Kazuhiro Katada, M.D.
Fujita Health University, School of Health Sciences
The introduction of helical scanning
in clinical practice had such a great impact that it fundamentally
changed the diagnostic paradigm of computed tomography (CT). In
addition, the recent introduction of real-time CT image display
has led to new applications, including biopsy under CT fluoroscopic
guidance, the SureStart function, and real-time helical scanning,
which are revolutionizing CT diagnosis.1) Given this environment
of rapid technological change, further innovations in CT technology
are eagerly anticipated,
in particular, an increase in scanning speed and the introduction
of a multislice detector. A new CT system, Aquilion (Toshiba Corporation,
Tokyo) (Fig. 1), was installed at Fujita Health University in
September 1998. This paper describes our clinical experience with
the systemｰｺs new functions, such as 0.5-s scanning, as well as
our investigational use of multislice helical CT, which is currently
under development, and also discusses the effects that these new
technologies can be expected to have on clinical practice.
Table 1 outlines the development
of CT technology over the last 10 years.
These technological innovations have dramatically altered the
basic paradigm of CT diagnosis. Before introducing the new-generation
CT system, Aquilion, I would like to briefly review the technological
features of different generations of CT scanners.
Helical scanning
Helical scanning was made possible by the introduction of slip-ring
technology, and this scanning method is now widely employed at
many medical institutions. Its advantages have already been described
in detail in earlier reports,2-4) and therefore will not be reiterated
here. Helical scanning is of great clinical value, and most currently
available CT scanners either have helical scanning capabilities
or can be upgraded to permit helical scanning. This is a major
shift from the situation only 10 years ago. In the future, it
is expected that virtually all CT scanners will have helical scanning
capabilities.
Subsecond scanning
CT scanners employing electron-beam scanning technology were the
first to achieve subsecond CT. Today, however, even conventional
helical CT scanner designs can achieve a scanning time of less
than 1 s, with high-speed CT
scanners reaching 0.8 s or 0.75 s. The main advantage of subsecond
CT is reduced motion artifacts, but another important benefit
is the further enhancement of helical scanning functions due to
a larger scanning area and improved longitudinal resolution.
Real-time CT
Real-time CT was developed jointly by Toshiba Corporation and
Fujita Health University in 1993. Initially, it was mainly applied
to interventional radiology (IVR) procedures such as CT fluoroscopy,1)
but later became more widely applicable due to the development
of a wide range of new clinical applications such as real-time
helical scanning (which usually refers to real-time monitoring
during helical scanning) and SureStart (which is a function for
optimizing the scan timing using real-time CT technology).5) Consequently,
by 1997, most major CT manufacturers offered real-time CT functions.
Real-time CT is of great significance because it makes it possible
to accurately monitor biopsy procedures performed under CT guidance
and simplifies setting of the optimal scanning area and scan timing
in contrast CT studies.
Submillimeter CT
In order to acquire high-quality volumetric data, it is necessary
to improve the longitudinal resolution by minimizing
the slice thickness. Conventionally, submillimeter CT has been
employed at a limited number of institutions for specialized diagnostic
applications such as the examination of the middle ear and brain
three-dimensional CT angiography (3D-CTA).6,7) A number of limitations
need to be overcome before this technology can be incorporated
into commercial systems for general use. These limitations are
related to geometric accuracy, system control, the smaller scanning
range due to the thin slices employed, a reduction in the number
of X-ray photons, and higher exposure dose.
As is clear from the above discussion, each technological advance provides distinct clinical advantages. Therefore, failure of a CT scanner to provide any of these features means giving up some of the clinical benefits that we are currently enjoying. This is unacceptable to patients, radiology technicians, and physicians. Next-generation CT systems should not only offer all the technological features mentioned above, but also extend them to the next level.
In developing the new-generation Aquilion, we paid particular attention to incorporating all of the advanced CT functions discussed above, and by further enhancing them, we have created an optimal platform for multislice-detector helical CT. The various technological features of Aquilion and their clinical significance are described below.
Half-second scanning
Aquilion is the first conventional CT scanner to achieve a 0.5-s
scanning time. This development target was set about 9 years ago.
