Computed Tomography: An Overview

CLRS 408: Introduction to CT Rebecca Keith, MS, RT(R)(CT)

Outline:

1. Meaning 2. Process 3. How CT Scanners Work 4. Historical Perspective 5. Digital Image Processing 6. Applications of Volume Scanning

CT is a Key Diagnostic and Imaging Tool Cross-sectional imaging Cancer treatment planning Trauma / Emergency Vascular imaging Cardiology ‘Fusion’ imaging

1 In the beginning, there was Radiography … Discovered by Wilhelm Roentgen in 1895 ◦ 2D photo of a 3D structure ◦ Great for internal anatomy

X-ray tube

Image receptor (IR)

Limitations of Radiography

◦Superimposed anatomic structures ◦Hard to differentiate similar tissues (poor contrast resolution) ◦Not quantitative

Tomography

“Process of recording a section” ◦ Body Section Radiography and Stratigraphy  1920s ◦ Tomography - 1935 Uses motion of x-ray tube and IR to blur out structures above & below level set by technologist ◦ Contrast resolution better than radiography but not as good as CT imaging ◦ Limited use today ◦ Nephrotomograms ◦ Digital Tomosynthesis (breast, chest, bone)

2 Conventional Tomography

Beginning of exposure End of exposure

Anatomy at this level will appear sharp on radiograph ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ Anatomy above and below will be blurred

End of exposure Beginning of exposure

Simulation

Patient with Right Flank Pain

◦Radiography order of KUB (Kidneys, Ureters, Bladder) ◦Radiopague structure seen overlying right transverse process of L3 and psoas muscle ◦Stone in the kidney? Ureter? Artifact from GI tract? ◦Poor Contrast Resolution

Patient with Right Flank Pain

◦Patient injected with iodinated contrast media for Intravenous Pyelogram study  conventional tomography ◦Image blurred above & below level of kidneys and ureters ◦Strictures in right ureter, but is stone inside? ◦Still has poor Contrast Resolution

3 Patient with Right Flank Pain

CT overcomes the limitations in detail and clarity by using computerized reconstruction Based on CT, we can tell the location of the stone is in the right UPJ

Image Reconstruction from Projections

Image Reconstruction from Projections: 1917  Radon 1980  Herman 1960s  Oldendorf, Kuhl, Edwards (NM) 1963  Cormack 1967  Hounsfield

2 Types of Computed Tomography

Emission CT ◦ To view function ◦ Receive emissions from radioactive agents that have been introduced into the patient’s body ◦ Single Photon Emission Computed Tomography (SPECT) ◦ Gamma emissions ◦ Positron Emission Tomography (PET) ◦ Positron emissions

4 2 Types of Computed Tomography

Transmission CT ◦ To view anatomical structures ◦ X-ray tube travels around patient at least 360˚ ◦ X-ray beam pulses on and off ◦ Transmitted x-rays are detected by detectors ◦ Computer reconstructs data to produce cross-sectional slices of body ◦ Image reconstructions are made…

Anatomic vs Functional Imaging

Patient is Alive Patient is Dead

CT Scan Anatomical information

PET Scan Functional information

A Little History…

Johann Radon (1917) ◦ Austrian mathematician who developed theory of image reconstruction ◦ 2D or 3D object from large number of projections from different directions ◦ The Radon Transform

From the AAPM

5 A Little History…

Dr. Allan Cormack – ◦ Solved mathematical issues of CT ◦ 1963 & 1964 – publishes mathematical analyses of tomographic image reconstruction in Journal of Applied , but received no interest at the time ◦ Unaware of Radon’s work ◦ Read more >>

From Journal of Applied Physics

A Little History…

Sir ◦ 1967 – applied reconstruction techniques to produce world’s first clinically useful CT scanner: ◦ If x-ray beam were passed thru an object from all directions, and measurements were made of all x-ray transmissions, information about the internal structures of that body could be obtained ◦ This info could be shown in form of images showing 3D representations ◦ Unaware of either Radon or Cormack’s work ◦ Read more >>

