X-rays and CT Scans

• X-rays are electromagnetic radiation with higher energy than visible light
• Wilhelm Röntgen discovered X-rays in 1895
• The X-ray tube produces X-rays by accelerating electrons to collide with a metal target
• Denser tissues like bones absorb more X-rays, while softer tissues allow more X-rays to pass through
• X-ray imaging captures images of the inside of the body
• Typical X-ray images are two-dimensional showing tissues and organs
• Image capture is based on how X-rays pass through the body
• Bones appear white, while soft tissues appear gray in X-ray images

CT Scans

• Computed Tomography (CT) scans use X-rays differently for detailed 3D cross-sectional images
• The X-ray machine rotates around the patient is used to capture images from multiple angles
• Detectors capture X-rays from all directions & specialized algorithms reconstruct 3D images
• CT provides clear images, eliminates superimposition of structures, and offers better contrast

Projected Rays

• Projected rays are X-rays emitted from the X-ray tube during CT scanning
• Rays passing through the body are absorbed to varying degrees
• The CT scanner records emitted rays to create cross-sectional images
• Projection refers to the data set obtained by measuring X-ray beam attenuation

Radiation Attenuation

• Radiation attenuation is the reduction in X-ray beam intensity as passing through tissues

• Bone: High density, effective at absorbing X-rays, appears white in CT

• Soft tissues: Less dense, less attenuation appear darker in CT

Energy Dependence

• Attenuation of X-rays varies depending on the energy
• Higher energy X-rays penetrate better, reducing contrast
• Lower energy X-rays are more likely to be absorbed, increasing contrast
• Selection of X-ray energy is important for good tissue contrast

Factors Affecting Radiation Attenuation

• Atomic Number: Higher atomic numbers attenuate more radiation

• Density and Electrons: More electrons mean more interaction with radiation

• Energy of Radiation: Higher energy X-rays are less attenuated

• Slice Thickness: Thicker slices lead to more attenuation

• Balancing energy involves choosing the kV (voltage) to ensure penetration while maintaining contrast

• High voltage improves penetration but reduces contrast

• Low voltage enhances contrast but may have difficulty penetrating denser areas

• Modern CT scanners use algorithms to adjust settings for clarity and radiation safety

CT Numbers

• CT Numbers or Hounsfield Units (HU) are numerical values assigned to each image voxel
• They represent the density of tissues and materials
• These numbers use water as a comparison point and quantify tissue density
• Water has a 0 HU reference point
• They are calculated based on the attenuation coefficient of a specific tissue and that of water
• Air: -1000 HU (very low density)
• Fat: Around -50 to -100 HU (low density)
• Soft tissues: Around 20 to 80 HU (moderate density)
• Bone: 300 to 1000+ HU (high density)
• CT numbers differentiate tissue types in the body and are the basis for diagnostic assessments
• Noise refers to the fluctuation in CT numbers from pixel to pixel
• CTnumber = (µtissue - µwater) / µwater * 1000

Accuracy of CT Numbers

• Accuracy refers to how closely the CT number reflects the actual tissue density
• Calibration: Regular calibration ensures CT numbers correspond to true tissue densities
• Incorrect CT numbers make it harder to distinguish between tissues
• Contrast resolution: The ability of a CT scanner to distinguish differences between tissue contrasts
• Consistency: CT numbers should be consistent across imaging sessions

Factors Affecting Accuracy CT Numbers

• Scanner calibration errors
• beam hardening
• patient movement
• artifacts

Uniformity

• Uniformity is the consistency of CT numbers across different areas of the same tissue
• Uniform CT numbers = homogeneous material throughout the region
• If the numbers are inconsistent there may be an underlying issue
• Good uniformity = consistent CT numbers across similar tissue types in different regions
• Poor uniformity can indicate Image noise or artifacts

Significance of Accuracy and Uniformity

• Allow for better diagnosis and treatment through clear planning: Accurate CT numbers enable the accurate determination of tumor density, bone fractures, and other abnormalities
• Help when comparing Scans: Allowing information from different scans to be combined and used comparatively: Uniform CT numbers allow comparison images from different times or different scanners
• Improve Image Quality: Uniformity and accuracy ensure high image quality for easier diagnosis