At that time, we were actively involved in the development of
helical scanning. From the earliest stages, we attempted to apply
this technology to obtain 3D images of the heart, and developed
a diastolic reconstruction method.8) In a study using 1-s scanning
conducted by Anno et al., acceptable images could be obtained
when the heart rate was 60 bpm or less, but at higher heart rates,
a marked deterioration in image quality was observed.8) It was
concluded that scanning at 0.6 s or less was required to overcome
this problem. For this reason, reducing the scanning time to 0.6
s or less had long been our target. In the fall of 1998, the introduction
of Aquilion made it possible to achieve a 0.5-s full scan, and
our target was attained. A number of technological innovations,
as described below, were required to achieve this goal, although
common wisdom led many in the CT field to consider these goals
unachievable.
a. Gantry
When the scanning time is reduced
by half, the forces applied to the gantry are increased by a factor
of four,9) reaching levels as high as 13g in 0.5-s scanning. The
gantry must be designed to withstand these intense forces. Aquilion
is a completely new design in which the gantry is a cylindrical
structure machined from a single block of aluminum. The safety
margin of this gantry is increased by a factor of ten, and it
can therefore easily withstand the forces generated during 0.5-s
scanning.
b. Drive system
The drive system of the gantry incorporates a linear motor like
that used in magnetic levitation vehicles. It offers significantly
higher mounting density, permitting 0.5-s rotation with ample
performance headroom.
c. Data transmission
In 0.5-s scanning, the amount of data generated per unit time
is markedly increased, exceeding 10 billion bits per second when
a single row of detector elements is used. Therefore, conventional
data transmission using the low-voltage slip-ring method cannot
be used. Aquilion employs asynchronous transfer mode (ATM) optical
transmission technology to achieve large-volume, high-speed data
transmission.
d. X-ray tube
The anode of the X-ray tube is subjected to a force of 13g during
0.5-s scanning, and so an entirely new tube design is required
in terms of precision, safety, and product life. The shorter scanning
time also requires an increased X-ray dose per unit time, so a
high-output X-ray generation system is needed. Furthermore, faster
scanning is expected to lead to an increase in the number of examinations,
which means that the X-ray tube must have improved cooling efficiency.
To satisfy these requirements, a completely new X-ray tube has
been developed for Aquilion. This tube has a straddle bearing
structure in which the anode is supported by bearings on both
sides. Therefore, compared with the conventional cantilever anode
type, a more precise and stable focus and a longer service life
are ensured. In addition, the new tube is anode grounded, so the
voltage is applied only to the cathode, increasing cooling efficiency.
This design also eliminates the risk of electrical discharge between
the anode and the housing, allowing them to be moved closer together,
further improving cooling. In an anode-grounded tube, recoil electrons
from the anode are prevented from re-entering the anode, minimizing
heat buildup. As a result of the above design innovations, the
new tube is able to offer a cooling efficiency comparable to that
of a conventional tube rated at 20 MHU. This new tube is used
in the Aquilion system installed at Fujita Health University,
and we have not found any problems with regard to stability or
performance in actual clinical practice.
Up to December 1998, Aquilion
was equipped with a single-row detector permitting a minimum slice
thickness of 0.8 mm. As of March 1999, a multislice detector (also
referred to as a two-dimensional detector or matrix detector)
has been available, and preliminary experiments are now being
conducted. The multislice detector is a two-dimensional solid-state
detector array incorporating a total of 34 rows of detector elements
arranged in a matrix, with four central rows of 0.5-mm detector
elements and 15 rows of 1-mm detector elements on either side
(Fig. 2). By selecting different combinations of detector rows,
scanning can be performed with various slice thicknesses (mm):
0.5 x 4, 1 x4, 2 x 4, 3 x 4, 4 x4, 5 x4, and 8 x4. When an appropriate
shifting speed is set, this detector can also acquire four standard
10-mm slices simultaneously
(10 mm x 4). The detector septa are specially designed to compensate
for the fact that a collimator cannot be placed in front of the
multislice detector.
