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 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

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? ??? Godfrey Paul, Ringo, George, & John

It Was the Late 1960’s

A lot of the money was going here

Electronic and Musical Industries LTD Follow the Money Electronic and Musical Industries LTD

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 image points 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 Math developed in 1910’s Other Applications Astronomy (sun spot mapping) Electron microscope imaging Nuclear medicine emission tomography MRI Acme Mini- Compu- ter Digital Image

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

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

Translate / Rotate

CT Early Units 4 minute scans 5 minute reconstruction 80 X 80 matrix 1 slice 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 monitor 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 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

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

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

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

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

Computer Improvements Virtually instantaneous reconstruction time Auto Window protocols Transmission to PACS Backup Image manipulation Slice reformatting 3D reconstruction And the ability to do it all simultaneously

Fundamental CT Tradeoff Typically phantom dose: 1-2 rad (10-20 mSv) To improve one requires compromise on another Resolution Noise Dose

CT Usage 16% of imaging procedures 23% of total per capita exposure 49% of medical exposure

CT Usage Annual growth ~ 67,000,000 procedures in 2006 U.S. Population: <1% CT Procedures: >10% ~ 67,000,000 procedures in 2006 about 10% pediatric CT Computed Tomography — An Increasing Source of Radiation Exposure David J. Brenner, Ph.D., D.Sc., and Eric J. Hall, D.Phil., D.Sc. New England Journal of Medicine, 2007

How many children’s lives are saved by CT? 6/19/2001 “Each year, about 1.6 million children in the USA get CT scans to the head and abdomen — and about 1,500 of those will die later in life of radiation-induced cancer …

Medical imaging procedures should be appropriate & conducted at the lowest radiation dose consistent with acquisition of desired information Discussion of dose risks should be accompanied by acknowledgement of procedure benefits

Risks of medical imaging at effective doses < 50 mSv (5 rad) for single procedures or 100 mSv(10 rad) for multiple procedures over short time periods are too low to be detectable & may be nonexistent. Predictions of hypothetical cancer incidence and deaths in patients exposed to such low doses are highly speculative should be discouraged

These predictions are harmful because they lead to sensationalistic articles … that cause some patients & parents to refuse imaging procedures, placing them at substantial risk by not receiving the clinical benefits of the prescribed procedures