Computed Tomography Basics Limitations of Radiography 3 D

Computed Tomography Basics

Limitations of Radiography 3 D body rendered in 2 D Structures superimposed on image Structure of interest viewed through underlying / overlying structures Multiple views used to adequately visualize structure. X-ray Beam Patient Film

Limitations of Radiography Density (gray shade) dictated by total attenuation encountered by beam Thin dense object Thin highly-attenuating objects have same density as thicker low-attenuating object. X-ray Beam Patient Thick less dense object Image

Radiography vs. CT Both based on differential attenuation of x-rays passing through body Radiography “Shadowgraph” using x-ray light source CT Cross-sectional image Image computed from pencil beam intensity measurements through only slice of interest

X-Ray vs CT View anatomy without looking through underlying / overlying structures Conventional improves contrast X-ray Beam Tightly collimated beam minimizes scattered radiation improves contrast Very small contrast differences visible CT X-ray Beam

Conventional (not Computed) Tomography Philips Polytome Predated CT by decades Application Examples IVP Inner ear

Body Section Tomography Not CT Body Section Imaging Goal keep plane of interest in focus blur all other plans enhances contrast Popularity decreased because of CT MRI

Conventional Tomography Blurring Image produced on film Objects above or below fulcrum plane change position on film & thus blur

Conventional vs Axial Tomography Conventional Cut CT Axial Cut

Limitations of Conventional Tomography Overlying / underlying structures blurred, not removed 5 -10% subject contrast difference required for objects to appear different many anatomic systems don’t have this much subject contrast

CT Advantages Excellent low-contrast resolution sensitive detectors small beam size produces little scatter Much better than film

CT Disadvantages Poorer spatial resolution than radiography Higher dose Artifacts at abrupt transitions bone / soft tissue interfaces metallic objects

CT Early Units 4 minute scans 1 slice 5 minute reconstruction 80 X 80 matrix head only water bag fit tightly around head

Data Acquisition cross sectional image reconstructed from many line transmission measurements made in different directions Tube Detector

Projection Measurements Radiation detector generates a voltage proportional to radiation intensity

Image Reconstruction Minicomputer does its thing Analog to Digital (A to D) conversion

CT - Improvements All CT generations measure same transmission intensities in many directions Improvements Protocol for obtaining many line transmissions # of line transmissions obtained simultaneously Detector location Overall acquisition speed

1 st Generation Translate / Rotate

3 rd Generation CT Wide angle fan beam Rotational motion only / no translation detectors rotate with tube 30 o beam Many more detectors

3 rd Generation CT Z-axis orientation perpendicular to page Patient

3 rd Generation (Non-spiral) CT �Tube rotates once around patient � Table stationary � data for one slice collected �Table increments one slice thickness �Repeat � Tube rotates opposite direction

Spiral CT (late 1980’s) Continuous rotation of gantry Slip ring technology Patient moves slowly but continuously through gantry No dead time as gantry reverses Much faster

Spiral CT Z-axis orientation perpendicular to page Patient

Data Aquisition Slice by slice One slice at a time Before ~ 1990 Since ~ 1990 Volume acquisition data for an entire volume collected patient moves in axial direction during scan tube traces spiral-helical path through patient

Spiral CT Advantages volume data acquisition in single breath hold no delay between slices improved 3 D imaging improved multi-planar image reformatting Special applications bone mineral content radiation treatment planning CT angiography

Multi-slice CT (2000’s) Multiple rows of fan beam detectors Wider fan beam in axial direction Table moves much faster Substantially greater throughput

Multi-slice CT Advantages Scans at much greater speed OR Allows scanning of same volume with thin slices Makes possible additional clinical applications