Together with this new multislice detector, a dedicated helical
scan interpolation reconstruction algorithm has been developed.4)
In multislice-detector CT, the X-rays must spread out in a cone
shape because the detector elements extend for some distance in
the longitudinal direction, and this cone angle results in artifacts
if the conventional reconstruction method is employed. The new
algorithm provides an optimized scan pitch and multipoint interpolation
reconstruction, and is therefore extremely effective in eliminating
these artifacts.10)
By selecting the optimal sampling scan, the pitch in the z axis
is set to non-integral values such as 2.5, 3.5, and 4.5. In this
way, the number of samples is doubled compared with the quarter
detector offset method in the x and y planes. (Fig. 3). Therefore,
the precision of interpolation is improved, resulting in superior
image quality compared with 180ｪﾑlinear interpolation in the single-slice
helical scan technique.11) One of outstanding characteristics
of multislice helical scanning is the significant reduction in
X-ray exposure dose. In the multipoint interpolation method, more
measurement data are used than in the conventional method,
and the signal-to-noise ratio (S/N) is approximately 20% higher
at the same dose. The total patient exposure dose can be reduced
by 40% or more by increasing the scan pitch and minimizing superimposition
at the margins.
In summary, multislice helical CT
will help to realize every radiologistｰｺ in achieving practical
CT-based lung cancer screening, which has recently been introduced.
Conventionally, 10-mm slice, 20-mm pitch scanning was employed
for lung cancer screening, but lesions near the thoracic wall
tended to be missed due to partial volume effects. Multislice
helical CT scanning permits imaging under ordinary conditions
(10-mm slice, 10-mm pitch) and is therefore expected to improve
diagnostic capabilities. In addition, a scanning time of 4 seconds
makes screening much more practical.
c. Whole-body, wide-area scanning
Half-second multislice helical scanning permits a larger area
to be scanned within the same scanning time, i.e., during a 10-
to 20-s breathhold, making it possible to perform simultaneous
thoracic and abdominal scanning during a single breathhold or
continuous neck-to-pelvis scanning. This has been found to be
useful for detecting abnormal lymph nodes in patients with malignant
lymphoma.
d. High-resolution longitudinal
images
The combination of thin slices and half-second multislice helical
scanning makes it possible to obtain high-resolution volume data
during a single breathhold. Consequently, high-resolution coronal
thoracic images, which cannot be obtained using conventional CT
scanning, have become available (Figs. 7 and 8).
e. Multilayer simultaneous dynamic
CT
Unlike magnetic resonance imaging (MRI), CT provides a contrast
enhancement effect proportional to the concentration of contrast
medium. In addition, CT has higher temporal and spatial resolution
than MRI. Furthermore, access to the patient in the event of an
emergency can be a problem with MRI. For these reasons, CT is
considered to be a more suitable modality for blood flow analysis
of various organs than MRI. However, conventional CT allows dynamic
scanning to be performed for only one slice at a time, and is
inferior to MRI in this respect. Multislice-detector CT scanning
permits dynamic CT to be performed for multiple slices, and is
therefore considered to be suitable for brain perfusion imaging.
In particular, it is expected to be superior to MRI in terms of
quantitative analysis.
f. Improved X-ray utilization efficiency
In multislice helical CT, with the same X-ray irradiation as in
conventional CT, the scanning area can be extended as far as the
number of rows of detector elements. Therefore, assuming that
the same number of slices is acquired, the service life of the
X-ray tube can be prolonged. In addition, patient exposure can
be significantly reduced using the new multiple interpolation
algorithm for multislice helical scanning, as described above.
Combination of real-time CT and multislice detector
The advantages of the multislice
detector can be further enhanced by the combined use of real-time
CT. A number of examples are presented below. a. CTA using a small
volume of contrast medium
The reduced scanning time made possible by multislice helical
scanning and the accurate determination of the scan timing made
possible by the SureStart function allow arterial 3D-CTA to be
performed with a relatively small volume of contrast medium, 50
mL or less (even 20 mL). In routine contrast studies, multislice
helical scanning reduces the scanning time to only several seconds,
which means that use of the SureStart function is essential.
b. Time-resolved 3D-CTA allowing
arteries and veins to be discriminated
Performing high-speed 3D-CTA with the minimum volume of contrast
medium makes it easier to visualize and discriminate between arteries
and veins in 3D perspective. For example, with a 4-row 1-mm slice
multislice detector, it can cover the circle of Willis in 5 s.
