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Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography
Comparison of Spiral Computed Tomography and
Cone-Beam Computed Tomography
Alexander Maninagat Luke, Krishna Prasad Shetty, SV Satish, Krishnarao Kilaru
ABSTRACT
Imaging is a highly dependent technology in the clinical
assessment of a patient. With respect to the dental field, imaging
has travelled a long way from the conventional radiographs to
modern techniques like the computed tomography (CT) and
the cone-beam computed tomography (CBCT). This article
describes the evolution made in the CT in the field of dental
imaging and attempts to compare the CT with the more
advanced CBCT which is fast gaining popularity in the field of
dental imaging.
Keywords: Radiology, Cone-beam computed tomography, Spiral.
How to cite this article: Luke AM, Shetty KP, Satish SV,
Kilaru K. Comparison of Spiral Computed Tomography and
Cone-Beam Computed Tomography. J Indian Aca Oral Med
Radiol 2013;25(3):173-177.
Source of support: Nil
Conflict of interest: None
INTRODUCTION
Imaging is an important diagnostic adjunt to the clinical
assessment of the dental patient. The introduction of panoramic radiograph in the 1960s and its widespread adoption
throughout the 1970 and 1980s herald major programs in
dental radiology.1
Nothing has captured the dental profession’s imagination
in the past few years like the introduction of cone-beam
volumetric imaging (CBVI), sometimes referred to as cone
beam computerized tomography (CBCT) or cone-beam
volumetric tomography (CBVT).2 Dental radiology has long
played an exciting and critical diagnostic role in dentistry.
This began with discovery of X-rays by Roentgen.3 Intraoral
and extraoral procedures, used individually or in combination, suffer from same inherent limitations of all planar
two-dimensional (2D) projections magnification, distortion,
superimposition and misrepresentation of structures. Efforts
were made toward three-dimensional (3D) radiographic
imaging and for the first time, practitioners had access to
X-ray devices that could generate narrow cross-sectional
images, usually perpendicular to the long axis of the human
body, hence, the term computed axial tomography or CAT
scan. Computed tomography acquisition has subsequently
been refined to incorporate a helical or spiral synchronous
motion, fan-shaped beam, and multiple detector acquisition
(MDCT), which enables fast scan times that provide highquality images that can be integrated to produce a volumetric dataset.
Image acquisition process in CBCT differs from that of
traditional medical computerized tomography (CT) scanners
in that the patient is not usually supine, the image gathered
is in a voxel (volume element) format, the X-ray dose
absorbed by the patient is substantially lower, appointment
availability is much easier, and it is less expensive.4-6
In 1972, the independent findings of Hounsfield and
Cormack revolutionized diagnostic imaging with the invention of the CT scanner.7,8 Willi Kalender (1970), who is
credited with the invention prefers the term spiral scan CT.9
An early volumetric CT predecessor of CBCT, the dynamic
spatial reconstructor, was developed in the late 1970s by
the Biodynamic Research Unit at the Mayo Clinic.10 CBCT
provided an alternate method of cross-section image production to fan-beam CT using a comparatively less-expensive radiation detector than conventional CT.
CBCT to dentistry first occurred in 1995. Italian coinventors, Attilio Tacconi and Piero Mozzo, developed a CBCT
system for the maxillofacial region and was designed and
produced by Quantitative Radiology, Inc. of Verona, Italy.
The NewTom DVT 9000 became the first commercial
CBCT unit, and was first introduced in Europe in 1999; its
design was similar to conventional CT.
The purpose of this article is to describe the important
differences between fan-beam CT and CBCT technologies
regarding beam geometry, image acquisition, image detection, image reconstruction, radiation dose and their application in different fields of dentistry.
EVOLUTIONS IN CT
The introduction of CT heralded 2 firsts in medical diagnostic imaging: (1) data acquisition and image display were
both digital in nature, with images viewed on special
monitors and (2) a series of single, discrete, contiguous crosssectional image slices of complete body sections in parallel
to the image acquisition geometry were generated instead
of planar projection images, which resulted in the superimposition of anatomic structures.11
A fan-beam CT scanner images a region of interest (ROI)
using an X-ray tube rigidly linked to a detector located on
the other side of the patients. Together the tube and the
detector initially scan across, but in later generations around,
patients on a fulcrum, sweeping a narrow X-ray beam through
one thin slice at a time. Contiguous image slices are acquired
from multiple X-ray projections around the object. The
Journal of Indian Academy of Oral Medicine and Radiology, July-September 2013;25(3):173-177
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Alexander Maninagat Luke et al
A
B
C
D
Figs 1A to D: Four basic scanning methods or systems: (A) first generation, (B) second generation,
(C) third generation, and (D) fourth generation
reconstruction of the transmitted X-ray attenuation data by
specific software algorithms produces adjacent image slices
of the individual’s imaged volume.12 This concept is better
understood if the human body (part thereof under examination) is made up of a stack of contiguous transverse slices.
