The utilization of Computed Tomography in paediatric examinations constantly
increases. During the procedure, a high amount of dose is delivered to children, which
could be avoided. This study examined whether the selection of scanning parameters –
tube voltage and tube current-time product- could be based on patient size instead of
patient age or weight aiming, for dose reduction.
The SRS78 spectrum processor software was employed to generate spectra with tube
voltage between 80kVp and 120kVp and with tube current-time product between 50mAs
and 165mAs. These spectra were attenuated by different thicknesses of polymethylmethacrylate
(PMMA) phantoms. The simulation technique was validated with
experimental measurements acquired on CTDI phantoms on a Siemens Somatom plus 4
scanner. The image quality was assessed in terms of noise, contrast and contrast-to-noise
ratio (CNR). Furthermore the contrast of iodine, adipose tissue and cortical-bone relative
to muscle were calculated in order to examine how the contrast of different materials was
influenced when tube voltage changed.
The data analysis shows that there is a definite relationship between image quality and
the size of a patient. When exposure settings are kept constant, the level of noise, contrast
and Contrast-to-Noise Ratio (CNR) depends on the size of the phantom. Noise is
increased exponentially and contrast is reduced linearly as the size of the phantom is
increased. CNR is markedly higher in small size phantoms. Moreover, when tube voltage
was reduced the noise level was increased less in the small size phantoms and the contrast
of high atomic number materials is reduced more when tube voltage is reduced. The CNR
for high atomic number materials presents modest improvements when tube voltage is
increased therefore examinations with contrast agents could be performed at lower tube
voltages. Furthermore the high CNR in small size phantoms could be traded off with
lower mAs. In particularly the mAs could be reduced by up to 95% while maintaining the
same CNR as for adults resulting in dramatic dose reductions for children.
Moreover, since Computed Tomography stands out from all the other X-ray techniques
due to its ability to detect structures of similar densities the detect ability of low contrast
details was investigated. The Catphan phantom and particularly the CTP515 module was
employed. The phantom was scanned with the Siemens Somatom plus 4 scanner at
80kVp, 120kVp and 140kVp and with tube current-time product between 43mAs and
165mAs. The image quality was assessed subjectively and objectively.
It is observed that when 120kVp and 140kVp are applied there are not sufficient
differences on image quality which justify the selection of 140kVp in paediatric
protocols. When 80kVp is applied structures with contrast lower than 10HU are not
detected. Concerning mAs does not contribute to the detection of low contrast details
except if it is combined with high tube voltages. However, mAs contribute to the
visualisation of smaller in size details but above a threshold value, higher mAs does not
serve any purpose and the value of 300mAs employ in many protocols is not justified.
In conclusion, the reduction of dose during paediatric Computed Tomography
examinations is more than probable since scanning parameters could be reduced without
degradation of image quality. However in order to assure the reduction of dose without
side effects, protocols must be constructed which will individualize the Computed
Tomography examinations. That is, the optimum spectrum must be selected relative to
the diagnostic task and the size of the patient. / -
| Identifer | oai:union.ndltd.org:upatras.gr/oai:nemertes:10889/1174 |
| Date | 15 December 2008 |
| Creators | Τζιωρτζή, Άντρη |
| Contributors | Παναγιωτάκης, Γεώργιος, Tziortzi, Andri, Παναγιωτάκης, Γεώργιος, Κωσταρίδου, Λένα, Σακελλαρόπουλος, Γεώργιος |
| Source Sets | University of Patras |
| Language | gr |
| Detected Language | English |
| Type | Thesis |
| Rights | 0 |
| Relation | Η ΒΥΠ διαθέτει αντίτυπο της διατριβής σε έντυπη μορφή στο βιβλιοστάσιο διδακτορικών διατριβών που βρίσκεται στο ισόγειο του κτιρίου της. |
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