COMPUTED TOMOGRAPHY

CHAPTER 33

FUNDAMENTALS

Creating a cross-sectional tomographic plane of any body part A patient is scanned by an x-ray tube rotating around the body A detector assembly measures the radiation exiting the patient 2

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FUNDAMENTALS

Exiting radiation: Primary data Primary data is collected by detectors The computer compiles and calculates the data based on preselected alogorithm and an image is 4

PRESELECTED ALGORITHM

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IMAGE

Each image is displayed in an axial form (usually) The images are displayed on a cathode ray tube 6

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CT

Conventional Radiographs: Frequently body structures are superimposed In CT: A tightly collimated x-ray beam is directed thought the patient from different angles – “cross sectional image” Essentially eliminating superimposition of body structures 8

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Claim to fame: CT Exceptional Contrast Resolution

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Contrast resolution = differentiation of densities, capable of differentiating among tissues with similar densities 12

CT

Due to the reduction in amount of scattered radiation  Reducing over lapping structures and 2 collimators Digitized image: because of this numerous image manipulation techniques can be used to enhance and optimize the diagnostic information.

Window/Level, Axial, Sagittal, Coronal 13

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Third Generation

Fan-shaped x-ray beam  960 detectors opposite the x-ray tube Complete 360 degree rotation  Rotate/Rotate movement One rotation = one slice Second data acquisition could be made as the tube and detectors move in the opposite direction. Time reduced to 1 sec per slice 16

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generation configuration

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Fourth Generation

Developed in 1980’s  Fixed ring of as many as 4800 detectors, completely surrounding the patient, Rotate only movement Rotating x-ray tube provides short bursts of radiation  Detectors collect the remnant radiation to reconstruct into an image 1 minute for multiple slices 18

4 th generation configuration 19

Modern Scanners

No longer categorized into Generations Contemporary CT scanners are either third or fourth generation designs  Scanners are categorized by tube and detector movement Slip Ring Technology: connects generator with tube (no cables) 20

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Technical Aspects

Optimum imaging: patient/area of interest and gantry are perpendicular to each other Tube rotates around the patient, irradiating the area of interest.

Detectors measure the transmitted x-ray values, covert them in to an electric signal, and relay the signal to the computer.

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Raw Data

The remnant radiation that is converted into an electrical signal values are called projections, scan profiles or raw data  Raw data is collected and digitized This process assigns a whole number to each signal.

 The value assigned is directly proportional to the strength of the signal.

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Map Indirect vs. Direct Digital Capture

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Digital Image

Array of numbers arranged in a grid of rows and columns called a matrix. Single square, or picture element, with in the matrix is called a pixel.

Slice thickness gives the pixel and added dimension called the volume element, or voxel 25

Voxel

Each pixel in the image corresponds to the volume of tissue in the body section being imaged. The voxel volume is a product of the pixel area and slice thickness 26

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Hounsfield units

Each pixel within the matrix is assigned a number that is related to the linear attenuation coefficient of the tissue within each voxel These are CT numbers or Hounsfield units.

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Hounsfield units

Defined: A relative comparison of x-ray attenuation of a voxel of tissue to an equal volume of water.

Water is used because it is in abundance in the body and has a uniform density Water is assigned an arbitrary value of 0 29

CT numbers

Tissue denser than water are given positive CT numbers Tissue with less density than water are assigned negative CT numbers The scale of CT numbers ranges from -1000 for air to +3,000 for dense bone 30

Displaying the image

On the CRT, each pixel within the image is assigned a level of gray The gray level assigned to each pixel corresponds to the CT number or Hounsfield units for that pixel 31

System Components

Computer, Gantry &Table & Operator’s Console Computer – provides the link between the CT technologist and the other components of the imaging system 32

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Computer

The computer has four basic functions:  Control of data acquisition  Image reconstruction  Storage of image data  Image display 34

Array Processor

Raw Data is sent to the array processor Array processor only performs algorithm calculations.

 Applies desired filters to raw data. 35

Gantry

Gantry is a circular device that houses the Data Acquisition system (DAS)  Tube  Detectors  Filters   Collimators Analog-to –Digital Converter (ADC) 36

Gantry

Can be tilted forward or backward up to 30 degrees to compensate for body part angulation.

The opening within the center of the gantry is termed the aperture 37

CT Tubes

X-ray tube for CT is similar in design to the conventional radiography tube,  but is specially designed to handle and dissipate excessive heat units – much higher heat loading Ceramic target backing  Decreases tube weight 38

Detectors

Function as image receptors for remnant radiation  then converts the measurement into an electrical signal proportional to the radiation intensity.

