Understanding X-Rays

X-Rays

• X-rays are a type of electromagnetic radiation
• They have similar characteristics to visible light but possess much higher energy
• Wilhelm Röntgen, a German physicist, discovered X-rays in 1895
• They are one of medicine's most important diagnostic tools
• X-rays are produced in a special tube called an X-ray tube
• Electrons are accelerated to high energy inside the tube
• When the accelerated electrons collide with a metal target such as tungsten, X-rays are emitted
• The X-rays pass through the body's tissues
• Denser tissues, such as bones, absorb more X-rays
• Softer tissues, such as muscles and fat, allow more X-rays to pass through
• X-ray imaging captures images of the inside of the body
• X-ray images are usually two-dimensional (2D) and show the various tissues and organs
• An X-ray beam is emitted from a tube and directed towards the body
• X-rays pass through the tissues and reach a detector or film on the other side of the body
• Denser tissues like bones absorb more X-rays than softer tissues like muscles
• Capture of the image is based on how the X-rays pass through the body
• Bones appear white in the final image because they absorb the most radiation
• Soft tissues appear gray in the final image

Computed Tomography (CT) Scan

• CT scan is a more advanced technique that gives detailed cross-sectional images (slices) of the body in three-dimensional (3D) form
• It also uses of X-rays, but in a different way
• An X-ray machine rotates around the patient, capturing images from multiple angles in a CT scan
• X-ray beams are emitted from different angles around the body
• Detectors capture the X-rays that pass through the body from all directions
• A detailed cross-sectional (slice) image of the body is created from information collected from many different angles
• Specialized algorithms are then used to reconstruct 3D images using the data
• This allows a detailed view of internal tissues and organs
• CT provides clear, cross-sectional images
• CT eliminates superimposition of structures
• CT offers better contrast and quantitative imaging

Projected Ray

• Projected rays refer to the X-rays emitted from the X-ray tube that pass through the body from different angles in CT scanning
• Tissues absorb the rays to varying degrees as they pass through them (such as bones and soft tissues)
• The CT scanner records the rays that come out of the body after they have passed through
• Data is used to create cross-sectional images of the internal structures of the body
• In CT, projection refers to the data set obtained by measuring the attenuation (or absorption) of the X-ray beam as it passes through the patient
• Projections are foundation for creating cross-sectional images

Radiation Attenuation

• Radiation attenuation refers to the reduction in the intensity of the X-ray beam as it passes through different materials or tissues in the body
• Different tissues attenuate the radiation in different amounts
• Bone: High density and more effective at absorbing X-rays; leads to more attenuation and appearing white or bright in the CT scan
• Soft tissues (like muscles, fat): Less dense, which causes less attenuation of X-rays, which makes them appear darker in the image
• Radiation attenuation allows the CT scanner to create contrast between different types of tissues
• The contrast is based on how much they weaken the X-ray beam

Energy Dependent

• Energy dependence refers to the attenuation of X-rays that varies depending on the energy (or strength) of the X-rays used
• The energy level of the X-ray beam affects how much it interacts with the tissues
• High-energy X-rays are better at penetrating the body and pass through tissues with less attenuation, which could reduce the contrast in the image
• Lower-energy X-rays are more likely to be absorbed by tissues, increasing the contrast between different tissue types in the image
• It is important to select the appropriate X-ray energy to ensure images are high-quality with good tissue contrast in CT imaging
• Radiation attenuation depends on several factors:
• Atomic number: Higher atomic numbers attenuate more radiation
• Density and electrons per gram of tissue: 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 in X-ray imaging involves choosing the right voltage (kV) to ensure enough penetration of X-rays while maintaining good contrast between tissues
• High voltage improves penetration through thicker tissues but reduces contrast, while low voltage enhances contrast between tissues but may have difficulty penetrating denser areas
• Modern CT scanners use advanced algorithms to adjust these settings intelligently to achieve the best balance between clarity and radiation safety, delivering optimal results with lower radiation exposure

CT Numbers (Hounsfield Units or HU)