Each scan aims to determine the composition of one transverse section. This slice or section can be imagined as composed
of discrete blocks of tissue known as volume elements
or voxels.11
The first generation of CT scanner in the early 1970s
incorporated a pencil-like X-ray beam and single detector
that acquired data in a translate-rotate motion (Fig. 1A):
The X-ray source and detector gradually scanned across
patients in a linear movement (translation) and then, after a
secondary rotational movement of the apparatus, the translation was performed again. After completing a 360 rotation,
supine patients were translated slightly through the apparatus on a gantry such that a new section was exposed. In
1972, second-generation scanners were introduced that
incorporated various efficiencies in image geometry acquisition. Although the motion remained a combination of
translation followed by rotation, the shape of the X-ray beam
changed from a narrow pencil beam to fan shaped, with
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gradual widening of the fan (Fig. 1B). The number of the
detector elements also increased from a single to a linear
array of detectors. Scan times, however, remained long,
partly because of the use of stationary anodes. In 1976, the
third generation of CT scanners, known as rotate-rotate, was
introduced. The linear array of detectors was modified to
an arc and their number increased to several hundred. In
addition, the X-ray beam shape was wide and divergent
enough to cover the arc (Fig. 1C). Finally the linear motion
of the X-ray tube was abandoned and a circular motion was
adopted with a fulcrum centered through the middle of
patients. During a single scan, the X-ray tube/arc-shaped
detector array combination continuously rotated around the
patients while the patients were translated on the gantry.
This design facilitated the incorporation of rotating anodes,
which resulted in higher generator output and reduced scan
times. In fourth-generation scanners, introduced in 1978,
multiple detectors (up to a few thousand) are used and form
a stationary ring around the patients (Fig. 1D). In this design,
only the X-ray tube rotates around the patients inside the
detector ring. During scanning, a wide fan-shaped beam
exposes only a portion of the detectors, which is different
for the various projections.
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Comparison of Spiral Computed Tomography and Cone-Beam Computed Tomography
The 4 generations of CT design incorporated innovations
designed to increase efficiency and reduce single-image scan
times; however, the resulting data set was a compilation of
single scans interposed by a stop/start action necessary for
table translation. In addition, extra time was required for
hardware adjustments during scanning (e.g. cable unwinding).
Both factors contributed in slowing down the overall
acquisition process and resulted in increased examination
time.13 The introduction of the spiral or helical CT acquisition in 1989 that image data would finally become isotropic. Unlike in sequential CT, the technique involves the
continuous acquisition of projection data through a 3D
volume of tissue by the continuous rotation of the X-ray
tube and detectors and simultaneous translation of the
patients through the gantry opening.14 The X-ray tube
continues to travel on a circular arc (Fig. 2); however,
relative to moving patients, it seems that it follows a spiral
or helical trajectory.11 The spiral motion of CT acquisition
has been facilitated by mechanical innovations, such as slip
ring technology, which prevents the winding and unwinding
of cables and electrical advances, including increased heat
capacity of the X-ray generators. Different reconstruction
algorithms (interpolation algorithms) were developed that
provided single-plane reconstructions despite the fact
that the projections were acquired in a spiral rather than
planar motion.13
The most recent technological advance in CT acquisition
occurred in 1998 with the incorporation of multiple rows
of detectors (initially 4) instead of a single row. MDCT
has revolutionized the versatility of CT data with the
partitioning of the information received from the incident
beam into multiple small segments.15 Current detector
configurations range from 16 to 64 and now 256 and even
512 channel systems.
The geometric configuration and acquisition mechanics
for the CBCT technique are theoretically simple.16-19 Four
technological developments converged to facilitate the
construction of affordable CBCT units small enough to be
used in the dental office: the introduction of X-ray detectors
capable of rapid acquisition of multiple basis images, the
development of suitably resilient X-ray generators, the
evolution of suitable image acquisition and integration
algorithms,20-22 and the availability of computers powerful
enough to process the enormous amount of acquired image
data. Fast data acquisition of the maxillofacial region with
a single revolution, shorter examination time, and the fairly
simple and inexpensive detection system are obvious
advantages of CBCT that have propelled the technology
for clinical applications in dentistry.
Is CBCT different to medical CT?