Two basic detector types are used: Scintillation (solid state) and Ionization (xenon gas) detectors.

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Detectors

Used to record photon activity Materials include: cadmium tungstate, cesium iodide, gadolinium or yttrium 40

Table

Automated device linked to the computer and gantry  Designed to move in increments after every scan according to the technologists scan program Accurate and reliable table movements is vital to image quality and accuracy  Has weight limits 41

Operator’s Console

Where the technologist controls the scanner  Keyboard  graphic monitor  Mouse 42

Display Monitor

For the CT image to be displayed monitor in a recognizable form, the digital CT data must be converted into a gray-scale image Each digital CT number is the matrix is converted into an analog voltage 43

Image Display

The brightness value of the gray-scale image correspond to the pixels and CT numbers of the digital data they represent Because the image is digital image manipulation can be performed 44

FOV The field of view determines the amount of data to be displayed on the monitor

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Image manipulation

Most common: windowing or gray-level mapping This technique allows the technologist to alter the contrast of the displayed image  by adjusting the window width and window level.

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Windowing

Window width: is the range of CT numbers that are used to map signals into shades of gray  Wide/Narrow or Long/Short Window level: determines the midpoint of the range of gray levels to be displayed  Darker or Lighter 47

Window Width

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Window Level

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Image manipulation

Multiplanar reconstruction or MPR Ability to reconstruct axial images into coronal, sagittal or oblique body planes 50

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Diagnostic Applications

Most common procedures: head, chest, abdomen, pelvis  CT is the exam of choice for head trauma  CT can evaluate the CNS for infarctions, hemorrhage, disk herniation, craniofacial and spinal fractures, tumors and other cancers 53

Biopsy & Abscess Drainages

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CT examinations

CT demonstrates abnormalities such as metastatic lesions, aneurysms, abscesses, and fluid collections from infection or trauma Interventional procedures: abscess drainage, tissue biopsy, cyst aspiration 56

CT Angiography

CTA: Uses three dimensional (3D) imaging techniques to evaluate the vascular system Advantages over conventional angio:  Image reconstruction without the use of more patient exposure to radiation or IV contrast  Overlying structures can be eliminated (post processing)  Does not require an arterial puncture 57

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Contrast Media

Is used in most CT imaging to distinguish normal anatomy from pathology  Iodine based IV contrast, similar to the IVU  Oral/Rectal contrast  2% Barium mixture is used  Iodinated oral contrast can be used (Hypaque) Gastrographen must be at low concentration to prevent contrast artifacts 59

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What are factors that affect image quality?

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Factors Affecting Image Quality

Four main factors contributing to image quality are: Spatial resolution, Contrast resolution, Noise, and Artifacts Diagnostic imaging has superior spatial resolution compared to CT  How do we measure spatial resolution?

 What is the range for plane films? 63

Technologist Determines Image Quality Factors The technologist choices will effect the resultant image quality”  slice thickness  focal spot size  display FOV  technique selection   pitch reconstruction algorithm 64

DYNAMIC SCANNING

Based on the principal that after contrast administration different structures enhance at different rates 65

Spiral Scanning Table and Tube moving throughout exposure 66

Helical Scanning only the tube moves during the exposure 67

SPIRAL/HELICAL CT

Spiral CT = Continuous Tube and Table movement  Pitch  .5 = more information more radiation  2 = less information less radiation Helical CT = Table moves in predetermined increments  Post processing is not possible 68

Pitch = 2

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Pitch = 0

How does the Pitch affect patient dose?

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MULTI-SLICE SPIRAL/HELICAL Multi-slice Helical CT (MSHCT) Multiple aligned row of detectors  4, 16, 32, 64  Improved Spatial resolution  Decreased scan times 71

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3D IMPROVES RAD THERAPY

This method helps the dosimetrist to plan treatment so that the radiation dose to the target (tumor) is maximized and the dose to the normal tissue is minimized.

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MRI vs CT

With the introduction of MRI the profession speculated if CT would become obsolete Advantages of CT : Metal is not affected, Claustrophobia, uncooperative patiens, obese patients, fast scan times, trauma, more cost effective Both modalities are used to provide as much diagnostic information as possible 78

CT in the Future

CT technologist has an increased responsibility to understand scanning dynamics This imaging modality will continue to be a highly respected diagnostic tool 79

Review/Questions

What does the window width control for image display?

Pitch how does this affect image quality and patient dose?

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