• CT numbers, also known as Hounsfield Units (HU), are numerical values assigned to each voxel (volume element) in the CT image
• The numbers represent the density of the tissues or materials that the X-rays pass through in relation to water
• Hounsfield units (HU) are used in CT imaging to quantify the density of tissues
• They are calculated based on the difference between the attenuation coefficient of a specific tissue and that of water, which has a value of (0) HU
• Different tissues and materials have different HU values:
• 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 help differentiate structures such as bones, muscles, and organs based on their density in the body
• They allow creation of the images and performing diagnostic assessments
• Noise refers to the fluctuation in CT numbers from pixel to pixel, which can be seen in a water phantom scan
• The formula for CT number is: CTnumber= (μtissue - μwater)/ μwater * 1000

Accuracy of CT Numbers

• Accuracy refers to how closely the CT number reflects the actual density of the tissue or material being scanned
• CT numbers need to be accurate for a CT scan to provide reliable diagnostic information
• Calibration: Calibrating CT scanners regularly ensures that CT numbers correspond to true tissue densities; drift or miscalibration could lead to incorrect CT numbers, making it harder to distinguish between tissues, or affecting the diagnosis; the ability of a CT scanner to distinguish differences between tissue contrasts is called contrast resolution, which determines how well tissues of different densities can be differentiated in an image
• Consistency: CT numbers should be consistent and stable across imaging sessions; the same tissue types should produce the same CT number values if a patient is scanned multiple times
• Factors that can affect the accuracy of CT numbers include:
• Scanner calibration errors
• Beam hardening (caused by dense materials like bone that distort X-ray beams)
• Patient movement during scanning
• Artifacts (such as from metallic implants)

Uniformity of CT Numbers

• Uniformity refers to the consistency of CT numbers across different areas of the same tissue or material
• For example, when imaging a homogeneous material like water or fat, the CT numbers should be uniform throughout the region of interest
• Good uniformity: CT numbers are consistent across similar tissue types in different regions of the body
• Poor uniformity: Can indicate issues such as:
• Image noise (random fluctuations that can distort the CT number values)
• Artifacts (such as streaks or rings caused by imperfections in the scanner or patient movement)
• Inconsistent calibration
• In a scan of the abdomen, fat should show a consistent CT number
• If the fat tissue shows varying CT numbers in different areas, it indicates that the scanner's uniformity is not optimal

Importance of Accuracy and Uniformity in CT Imaging

• Diagnosis and Treatment Planning: Accurate CT numbers are essential for physicians to assess the size, density, and location of tumors, bone fractures, or other abnormalities; inaccurate CT numbers can lead to incorrect diagnoses or treatment plans
• Comparison of Scans: Uniform CT numbers across scans allow doctors to compare images from different times or different scanners, helping in monitoring the progression of diseases or effectiveness of treatments
• Image Quality: Uniformity and accuracy ensure high image quality, making it easier to interpret and diagnose potential medical conditions; to reduce noise by half, the dose must be increased by a factor of 4; noise reduction is proportional to the square root of the dose increase
• Motion during CT scanning can cause artifacts including blurring, double images, streaks, and noise that are all due to patient movement

Methods to Ensure Accuracy and Uniformity

• Routine Calibration: Regular calibration of CT machines ensures that the CT numbers are consistent and accurate
• Quality Control: CT scanners undergo periodic quality control procedures to test for any inconsistencies in the numbers and uniformity; phantom tests (scanning test objects of known density) are used to check uniformity and accuracy
• Software and Algorithms: Advanced software helps to correct artifacts, reduce noise, and ensure that the CT numbers reflect the true tissue densities as closely as possible

CT Image: Windowing

• Windowing is a technique used to control the contrast in a CT image
• This allows a focus on specific tissues or areas that are important
• The two main components of windowing include:
• Window Width (WW): The range of contrast shown in the image:
• A narrow window width: Means higher contrast, making similar tissues with close densities easier to see
• A wide window width: Means lower contrast, which shows a broader range of tissues but might make differences harder to distinguish
• Window Level (WL): Window level is the middle value of the range of CT numbers
• The window level determines the center of the gray scale
• Increasing the window level focuses on higher-density tissues, such as bones
• Decreasing the window level focuses on lower-density tissues, such as air
• Window controls (Window Width - WW) control the contrast in CT images by adjusting the range of pixel values displayed

Data Acquisition

• CT (Computed Tomography) refers to the process of collecting raw data from the patient using X-rays, detectors, and other components of the CT system
• The data is then processed to generate the final images that are seen in the CT scan

