
imaging after the ESWL is very important. We implemented
LDCT with the ED reduced by approximately 75% compared to
SDCT. The accuracy of LDCT for urinary calculi after ESWL was
100%. The accuracy of LDCT for urinary calculi greater than 3
mm in size has also been reported to be 99-100% in other studies
. On the other hand, the accuracy of KUB was 78.6% in our
study. LDCT was also able to accurately diagnose the treatment
effect and evaluate hydronephrosis and hematoma. The ED of
LDCT was around 25% that of SDCT and equal to that of KUB. As
a result, LDCT other than KUB is recommended for examination
of urinary calculi following ESWL.
In the U.S., the annual number of CT examinations has
increased 20 fold in the last 30 years [17,18]. Medical service providers
have to pay sufficient attention to the potential risk of second
primary cancer in patients due to frequent implementation
of CT . The number of CT scanners per million populations in
Japan is 2.5 times higher than in the U.S. . Thus, decreasing
the exposure dose in CT is a serious matter. The ED of our LDCT
was 1.5 mSv, approximately equal to KUB. Recently, there have
been some reports regarding CT with an extremely low dosage,
less than the ED of KUB [20,21]. It is possible to decrease the ED
of LDCT by adjusting the NI. However, this decreases the image
quality and prevents accurate diagnosis. Diagnosable image quality
must be maintained and the dosage should not be blindly decreased
in CT. The purpose of inspecting urinary calculi following
ESWL is to evaluate the existence, size and location of residual
calculi, along with the improvement in hydronephrosis, and the
impact on surrounding organs. Optimization of the imaging conditions
is vital to obtain sufficient image quality to achieve these
There are several limitations of LDCT screening. First,
the condition setting of LDCT depends on the model of the CT
device. The exposure dose of patients differs even when using the
same LDCT due to differences in CT devices . Second, the appropriate
value of the ED for urinary calculi cannot be fixed. The
ED of our LDCT was 1.5 mSv. In prospective diagnoses for urinary
calculi, lower dosages may be sufficient for diagnoses by LDCT.
We should use SDCT and LDCT in accordance with the purpose
of the examination and pay sufficient attention to the exposure of
patients to radiation.
In conclusion, LDCT with the exposure dose reduced by
75% is an effective means of examining the effect of ESWL. For
standardization of LDCT, the appropriate setting of the dosage
and selection of indications are problems that remain to be resolved.
Once this is done, LDCT will be the major means of examining
for urinary calculi.
We acknowledge the support and assistance provided
by the radiologists and technicians of the Department of Radiology,
JR Tokyo General Hospital.
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