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PatientPatientpositioning andpositioning and
centering in CTcentering in CTEtienneEtienne GrechGrech
CT Unit, Mater Dei Hospital.CT Unit, Mater Dei Hospital.
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Aim of presentationAim of presentation To differentiate between the term
positioning and centering.
To determine the effects on patientdose and image quality of impropercentering and positioning.
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Radiation dose & reductionRadiation dose & reduction
techniquestechniques Radiation doses in CT range between 1-
24mSv for adults and 2 to 6.5mSv. Heightened concerns and awareness ofradiation induced cancer from CTprompted to the development of
strategies and techniques for dosereduction. In order to obtain appropriate benefits
from these techniques, it is important toappropriately centre/position patients tothe isocenter of the gantry.
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Positioning and centeringPositioning and centering Positioning: the proper placement of
the patient on the CT table in the z-axis or along the length of patient forscanning an anatomical region.
Centering: the appropriateplacement of the patient with respect
to the gantry isocenter in the x-yplane.
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Patient positioningPatient positioning Improper positioning: may result in the
acquisition of a reference radiograph beyond theROI and/or require a repeat of the latter.
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PositioningPositioning cont.cont.
Proper positioning in head
examinations is important so as toadequately ensure the reproductionof any asymmetry due to anypathological process.
Avoiding the eyes by depressing the
patients chin may reduce lens dose.
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Improper positioning cont.Improper positioning cont. Navasivayam et al., (2006) have reported that up
to 13cm are sometimes exposed unintentionallyin reference radiographs due to inadequatepositioning.
Increase in radiation dose due to improperplacement of the upper limb in systems usingtube current modulation (up to 45% higher).
Degradation in image quality usually occurs.
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What is tube currentWhat is tube current
modulation??????????modulation?????????? Automatic tube current adjustment along the x-y
axis (angular) and z-axis (or both) according topatient thickness.
Philips uses DoseRight-consists of two types of
modulations ACS and DOM.
ACS uses a reference radiograph to asses thepatients attenuation. It suggests the lowest mA
to maintain adequate image quality. mA is set sothat 90% of images will have noise levels equalto or higher than that of a reference image.
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Tube current modulationTube current modulation cont.cont.
D-DOM provides angular modulation.
Z-DOM modulates mA in the z-axis
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Reasons for off centeringReasons for off centering Awareness/attention: lack of education,
increase in workload etc. Kalra et al.,(2008) demonstrated that in 240 patients,mean mis-positioning averaged 33mm in
95% of patients (range 5.1-97mm). Adjustment of DFOV: acquisition of
reference radiographs at different planes.
Patient related factors: physicalabnormalities, obese individuals etc.
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Off centering: effect onOff centering: effect on
scout datascout data Inappropriate centering effects size of reference
radiographs. Magnification or reduction of thelatter accompanies mis-operation of DOM.
The latter leads to erroneous calculations formA settings
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Magnification and offMagnification and off
centeringcentering
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Effects of off centeringEffects of off centering CT uses a bow tie filter to shape X-
ray beam to the body, i.e. restrictphotons at the periphery (areas withless attenuation) and allow morephotons to pass trough the centralportions i.e. those with greater
attenuation and contribution toimage noise.
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Off centering and bow tieOff centering and bow tie
filtersfilters Shaping the radiation beam allows
for:1)A reduction in scattered radiation
from peripheral object zones.2)Reduce patient dose.
3) Improve image quality.
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Effects of off centeringEffects of off centering cont.cont.
BF presumes that the centre of the
body is at the isocenter. If thepatient is off centre, portions of thebody other than the peripheral
portions receive fewer photon.
Increase in image noise and
radiation dose to peripheries.
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With correct centering, more photons(X+++) traverse the thicker central
parts, resulting in adequate imagenoise and radiation dose.
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More photon (X+++) passing throughperipheral parts, increase noise centrally
and dose peripherally.
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Effects of off centeringEffects of off centering cont.cont. Phantom studies have shown a 50%
increase in surface and peripheral doses(12%-18%@30mm.41%-50%@60mm).
Actual patients: 2%-30% (older study). Organs mostly at risk: breast, eye lens,
thyroid. Such phenomena are morecommon when using lower mA, forexample when imaging children.
Such phenomena are more common whenusing lower mA, for example whenimaging children.
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Off centering and imageOff centering and image
qualityquality
Difference in image noise @30mm (left)and 60mm (right).
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Effect on image qualityEffect on image quality
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ConclusionConclusion Radiographers must pay attention
whilst position/centering. Patients arms should elevated on
top of the head. When not possible
improvised arrangements should bemade.
Radiographers are directly
responsible for patient doseso thinkbefore you act!