Thoracic Imaging 2003 - Society of Thoracic Radiology

Multidetector CT: More Slices and Beyond
Patricia J. Mergo, M.D.
Associate Professor
University of Florida
The development of multidetector CT (MDCT) represents a
quantum leap in CT technology. The clinical advances made possible
by the advent of MDCT represent one of the largest developments
in current technology in radiologic imaging. New 16
slice scanners are in production by several vendors including GE,
Siemens, Philips and Toshiba. The biggest clinical advance made
possible with MDCT is CT angiography. The capabilities of thoracic
imaging are affected significantly with multislice CT with
subsequent improved pulmonary arterial CT angiography (CTA),
as well as thoracic aortic CTA and more recently coronary artery
CTA, using cardiac gating. All these are accountable to the ability
to obtain volumetric scanning of the region of interest, with
imaging approaching now or equaling isotrophic voxels.
When considering multislice CT, important technical considerations
need to be taken into account including considerations
for pitch, collimation, scan slice profile, noise and radiation
dose.
Pitch is an important consideration since with a higher pitch,
a given volume of tissue can be scanned in a shorter time period.
The advantages for scanning at a higher pitch are well
appreciated when fast scanning is needed as in CT angiography.
The scanning at a higher pitch does result in widening of the
slice width profile, however. The dosage should remain the
same, technically speaking, but dosage is in reality potentially
increased if adjustments are made in the mA to compensate for
limitations in slice width profile. In single slice CT pitch is
defined as table movement per rotation / slice collimation. For
multislice CT, pitch is defined as table movement per rotation /
single slice collimation. This implies that with a 0.5 second
scanner, with 2 rotations per second, that if the table travels 16
mm in a second and 1 mm collimation is used, then the pitch
would be 8.
With SDCT the true width or the "effective slice thickness"
of the reconstructed image is influenced by pitch and the reconstruction
algorithm used (wide vs. slim). With a pitch of 2 and a
slim algorithm the "effective slice thickness" may increase by
27%. With MDCT these factors are effectively corrected using
axial interpolation algorithms.
MDCT yields increased flexibility of scanning relative to
slice collimation and slice width. With single detector CT the
slice collimation and the slice width are the same and they cannot
be modified once the scans are obtained. In MDCT, the
detector collimation is chosen prior to scanning. Once the
images are obtained, the images can be reconstructed at varying
slice widths, such as 1 mm, 2.5 mm, 5 mm, and 10 mm slice
thickness. The thinner section imaging allows higher resolution
scanning, albeit with increased noise, for high resolution scanning
of pulmonary parenchyma or CTA. The data obtained can
be considered true volume data sets for reconstruction at varying
slice width. The ability to volumetrically manipulate the
data and reconstruct it at various slice widths is dependent on
the initial collimation.
Image noise is altered by several factors including mAs, kV,
slice thickness, collimation and patient size. These factors are a
trade of for thinner section imaging that is often required with
CT angiographic studies.
Since with MDCT the acquisition time for CT is significantly
shortened, for instance, enabling acquisition of imaging of
the thorax for pulmonary embolism detection in time periods as
short as 9 seconds, contrast material administration profiles are
subsequently altered. New scan protocols are optimized for contrast
injections depending on the organ or structure scanned. In
general, higher injection rates result in higher level of arterial
enhancement suitable for CT angiography. Similarly, protocols
with shorter injection times with higher osmolality contrast have
been proposed to yield improved imaging. Scan timing can be
optimized using bolus triggering or test injection techniques.
The changes in scanning that we have talked about can
quickly result in information overload in terms of the amount of
data generated from the scans. For this reason, 3D processing of
scans becomes crucial in order to adequately display the findings
to the clinician. This is especially important in CT angiographic
studies, and in the chest can be most helpful for the
evaluation of the airways or for the detection of pulmonary
embolus.
Ultimately, for many studies, although thin section imaging
is obtained, reconstruction at thicker slices for interpretation
results in a compromise for ease of interpretation of the axial
images, while 3D reconstructions are performed on the thinner
section images for improved display of the pertinent findings.
47
SUNDAY