Methods to Ensure Accuracy and Uniformity

• Routine Calibration: Regular CT machine to make sure numbers stay accurate
• Regular Quality Control: Checks the quality of CT images over time
• Computer software and Algorithms can reduce image noise and artifacts

CT windowing

• Windowing controls the contrast in a CT image for specific focus
• The Two Main Components of Windowing: Window Width and Window Level

Window Width

• Window width defines this amount of CT numbers in the image
• Higher contrast: A close range of numbers means the resulting image has a high contract
• Higher detail: A wide rang of numbers displayed means there s broader range of detail, but it makes it more diffcult to distinguish detail than smaller numbers
• Window controls adjust range of pixel values displayed

Window Level

• Window level refers to where on the range to focus. For examples the center of the grayscales
• Higher Density: Increasing the window level focuses on higher resolution images like bone
• Lower Density: Decreasing the window level focuses on lower resolution images like air

Data Acquisition

• CT Data acquisition of CT (Computed Tomography) is the process of of collecting data from the patient, then processing data from that patient by computer

Raw data Collection

• X-ray beams begin the data process and runthrough the body
• Detectors read the information and record levels of attenuation, the reduction of strength of X-rays

Detectors:

• Collection and Transmition: Collect information about the X-rays and transmit the data at various angles to the computer for image creation
• Process the data internally: Convert the internal signals from X-rays into electronic signals, then pass the signal to the computer to work with
• Electronic signals are first prepared by a preprocessor for more filtering.
  • From there the data is sent to the host for reconstruction

CT Scanning

• Scanner data processing and images are all about good capture efficiency
• CT scanners work because they can capture x-ray photons, and turn them into electronic images
• Rotation is key: X-ray must come from all angles of patients for good image construction
• The collected data is used to reconstruct cross-sectional images of the body.

Algorithms

• Collected data is converted into a visual by mathmatical alogrithms

Important points for 3d scanning

• The individual images must be stacked to create a 3d model.

Data Sampling

• Sampling is measuring data to be converted: Sampling converts continous data into non continous data units
• CT data sampling: Data is catpured specific intervals dependent on the rotation and movement rate of detector mechanisms and rotation of the device

Nyquist

• The Nyquist Sampling Theorem expains the importance of pixel size for good resolution of CT scans and how many measuremnt instances

Image element

• Is a single element, in 2d called a pixel and in 3d called Voxel

CT Scanning geometry

• Good scanning geometry of the patient is critical in the scan quality
• Must know how to measure patients correctly and how best to use motion to optimize
• Data Acquisition Geometries (DAS):
• the patient should be as stillas possible

Generation of Scanners:

• The different types of scanners vary in capture rate, resolution, speed and image quality
• The first generation scanner used X-ray tubes that rotated with a detector in only 1 increment, and only stopping with an X-ray needed
• The old method was slow and only stopping at the right angle was time consuming
• Scanners used to go around the patient in a complete circle
• First generation scanners used a rotate-translate method
• Second generation scanners used multple detectors in a linear array
• With the second generation scan you could rotate the camera for capture
• The 1st and 2nd generation scanners take individual pictures and merge them
• Third generation scanners feature a rotating X-ray and a curvine detector array The X-rays are able to use the third generation systems at a faster rate compared to older technology
• Forth generation Scanner set ups use detectors that completely encircle the patient
• In this set up the only thing rotating is the detector
• This increased speed while fixing the mechanical issues with previous models.
• Fifth Generation scanner is the most advanced with significant increases in speed, function and image
• These types of scanners come in Spiral Helical and Dual Source models

Helical CT Scanning

• During Spiral Helical data capture, a single spot rotates with the detector around the patient while it is in uniform helical motion
• It is a good motion becaue it does fast data adquistion, resolution and the ability to reduce image overlap during capture
• This model is easier to use and work better than other techonology due to high scan rate, image quality, and the scan's ability to adjust

Dual Source CT Scanning

• This model utilizes 1 tube at at higher enery level and the other as the lower
• This set up is benefical for differentiation of tissues
• The system also can be high quailty scans in patients with medical needs thanks to the design
• commonly used for cardiac imaging