This makes it possible to perform time-resolved 3D-CTA, in which
the target region is scanned from bottom to top during the arterial
phase, immediately followed by scanning from top to bottom during
the venous phase. In such studies, SureStart is once again indispensable
for ensuring the optimal scan timing.
c. Measurement of arterial blood
flow velocity and volume
The difference in time for contrast medium to reach the region
of interest can be determined by monitoring the image slices at
the ends of the scanning range in real time and observing the
enhancement effect in the region of interest where the target
blood vessels are located. From this information, the arterial
blood flow velocity and volume can be calculated. This is thought
to be particularly effective for evaluating blood flow in vessels
that run parallel to the body axis, such as the aorta, the carotid
artery, and the femoral artery.
d. Multilayer simultaneous CT fluoroscopy
In conventional CT fluoroscopy, only one slice is displayed in
real time, and if the biopsy needle moves out of the slice, it
is not possible to monitor its position. In multislice-detector
CT, three or more slices can be reconstructed simultaneously in
real time, permitting the needle position to be ascertained without
shifting the couchtop. This is helpful in various biopsy and drainage
procedures performed under CT-fluoroscopic guidance. In the future,
if simultaneous real-time reconstruction is realized with more
rows of detectors, the combined use of
the real-time multiplanar reconstruction (MPR) function should
make it possible to perform oblique puncture while observing longitudinal
images.
Combination of submillimeter slice scanning and multislice detector
As mentioned above, submillimeter helical scanning is a powerful tool for obtaining high-quality volume data. However, in conventional helical scanning using a single-row detector, when the slice thickness is reduced, the imaging area is restricted, resulting in limitations in clinical applications. The use of a 4-row, 0.5-mm multislice detector and half-second scanning in Aquilion will make it possible to perform submillimeter slice helical scanning as easily as conventional 5-mm slice helical scanning. Moreover, the multipoint interpolation reconstruction algorithm provides a high S/N, and may permit visualization of relatively low-contrast structures. Its range of applications is expected to extend from middle ear examinations (Fig. 9) to brain 3D-CTA studies. Submillimeter slice scanning is considered practical only when used in combination with multislice-detector, half-second scanning.
This paper has discussed the current
status and future developments of various CT technologies. It
should be emphasized that the combined use of different technologies,
such as helical scanning, submillimeter slice scanning, real-time
CT, half-second scanning, and a multislice detector, enhances
the effectiveness of these functions, compared with any one of
them used alone (Fig. 10). In other words, the combined use of
these technologies results in a synergistic effect. It should
be understood that simply installing a multislice detector in
a conventional CT system is not sufficient.ｰnｰnHalf-second, half-millimeter
real-time multislice helical CT scanning, as supported by Aquilion,
is expected to lead to the development of a number of new applications
that have been considered impossible by conventional CT scanning.
The range of new possibilities is limited only by the imagination
and creativity of CT users. What can we achieve using Aquilion?
We must expect a period of trial and error, but this should lead
to remarkable progress in CT diagnosis. The introduction of a
multislice detector in
the next few years should lead to a period of innovation comparable
to that observed when helical scanning was first introduced.
We had dreamed of helical scanning CT since the mid 1980s, and
have thought about real-time CT and its applications since the
early 1990s. There has been always an ideal beyond our immediate
goals. The ultimate goal of CT for us is to be able to visualize
the 3D distribution of X-ray absorption in the human body within
the shortest possible time and with the highest possible accuracy.
This has been our goal since we first became involved with the
development of helical scanning in 1986. It is the unwavering
pursuit of this goal by Toshiba engineers that led to the development
of the 0.5-mm/1-mm matrix detector and 0.5-s scanning. If we succeed
in realizing our dream, CT can be expected to become the primary
X-ray-based diagnostic imaging modality in all but exceptional
cases. The development of Aquilion was a true milestone in CT
technology. Aquilion is a dream CT scanner that looks to the future.
This work was supported in part by a Grant for Scientific Research Expenses for Health and Welfare Programs from the Foundation for the Promotion of Cancer Research, and also by the second-term Comprehensive 10-year Strategy for Cancer Control.
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