vol. 6 2/2017 Inżynier i Fizyk Medyczny
88
radiologia
\
radiology
artykuł naukowy
\
scientific paper
Introduction
A CT simulator is a component in the radiation oncology facility.
It enables a radiotherapy staff to set/simulate the radiotherapy
treatment as well as a clinician to determine a location, size and
shape of the tumour/treated region/OARs..
3D planning scans are carried out on “regular diagnostic”
CT scanners. Usually they have “wide-bore” sizes from 80cm
to 90cm. It makes a space to scan patients in the radiotherapy
treatment position with appropriate immobilisation equip-
ment. A flat table-top with indexing for immobilisation devices
is required to replicate the linac table top to get a simulation of
a patient position and treatment setup during radiotherapy ses-
sions. Additionally a table movement and sag effect are restrict-
ed more than for a standard diagnostic system as a geometrical
reconstruction precision is required.
Image quality is less restricted in radiation oncology than in
diagnostic imaging. There is because patients undergoing CT
simulation in radiation oncology have already diagnosed disease
and clinician contouring is supported by other imaging modal-
ities (MRI, nuclear imaging). Quantity data of the CT image is
more concern in radiation oncology. There is still very import-
ant to get a proper setting protocols/facilities (gating, metal
artefact correction, contrast etc.) in relation to radiotherapy
purposes and to reduce artefacts which can change significantly
quality or quantity parameters in the image.
Scan protocols are similar to diagnostic but should be altered
to increase geometric accuracy and linearity of Hounsfield
Units. Each voxel has a HU value proportional to it’s x-ray ab-
sorption, which is proportional to its electron density, which is
fairly proportional to its physical density. A CT scan is thus a 3D
map of x-ray absorption in the patient which is used by treat-
ment planning software to determine the absorption of radia-
tion anywhere in the patient taking into account the 3D shape
of the patient and the different absorption of different tissues
within the patient. The treatment planning software uses the
CT scan to calculate the dose absorbed anywhere in the patient
and produce a 3D treatment plan and dose distribution.
External laser systems are installed as the internal lasers on
CT scanners characterizes high uncertainty of a setup in relation
to radiotherapy requirements. A full set contains moving side-
wall lasers and overhead sagittal laser.
A radiotherapy facility built in the CT scanner system is a vir-
tual simulation software developed to replace the physical sim-
ulator. Using it treatment planners can import the patient CT
planning scan and then:
––
position beams around the virtual patient with different
gantry & collimator angles and table rotations,
––
set field sizes and MLC shapes, generate DRRs (digitally re-
constructed radiographs),
––
visualize the patient surface, showing the light field of the
treatment field on the patient surface and showing patient
marks that have been placed on during the planning CT scan,
––
contour patient anatomical structures within the patient CT
dataset which can be used by the TPS,
––
calculate dose distribution in the patient and DVHs.
(AEC) System
AEC (Automatic Exposure Control) systems for CT scanners
have different capabilities and operate in a variety of ways, de-
pendently on a manufacturer. Their main purpose is to adjust the
x-ray tube current/time of a rotation to compensate for differ-
ent levels of attenuation (patient size/shape) of the CT scanner’s
x-ray beam. This has a number of potential advantages – consis-
tency of image quality, optimization radiation dose provided to
patient and image quality, avoidance of certain types of image
artefacts. There is very important to find out a new concept and
understand facilities of those systems to use them correctly.
A range of exposure parameters can be adjusted – tube volt-
age, tube current, exposure time and other factors. The reason
that a single set of exposure parameters can’t be used for all
patients is their size or shape as well as composition of the ana-
tomical regions. The penetration of x-ray through a patient de-
pends on a range of factors, but in an average way, x-ray beam
halves for each 3 cm of soft tissue. A difference in patients sizes
and composition – eg. soft tissue, bone and lung tissue - causes
difference of levels of image noise.
CT scanners produce digital images as a result of a mathemat-
ical reconstruction from a series of attenuation measurements
made during the rotation of a x-ray tube and detector around
of the examined patient. These images don’t suffer from under-
and over-exposure in the same way as standard imaging sys-
tems. All digital images systems use image presentation tools
– window level and width controls – allowing viewing in a highly
flexible manner. The effect of differences in patient size and/
or composition is to increase/decrease the intensity of the x-ray
beam getting the CT detectors. The statistical nature of x-ray
transmission, the accuracy of the attenuation measurements
made by detectors depends on the beam intensity – the lower
intensity causes the worse accuracy of measurements resulting
in higher noise in the image. The worse statistic can result the
worse noise “distribution” and “streaking” artefacts appearing
(especially in asymmetric regions of the body – eg, shoulders).
The usual method for adjusting CT exposure levels to com-
pensate for patient size is to adjust the tube current or rotation
time in order to change the mAs. Currently CT scanners have
a range predefined protocols for different examination types,
anatomical regions, used facilities enhancing functionalities
matching to diagnostic or radiotherapy purposes. These are
generally set up for an “average” sized patient, and the user can
vary these parameters on a patient–by-patient basis.
The AEC system works at three levels: patient size, Z-axis, rota-
tional/diagonal separation. The patient size determines a mA set
up. The same mA is used for an entire examination or scan series as
an average assessed froma scannogram. The Z-axis AEC adjustsmA
