Computed tomography basics

Maulik R. ShahB. E. (Inst & Control Engg.)M. S. (Biomed Engg., USA)PGDHA (Apollo Hospital)

Consultant Biomedical EngineerMedical Equipment PlannerHospital Project Consultant

Mobile : 9824019971Email : [email protected]

www.atozforhospital.com

Basic & Conceptual Information

aboutComputed Tomography.

Equipment Arrangement

Radiography v/s

Tomography

Tomo = image // to long axis of the body CT = image is transverse to the body

Basic C T Principles

• Computed Tomography has the x-ray tube move across the so the image is called a transverse image or one perpendicular to the long axis of the body.

Computed Tomography Development

• Computed tomography has gone through five major design advancements since 1970

• Each development improved both scan time and resolution or image quality.

• Scan time have been reduced from 5 minutes to 50 ms.

• First scanner used a very tightly collimated pencil beam.

Generations

• First Generation Scanners– Translation/Rotation– Tube produced a finely collimated beam or pencil

beam– 1 to 3 detectors were placed opposite the tube for

radiation detection– 4.5 minutes to gather enough information for one

slice– Tube was only able to rotate 180 degrees

First Generation

• 1971• Pencil Thin Beam of Radiation• Translate and Rotate

– X-ray source and detector fixed relative position

– One degree of rotation possible

• Scan Time (Single Image)– 5 min

• 128 x 128 images

Patient

X-ray tube

Detector

First Generation CT Scanner

• Pencil Beam• Translate-Rotate Design• 180 one degree images

or translations.• One or two detectors.• 5 minutes scan time

Second Generation

• Fan-shaped x-ray beam• 30 or more detectors• 20 seconds per slice or 10 minutes for a 40

slice exam• 180 degree rotation• Long data reconstruction time

Second Generation

• 1974• X-rays

– Multiple Pencil– Fan Shaped 3 - 26 degrees

• Still Translate and Rotate– X-ray source and detector still fixed

relative position– Rotation increased from one to three

degrees• Scan time (Single Image)

– 3.5 min– Eventually 5.3 sec

X-ray tube

Detectors

Second Generation CT Scanner

• Translate-Rotate• Fan beam collimation so

there is more scatter radiation.

• 5 to 30 detectors• 10 degrees /translation

18 per scan.• 30 second scan times• Faster scan time

Third Generation

• Fan-shaped x-ray beam• 960 detectors opposite the x-ray tube• Complete 360 degree rotation Rotate/Rotate

movement • One rotation = one slice• Second data acquisition could be made as the

tube and detectors move in the opposite direction.

• Time reduced to 1 sec per slice

3rd generation configuration

Third Generation

• 1977• X-ray

– Fan Shaped– 21 - 45 degrees

• Rotation Only– Array of Detectors

• Scan Time (Single Image)– 4.8 sec– Eventually reduced to 2.5

seconds

Detectors

X-ray tube

Third Generation CT Scanner

• Rotate-Rotate• Fan shaped beam of 30

to 60° for full patient coverage.

• Constant Source to detector distance due to curvilinear detector array.

Third Generation CT Scanner

• If one detector fails, a ring artifact appears.

• 1 second scan times• Superior reconstruction

and resolution.

Fourth Generation

• Developed in 1980’s • Fixed ring of as many as 4800 detectors,

completely surrounding the patient, Rotate only movement

• Rotating x-ray tube provides short bursts of radiation

• Detectors collect the remnant radiation to reconstruct into an image

• 1 minute for multiple slices

• 4th generation configuration

Fourth Generation

• 1980• X-ray

– Wide Fan– 48 - 120 degrees

• Rotation Only– Full Ring of Detectors

• Scan Time (Single Image)– 5 Sec– Eventually reduced to ~1 sec

X-ray tube

Ring of Detectors

Fourth Generation CT Scanner

• The tube rotates around a stationary ring of detectors.

• Fan beam• Variable slice thickness

with pre and post patient collimation.

Fourth Generation CT Scanner

• As many as 8000 detectors.

• 1 second scan time.• Auto-detector

calibration so no ring artifact.

• High radiation dose compared to earlier scanners.

Fifth Generation CT Scanner

• This is the latest generation of CT.

• Allows for continuous rotation of the tube for spiral CT.

• 5th Generation also includes two novel designs: