CT Seeram: Chapter 1 Computed Tomography - An Overview

Early History “tomos” Greek word meaning section Sectional imaging methods first developed in 1920’s

Early History: Conventional Tomography first used in 1935 image produced on film Image plane oriented parallel to film Anatomy in plane of fulcrum stays in focus anatomy outside of fulcrum plane mechanically blurred

Conventional vs Axial Tomography Conventional Cut CT Axial Cut

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

CT Image Not produced on film Mathematically reconstructed from many projection measurements of radiation intensity Digital Image calculated Acme Mini- Compu- ter Digital Image

How Did We Go From…

The story concerns these men. What was their Link? ??? Geoff Paul, Ringo, George, & John

It Was the Late 1960’s

A lot of the money was going here

Follow the Money

Measure Intensity of a Pencil Beam X-Ray Source Radiation Detector

CT Image Measure a bunch of pencil beam intensities

CT Image Now make measurements from every angle

CT Image When you get done, multiple pencil beams have gone through every point in body

Image Reconstruction Pixel (calculated) Data Projection (raw) Data X-Ray Source Acme Mini- Computer Pixel (calculated) Data Radiation Detector Projection (raw) Data

Digital Image 2-dimensional array of individual image points calculated each point called a pixel picture element each pixel has a value value represents x-ray transmission (attenuation)

Digital Image Matrix 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 120 145 301 256 223 287 225 178 322 325 299 353 333 300

Numbers / Gray Shades Each number of a digital image corresponds to a gray shade for one pixel

Image Reconstruction CT math developed in 1910’s Other Applications astronomy (sun spot mapping) electron microscope imaging Nuclear medicine emission tomography MRI

CT History First test images in 1967 First clinical images ~ 1971 First commercial scanner 1972                                                                      

CT History CT math developed in 1910’s First commercial scanner 1972 What took so long?                                                                      

CT History CT made possible by high speed minicomputer                                                                      

CT Computers Old mainframe computers too expensive & bulky to be dedicated to CT

The 1st Computer Bug

CT history - Obsolete Terminology CTAT computerized transverse axial tomography CAT computerized axial tomography CTTRT computerized transaxial transmission reconstructive tomography RT reconstructive tomography

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

Translate / Rotate

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

Beam Translation beam collimated to small round spot collimated at tube and collimator X-ray Tube Detector

Beam Translation Tube/detector translates left to right Entire assembly rotates 1o to right Tube/detector translates right to left X-ray Tube Detector

Translate - Rotate 180 translations in alternate directions 1 degree rotational increments between translations

Projection Measurements Radiation detector generates a voltage proportional to radiation intensity

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

Digital Image Matrix Digital Matrix contains many numbers which may be Displayed on CRT Manipulated Stored 125 25 311 111 182 222 176 199 192 85 69 133 149 112 77 103 118 139 154 120 145 301 256 223 287 225 178 322 325 299 353 333 300

Digital Image Manipulation Window Level Smoothing Edge enhancement Slice reformatting 3D derived from multiple axial slices

Digital Image Storage Magnetic Disk CD Tape Optical Disk PACS archive picture archival and communications system not part of CT contains images from many modalities allows viewing on connected computers

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

2nd Generation CT arc beam used instead of pencil beam several detectors instead of just one detectors intercepted arc radiation absorbent septa between detectors reduced scatter acted like grid Tube Detectors

2nd Generation CT arc beam allowed 10 degree rotational increments scan times reduced 20 sec - 2 min 2 slices obtained simultaneously double row of detectors

3rd Generation CT Wide angle fan beam rotational motion only / no translation detectors rotate with tube 30o beam Many more detectors scan times < 10 seconds

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

4th Generation CT Fixed annulus of detectors tube rotates (no translation) inside stationary detector ring only a fraction of detectors active at once

3rd & 4th 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

3rd / 4th Generation Image Quality Improvements Faster scan times reduces motion artifacts Improved spatial resolution Improved contrast resolution Increased tube heat capacity less wait between scans / patients better throughput

Spiral CT Continuous rotation of gantry Patient moves slowly through gantry cables of old scanners allowed only 360o rotation (or just a little more) tube had to stop and reverse direction no imaging done during this time no delay between slices dynamic studies now limited only by tube heating considerations

Z-axis orientation perpendicular to page Spiral CT Z-axis orientation perpendicular to page Patient

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

Computer Improvements Reconstruction time Auto-printing protocols Image manipulation Backup time Slice reformatting 3D reconstruction And the ability to do it all simultaneously

Cine CT (Imatron) four tungsten target rings surround patient replaces conventional x-ray tube no moving parts like 4 moving focal spots electron beam sweeps over each annular target ring can be done at electronic speeds 2 detector rings 2 slices detected maximum scan rate 24 frames per second

(scanned from Medical Imaging Physics, Hendee) Imatron Cine CT (scanned from Medical Imaging Physics, Hendee)

CT Patient Dose In theory only image plane exposed In reality adjacent slices get some exposure because x-ray beam diverges interslice scatter

Dose Protocols Plain X-ray Mammography CT entrance skin exposure mean glandular dose CT Computer tomography dose index (CTDI) Multiple-scan average dose (MSAD)

CT Dose depends on Noise detector efficiency collimation kVp mA time slice thickness filtration Noise detector efficiency collimation matrix resolution reconstruction algorithm

CT Patient Dose Typically 2 - 4 rad AAPM has single slice protocol for measuring head & body doses More dose required at higher resolution for same noise level More dose required to improve noise at same spatial resolution Resolution Noise Dose

Fundamental CT Tradeoff To improve one requires compromise on another Resolution Noise Dose

New Stuff CT Angiography CT fluoroscopy CT virtual endoscopy / colonoscopy / ??scopy