
Japanese Journal of Radiology
1 3
Discussion
As expected, we observed the highest maximal recogniz-
able bronchial bifurcation order in CTVB by UHRCT uti-
lizing matrix size of 1024
2
and slice thickness of 0.25mm,
and that order was significantly higher than that obtained
using the values currently recommended for CTVB using
standard MDCT scanners (matrix size, 512
2
; slice thick-
ness, 0.5 or 1.0mm) [2]. The UHRCT scanner used in
our study has been in clinical application since 2017 and
achieved higher spatial resolution (maximal spatial resolu-
tion, approximately 0.15mm or less) than that of standard
MDCT scanners, even with the same voxel size [7–10].
Physical specifications improved by UHRCT included
the SHR scan mode (slice thickness, 0.25mm; number
of channels, 1792) and smaller x-ray tube focus (small-
est, 0.4 × 0.5mm). In fact, delineation of the anatomy of
the temporal bone has been reported more conspicuous
utilizing the improved detector of UHRCT than depic-
tion achieved using standard MDCT, even with the same
voxel size [11]. In addition, UHRCT facilitates the use of
smaller voxel size to decrease partial volume averaging,
so the superiority of CTVB by UHRCT to that utilizing
standard MDCT has been shown in delineating more distal
bronchi while preserving the continuity of the bronchial
inner surface [4, 6].
The maximal recognizable bronchial bifurcation order
by UHRCT ranged from 7.9 ± 1.4 to 13.1 ± 1.7 (median,
10; mean, 10.2 ± 2.4) in Group C, higher than that reported
by standard MDCT [4, 6]. Specifically, in the study by
Asano and colleagues, the median order was 6 using 16-
or 64-detector-row CT with matrix size of 512
2
and slice
thickness of 0.5 to 1.0mm; in the study by Khan and col-
leagues, the mean order was 6.5 ± 0.3 using 16-detector-
row CT with matrix size of 512
2
and slice thickness of
0.75mm. An ultrathin bronchoscope allows more distal
insertion than a larger conventional bronchoscope with
external diameter of approximately 5 to 6mm, and maxi-
mal insertion of the thinner scope to the ninth order has
been reported (median, fifth order) [4]. Thus, use of
UHRCT can better assist this maximal insertion of the
ultrathin bronchoscope. For transbronchial biopsy, diag-
nostic yield can be improved and examination time and
risk of complication reduced by insertion of an ultrathin
bronchoscope to PPLs with the aid of CTVB navigation by
UHRCT employing matrix size of 1024
2
and slice thick-
ness of 0.25mm [1, 4].
The maximal recognizable bronchial bifurcation order
was higher in the left S1 + 2 than in the right S1, pre-
sumably because the bronchial anatomy tends to detour to
the apex more prominently in the left S1 + 2. Image noise
and beam-hardening artifact caused by surrounding bony
structures in this apical area might diminish bronchial
delineation compared with both S10. However, the use of
UHRCT in combination with model-based iterative recon-
struction can improve the maximal recognizable bronchial
bifurcation order in this apical region. We excluded from
analysis a patient with poor breath-hold but did not per-
form electrocardiographically gated chest CT scanning,
which offers higher radiation exposure to patients. Thus,
the maximal recognizable bronchial bifurcation order was
lower in the left S10 than the right S10 and comparable
between Groups B and C only in the left S10, presumably
because bronchial delineation might be more susceptible
to motion artifacts from cardiac pulsation in the left S10.
According to the vendor of our workstation, its automated
tracking function permits the automatic drawing of a track-
ing line into bronchi with inner diameter of at least one
mm. Thus, improvement of this function will even further
increase the maximal recognizable bronchial bifurcation
order in CTVB by UHRCT.
Study limitations
Our study was limited because it was retrospective and
included only a small study population at a single institution,
and we restricted our pilot assessment of maximal recogniz-
able bronchial bifurcation order to only the right S1, left
S1 + 2, and right and left S3 and S10, whereas we selected
the right S1 and left S1 + 2 as the most apical segments, both
S10 as the most basal segments, and both S3 where the bron-
chi run almost parallel to the axial CT plane. We did not use
actual bronchoscopy as a reference to confirm delineation of
bronchial orifices, and insertion of even an ultrathin bron-
choscope to the maximal recognizable bronchial bifurcation
order delineated using CTVB navigation by UHRCT may
not be possible [4]. Confirmation of the clinical utility of
CTVB navigation by UHRCT to assist actual bronchoscopy
and thus transbronchial biopsy may warrant a large-scale
multicenter prospective study. Further, we used the only
workstation at our institution that was capable of generating
CTVB by UHRCT with matrix size of 1024
2
or more, but its
limited capacity to process high-volume data did not permit
reconstruction of UHRCT images with maximal matrix size
of 2048
2
. Our findings may also have been influenced by
the smaller body weight and BMI of our Japanese patients
compared to that of average-sized patients in Western coun-
tries, and the noise index in our study was that commonly
used for routine chest CT at our institution and might be
relatively small. Nevertheless, both the CTDI
vol
and DLP
complied with the criteria for radiation dose to patients for
standard chest CT (CTDI
vol
, 30mGy; DLP, 650mGycm)
according to European guidelines on quality criteria for
CT [12]. The lower radiation dose may have affected our
results by increasing image noise, whereas more advanced