Yes, CBCT is different in the following ways,
Shape of beam: In CBCT the shape of the X-ray beam
is divergent, like a cone, hence the name. In medical CT,
the shape of the beam is flat, like a fan.
Patient posture and movement of X-ray tube: In CBCT,
the patient is usually sitting up in a dedicated chair, although
with some machines the patient is standing up, while with
one particular machine, the patient is lying down. Once the
patient is in position, the X-ray tube head then revolves
around the patient’s head in a single 360º rotation. In medical
CT, the patient lies supine on the CT table. The table then
moves the patient into the CT scanner. Depending on the
type of CT scanner (Fig. 3), the table may move slowly as
the X-ray exposure takes place, or the table stays static as
Fig. 2: Trajectory and the axis of rotation of the X-ray source and
the detector around the object
Fig. 3: Path of the motion of the X-ray source, detectors and
the patient
Journal of Indian Academy of Oral Medicine and Radiology, July-September 2013;25(3):173-177
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Alexander Maninagat Luke et al
the X-rays are generated. A modern medical CT scanner
(multislice spiral CT) can scan a whole person in just a
few minutes.
Software processing: In CBCT the initial raw data
(image dataset) from one scan has to be processed
immediately to allow reconstruction into different planes
for viewing. This usually takes a few minutes or sometimes
quicker depending on the volume of data. Thereafter,
depending on the patient’s treatment needs and the dentist’s
treatment plan, the image data can be further processed,
e.g. to help with implant planning, construction of surgical
drill guide, image-guided surgery. These specialized applications require additional resources. Medical CT datasets
can be processed in a similar way too because both CBCT
and medical CT have their raw data in a standard universal
format known as DICOM. Radiation dose CBCT scans
involve much less radiation dose compared to medical CT
because a smaller volume of tissue is irradiated. CBCT
machines also use additional means to reduce dose, such as
pulsing the X-ray beam (instead of continuous emission of
X-rays), 180º scan (instead of 360º). Field of view (FOV)
and image resolution one of the advantages of CBCT is
that the irradiated volume, or field of view, can be as small
as 4 × 4 cm. At this size, up to two adjacent molars can
be included in their entirety. At this small volume size,
the image voxel size can be as small as 0.076 × 0.076 ×
0.076 mm. In CBCT, the voxels are isotropic (equal lengths
in all three dimensions), whereas in medical CT the voxels
are anisotropic rectangular blocks, up to 2 mm long. The
smaller the voxel size the better the image resolution and
CBCT is better than medical CT.
DISADVANTAGES OF CBCT
Artefacts: Is defined as a visualized structure in the reconstructed data that is not present in the object under investigation.
The following artefacts are reported extinction, beam
hardening, partial volume, ring and motion (misalignment).
Noise: Two sorts of noise are considered in the reconstruction
images: (a) additive noise (b) photon-count noise. CBCT
machines for dose reduction reasons are operated at mill
amperes that are one order of magnitude below than those
of medical CT machines. Thus, the signal-to-noise ratio is
much lower than CT. A high noise is present in CBCT images.
CBCT is not able to demonstrate nor differentiate soft
tissues and soft tissue lesions.
CONCLUSION
CBCT, an emerging technology, is sure to make prominent
inroads into imaging in dentistry. It is a unique technique
that allows 3D visualization of dental tissues there by permitting accurate diagnosis and treatment planning for all
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specialties of dentistry. CBCT makes clinical decision
making easier and more precise, patient treatment decisions
more accurate and visualization of the X-ray data more
meaningful. Dentistry is moving away from ‘radiographic
interpretation’ and into ‘disease visualization,’ and it could
not have come at a better time.
CBCT is capable of providing accurate, submillimeter
resolution images at shorter scan times, lower dose, and lower
costs. Increasing availability of this technology provides an
imaging from diagnosis to image guidance of operative and
surgical procedures.
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ABOUT THE AUTHORS
Alexander Maninagat Luke (Corresponding Author)
Radiologist, Department of Oral Medicine and Radiology, College of
Dentistry, Ajman University of Science and Technology, UAE, Phone:
00971555171094, e-mail: [email protected]
Krishna Prasad Shetty
Professor, Department of Conservative Dentistry and Endodontics
Navodaya Dental College and Hospital, Raichur, Karnataka, India
SV Satish
Professor, Department of Conservative Dentistry and Endodontics
Navodaya Dental College and Hospital, Raichur, Karnataka, India
Krishnarao Kilaru
Assistant Professor, Department of Restorative Dental Sciences, King
Khalid University, Saudi Arabia
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