RAD 254 –Chapt 23 Mulit-slice Computed Tomography

In short – CT is a thin band (fan) of radiation directed toward the

patient and the remnant radiation emitted from the patient is

measured by a receptor and the response transmitted to a

computer.

Dates/activities to remember

• 1970 Dr. Godfrey Hounsfield demonstrated the technique – “EMI” unit

• 1982 – Dr. Hounsfield and Dr. Alan Cormack share the Nobel Peace Prize in physics

CT Generations and important items

• 1st – translate-rotate – pencil beam – one rotation for 1 image – Head only – 5 min/image

• 2nd – translate-rotate, but with a fan beam – produced increased intensiity at the edge of the beam – “bow-tie” filter used to remedy this. 30 sec image time

Generations con’t

• 3rd used array of detectors - produced “ring artifacts”

• 4th rotate – stationary, fan beam, detector array, short image time and no re-con time

• 5th Is now here! “spiral/slip ring” (it is slip ring technology that has enabled this latest generation!)

Major components of CT

• Gantry – tube-receptor array, collimators (pre-patient and pre-detector), generator, patient couch

• Computer – microprocessor – recon time a big issue until recently

• Operating console – usually two monitors – one for RIS/HIS information and the second for the imaging portion (often times a remote monitor is set up for the radiologist – can be a hassle)

Spiral CTSprial

Helical

16 slice

Slip Ring CT

Interpolation Algorithms

• Interpolation – information along the axis within established values

• Extrapolation – information outside axis values established, but an estimated value if parameters continue as previously meassured

Spiral Terms

• Pitch – (pitch ratio) relationship between couch movement and x-ray beam collimation– Increasing pitch ratio enables much more of

the patient to be imaged in one breath hold

Slip Ring Technology(Made spiral possible – like brushes on s starter motor)

• Three slip rings in the gantry (usually)– One for hi voltage power to the tube and hi

voltage generator– Low voltage power to the rotating gantry– Transfer of digital data from gantry receptors

Multi slice CT

• Multi detector arrays– Two at once with one beam = two slices per

360 degrees– With “spiral” enable two detectors to produce

16 slices

• Envision a person on the table/couch in a screw/nut fashion going through the gantry

Maximum Intensity Projection

• Pre selecting pixel levels to be viewed AFTER obtaining a “rubic’s cube of information”– Then exhibit only those at the pre-determined

value – like MRA/CTA

Spiral Advantages over Conventional

• Motion blur is reduced/fewer artifacts

• Lowered imaging time (fewer breath holds required/less motion/miss-representation)

• Larger volumes can be imaged with less part overlap artifacts