centraggio in ct

7/29/2019 centraggio in ct

1/25

PatientPatientpositioning andpositioning and

centering in CTcentering in CTEtienneEtienne GrechGrech

CT Unit, Mater Dei Hospital.CT Unit, Mater Dei Hospital.


2/25

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.


3/25


4/25

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.


5/25

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.


6/25

Patient positioningPatient positioning Improper positioning: may result in the

acquisition of a reference radiograph beyond theROI and/or require a repeat of the latter.


7/25

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.


8/25


9/25

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.


10/25


11/25

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.


12/25

Tube current modulationTube current modulation cont.cont.

D-DOM provides angular modulation.

Z-DOM modulates mA in the z-axis


13/25

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.


14/25

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


15/25

Magnification and offMagnification and off

centeringcentering


16/25

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.


17/25


18/25

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.


19/25

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.


20/25

With correct centering, more photons(X+++) traverse the thicker central

parts, resulting in adequate imagenoise and radiation dose.


21/25

More photon (X+++) passing throughperipheral parts, increase noise centrally

and dose peripherally.


22/25

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.


23/25

Off centering and imageOff centering and image

qualityquality

Difference in image noise @30mm (left)and 60mm (right).


24/25

Effect on image qualityEffect on image quality


25/25

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!

centraggio in ct

Documents

Transcript of centraggio in ct