CT Chapter 1 Computed Tomography: An Overview

Tomography

- 1920 originally termed "body section radiography" Used to describe imaging a specific layer or section of the body - 1935 the term was refined to tomography derived from the greek word tomos meaning section - 1937 transverse axial tomography was the term used to refer to the imaging ability of cross-sectional images to be acquired

Dual Source (dual energy) CT Scanner

- 2 x-ray tubes and two detector array components - Primarily in cardiac imaging - Provide improved temporal resolution - Each x-ray tube operates at different energy levels

3D Imaging

- Algorithms that transform transaxial CT data into simulated 3D images - Transformation consists of three steps 1. Volume formation (stacking images to form volume with preprocessing) 2. Classification (determining the tissue types within the slices) 3. Image projection

Multislice CT (MSCT) Scanners

- Based on the use of multidetector technology - 4 or more slices per revolution - Increasing the # of slices per revolution--> increases the volume coverage-->reduces exam time

Quality Control

- CT subject to QC - Preforms quantitative and qualitative tests to ensure perfect working conditions - QC preformed by technologists and physicists

1. Data Acquisition

- Collection of x-ray transmission measurements from the patient - Electronic detectors measure the attenuation of the x-ray beam as it passes through the patient

2. Image Reconstruction

- Data collected during acquisition are sent to the computer for processing - Computers then use mathematical techniques known as image reconstruction algorithms which are applied to the data to produce and image

2. Detector Technologies

- Detector responsible for absorbing attenuated radiation and then converting it into electrical signals that become digitized and are sent to the computer to create the CT image - Two categories of detectors 1. Solid sates detectors 2. Gas-ionization detectors

Digital Image Processing

- Digital computer used to process and manipulate digital images - Image enhancement

Other MSCT Improvements

- High-speed imaging - Lare bore gantry apertures - Improved z-axis resolution - Dose modulation techniques - Cone beam image reconstruction algorithms - Pitch Improved isotropic resolution of less than 0.4mm

Cardiac CT Imaging

- Imaging the beating heart Electron beam scanners Multislice scanners Dual source scanners

Volume Visualization

- Involves the use of certain computer tools that change how the CT image is displayed EX: Windowing Interactive Cine visualization

Radiation Dose Optimization

- Keeps with ALARA philosophy - Striving for dose reduction while maintaining diagnostic quality images

CT Fluoro

- Real time Required three advances 1. Fast, continuous scanning 2. Fast image recon 3. Continuous image display in cine mode, 2-8 images per second

Imaging Parameters Affecting CT Dose

- Slice thickness - Noise - Resolution detector efficiency - Reconstruction algorithm - Collimation (2 axis) - Filtration

Spiral/Helical CT Scanners: Volume Scanning

- Technique of CT imaging in which a volume of tissue is scanned by moving the patient continuously through the gantry of the scanner while the x-ray tube and detectors rotate continuously for several rotations - Continuous motion of the x-ray beam and patient result in a spiral or helical path

CT Screening

- Used to detect diseases early in healthy individuals ( somewhat controversial) Uses include: Cardiac screening Lung cancer screening Whole body imaging

Spectral CT Imaging (Dual energy CT)

- Utilizes the x-ray tube's wide energy spectrum that reaches the detector in a way to optimize all structures within an imaged object

Hounsfield

-Applied reconstruction techniques to produce worlds first CT for the brain

Basic Principle of CT

-Produces sharp, clear cross-sectional images by reconstructing a large number of projections from different locations

3. Image Display

-Reconstructed image can be displayed for viewing or stored for later reanalysis - Occurs on monitors

3. Image Communication

-The electronic transmission of text data and images from the CT scanner to other devices - Electronic image communication requires a standard protocol to facilitate networking among the sending and receiving computers DICOM PACS

Image Reconstruction from Projections

-derived by mathematician Radon -two or three-dimensional objects could be "built up" from several projections from different directions

Formation of CT Images (3 steps)

1. Data acquisition 2. Image reconstruction 3. Image display, manipulation, storage, recording, and communication

CT Trends

1. Iterative reconstruction (IR) algorithms 2. CT detector technologies 3. Radiation dose optimization 4. Spectral CT imaging 5. Graphic processing unit (GPU) computing

1. Data Acquisition (Cont.)

1st CT scanners= x-ray tube and detectors move in a straight line across the patients head -left-->right, rotate 1 degree, right-->left (repeat for 180 degrees) -Termed translate-rotate-stop-rotate or scanning -Problematic due to length of exam time Newer generation CT scanners acquire a volume of tissue (volume scanning) with continuous rotation of the x-ray tube and detectors (spiral/helical)

How CT Scanners Work (Basic Steps)

1. turn on and QC 2. place patient in gantry 3. set technical factors 4. begin scanning 5. attenuated x-ray beam is measured by detectors that convert the attenuated data into electrical signals 6. electrical signals are converted into digital data for the computer 7. computer preforms image reconstruction process 8. digital reconstructed image is converted into electrical signals in order to be viewed by the technologist on a monitor 9. images are sent to PACS or stored on optical disks

CT Generations (method of scanning)

1.Pencil beam 2.Multiple pencil beam 3.Fan beam 4.Stationary ring of detectors (all single slice)

Applications of Volume CT

CT fluoro CT angio- CT imaging of blood vessels with contrast 3D imaging Virtual reality imaging Cardiac CT imaging CT stimulators in radiation therapy PET/CT

1. Iterative Reconstruction Algorithms

Creates simulated projections that are true representations of the scanned object

3. DICOM

Digital Imaging and Communication in Medicine

Historical Perspectives (contributors)

Godfrey Newbold Hounsfield - discovered that measurements of x-rays passing through an object from all directions could be formed into a 3D object - 1st clinically useful CT (brain) Allen MacLeod Cormack - developed solutions to the mathematical problems that plagued early CT development

3. Image Storage and Recording

Images can be stored and recorded in some form of archive - x-ray film - magnetic tapes/disks - optical storage media (disk, tape, and card)

3. Image Manipulation (digital image processing)

Images may be modified to make them more diagnostic EX: Image reformatting Smoothing Edge enhancement Gray-scale manipulation 3D processing

Radiation Dose Schemes

Manufacturers have developed various schemes to reduce patient dose in CT based on these radiation dose studies -Prepatient filtering technique -Ultrafast ceramic detectors -Automatic tube current modulation technique that adjusts mA based on patient characteristics *Peak efficiency @ ISO CENTER*

Introduction

Meaning of CT Fundamental principles History of CT

High-Speed CT Scanners

Multislice CT (MSCT) introduced at RSNA meeting (1998)

3. PACS

Picture Archiving and Communication System - uses DICOM for the networking and communication of images

Radiation Dose Studies

Provide maximum information while keeping radiation dose minimized Developments in how to measure and describe radiation dose - Single-scan dose profile - Multiple-scan dose profile - CT dose index - Multiple-scan average dose - Isodose curves