Basic Concepts of Data Acquisition

• Raw Data Collection:
• Data acquisition in CT begins when X-ray beams pass through the patient's body
• The detectors then detect the X-rays by measuring the attenuation (reduction in strength) of the X-rays as they pass through different tissues in the body
• The amount of attenuation depends on the density and composition of the tissues being scanned (bones, muscles, fat, air)
• Detectors collect information about how much of the X-ray beam is transmitted through the body at various angles
• These detectors convert the X-ray data into electrical signals, which are sent to the computer for further processing
• Data from the CT detectors are first sent to the preprocessor, where they are prepared for further processing, filtering, and then sent to the host computer for image reconstruction
• Capture efficiency refers to how well a detector can capture incoming X-rays (photons) and convert them into electrical signals

X-Ray Source

• The X-ray source emits X-rays that pass through the body and reach the detectors in CT imaging
• The rotation of the X-ray source and detectors around the body helps in capturing data from multiple angles
• Then the collected data is used to reconstruct cross-sectional images of the body
• Collection of raw data processes the use of mathematical algorithms (such as filtered back projection or iterative reconstruction) to generate crosssectional images
• These images can then be stacked together to create a 3D model of the body

Sampling

• Sampling serves as an especially important concept in data acquisition, and refers to processing data into various discrete data points
• Sampling involves capturing measurements at specific intervals or angles as the X-ray tube and detectors rotate around the body in a CT

The Nyquist Sampling Theorem

• It explains the importance of pixel size in CT imaging to improve resolution
• Likelihood of accurately representing the object increases if pixel size is smaller, and the object does not fit entirely within a single pixel
• If the object is the same size as a pixel, it can either fit entirely within the pixel, straddle two pixels, or even intersect four pixels
• Likelihood of the object being represented accurately also increases by reducing the pixel size, since there is more spatial resolution
• The pixel-size is determined by field of View (FOV)

Data Acquisition Geometries

• Data Acquisition Geometries (DAS) stands for Data Acquisition System in CT
• DAS refers to getting data from a patient and scanning through x-ray tubes
• Data collection is used for creating images
• It is broken up into multiple generations of CT scanners
• Below are different generations of CT scanners
  • First Generation Scanners:
  • Geometry: Used single X-ray tube and single detector
  • How it worked: Tube and the detector rotated 1 degree around the patients, before stopping to receive data at a fixed angle
  • Used pencil beams
  • Caused long scan times because the system had to stop for each angle

Rotating Source with Translate

• First generation scanners used a rotate/translate method, where the X-ray tube
• The detector moved into a linear fashion to capture the images
• Second Generation Scanners:
• Geometry: introduced multiple detectors
• Replaced pencil beam
• How It Works: Captures patients at multiple angles
• Improved data collection
• First and second generation take individual slices of images and slice by slice scanning
• Multislice/spiral scanning came later
• Third Generation Scanners:
  • Geometry: CT scanners featured a rotating X-ray tube and a curved detector array (ring of detectors)
  • Utilized 360 rotation
  • Improved with better image quality
  • This design improved scanning speed with higher images
  • Drawbacks include having mechanical complexity, which resulted in image artifacts
• Fourth Generation Scanners:
  • Geometry: Stationary x-ray tube and rotating ring
  • Fixed x-ray reduced complexity
  • Improved on speed
  • Fourth-generation images were very costly though that performance justified
• Fifth Generation Scanners:
  • Represents great leap for in terms of speed quality and functionality
    • Featured in spiral helical

Dual Source CT

• Spiral Helical Geometry:
  • X-Ray source and rotating detectors while patient scans
  • Tube and detectors rotate 360 degrees while patient is scanning with continuous Spiral
• It can allow fast data acquisition and high resolution with less overlap
  • Faster-scanning improves with earlier generations scanners
• Resolution is for having images stay open and stable with periods of time
• High-resolution scan is designed for extended periods of time
• Dual Source CT Scanners:
  • 2 X-ray tubes and detectors and tubes in a way to achieve more simultaneously

One Tube

• Runs at high energy level that allows for a better differentiation of tissues
• Utilized for cardiac functions such as acquiring imaging with heart
• Fast scanner and high resolution -Dynamic is used for high speed for movements