A Little History…

Sir Godfrey Hounsfield 1967  1st scan ◦ 9 days  2.5 hours to process 28,000 measurements collected by gamma detector ◦ Replaced by x-ray tube, which was more accurate but still took 1 day to produce an image ◦ Worked with Dr. James Ambrose (radiologist) 1971  1st clinical prototype CT brain scanner (EMI Mark 1) ◦ Required water bath ◦ Processing time reduced to 20 mins ◦ Addition of Microcomputers – reduced to 4.5 mins 1972  1st patient scanned ◦ Suspected brain lesion

6 A Little History…

Robert Ledley from ◦ Dentist turned biomedical researcher and trailblazer 1974 – 1st whole body CT scanner ◦ Automatic Computerized Transverse Axial (ACTA) scanner ◦ No water bath ◦ Original prototype of the ACTA scanner is at the Smithsonian Institution

A Little History…

Dr. Willi Kalendar – German Medical Physicist ◦ 1989 – 1st to report on a practical spiral CT scanner at RSNA ◦ Dr. Kalender has made significant contributions to the technical development and practical implementation of spiral/volume CT

7 Evolution of Terms

◦ Hounsfield called his new technique “Computerized Transverse Axial Scanning (Tomography)” in British Journal of Radiology in 1973 ◦ Other terms seen in journals: ◦ “Computerized Transverse Axial Tomography” ◦ “Computer-Assisted Tomography” ◦ “Computerized Axial Tomography” ◦ “Computerized Transaxial Transmission Reconstructive Tomography” ◦ Patients say they are coming for a “CAT Scan” ◦ “Computed Tomography” established by RSNA in its journal Radiology ◦ This is what we use today!

Overview of CT Imaging Process

1. Data acquisition 2. Image Reconstruction 3. Image Display, Manipulation, Storage, Recording and Communication

Data Acquisition How we collect information from the patient! X-rays pass through patient ◦ Some are absorbed by the patient ◦ Some are scattered ◦ Some are transmitted onto detectors that measure the transmission values (attenuation values) ◦ Enough x-rays must be transmitted to meet requirements of recon process!

8 Image Reconstruction

Image reconstruction algorithms  transmission data sent to computer for processing Hounsfield used algebraic reconstruction technique Today we use: Filtered Back Projection (FBP) Adaptive Statistic Iterative Reconstruction (ASIR)

Image Display, Storage, and Communication

After processing, reconstructed image is displayed ◦ Text says CRT monitors but …LCD and Flat panel being used  high-res is key! Post-processing occurs here ◦ Reformattes to coronal, sagittal, oblique, para-axial, etc ◦ 3D, edge enhancements, other manipulations Storage ◦ PACS  has to be DICOM image ◦ Magnetic Tapes / Laser Disks?

9 First Generation

Pencil beam Single detector Translate-Rotate 180 /1 degree 5 minutes

Second Generation

Multiple pencil beams Multiple detectors Translate-Rotate 180/10 degree 30 sec

Third Generation Fan beam Multiple detectors 360 rotate 1 sec

10 Fourth Generation Stationary detectors Moving tube Fan beam Thousands of detectors 1 sec or less

Rapid Advances

1990 - Spiral CT 1994 - Sub-second spiral 1998 – Multislice (Multidetector) Spiral 2005 – Dual source CT

Volume Scanning Lots of data Quick acquisition Leads to: ◦ CT angiography/cardiac CT ◦ 3D/4D imaging ◦ Virtual reality imaging ◦ PET/CT; SPECT/CT ◦ CT simulation

11 Advances or Generations?

•Electron Beam CT (EBCT) • Cardiac Uses • 5th Generation?

Advances or Generations? • Dual Source CT • Multiple Sources/Detectors?! • Sixth Generation?

Advances or Generations? • Flat Panel Technology • High spatial resolution volumetric imaging and dynamic CT scanning • Diagnostic and interventional clinical purposes • 7th Generation?

12 Uses outside

Lumber industry Paleoanthropology Stock breeding Explosives detection

The End

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