CT Scanner Generations

• Unit 1 concerns a CT scan

History of CT Scan

• Godfrey Hounsfield, a British engineer, invented the CT scan
• The first patient scanned was a middle-aged woman with a suspected brain tumor
• The scan time was 30 minutes
• It took 2.5 hours to process the scan on a mainframe computer
• The CT revealed a cystic mass the size of a plum in the left frontal lobe

CT Scan Generations

• 1st Generation: Rotate/Translate, Pencil Beam
• It uses a single X-ray source
• Detectors moved around the patient in 1° increments
• Data is collected through a 180° rotation
• 2nd Generation: Rotate/Translate, Narrow Fan Beam
• It uses a narrow fan beam and a linear detector array
• The gantry rotated 30° between linear movements
• 3rd Generation: Rotate/Rotate Type uses the most modern CT scanners
• It uses a fan beam with multiple small detectors arranged in an arc
• Data is continuously collected through a full 360° rotation
• 4th Generation: Fan Beam
• Its wide fan beam covers the entire patient
• Scan time is less than 2 seconds
• Only the X-ray tube rotates, detectors are static
• Ring artifacts seen in 3rd generation scanners are avoided
• 5th Generation: Electron Beam CT (EBCT)
• It is designed for cardiac imaging
• Produces high-resolution images of moving organs and minimizes motion artifacts
• 6th Generation: Helical/Spiral CT Scanner
• It uses slip-ring technology
• There is continuous rotation of the X-ray tube & detectors
• The table moves continuously through the gantry
• 7th Generation: Multislice CT (MSCT) / Flat Panel CT Scanner
• Multiple detectors are exposed simultaneously
• Thousands of parallel detector bands operate at the same time

CT Scanner Components

• Gantry:
• The aperture size is 70 – 90 cm
• It is tiltable forward/backward (15° – 30°)
• Components include laser light, a microphone, and control panels
• Generator:
• A high-frequency generator provides high voltage to the X-ray tube
• Produces high kV (120-140 kV):
• Increases beam intensity & penetration
• Reduces patient dose
• Reduces heat load by allowing lower mA settings
• X-Ray Tube
• It is composed of a cathode assembly, anode assembly, and rotor
• 99% of electrical energy is converted to heat, 1% to X-ray photons
• Cooling oil between the tube envelope & housing absorbs excess heat
• Detectors:
• Detect and converts X-ray photons into electrical signals
• There are two types:
• Solid-State Detectors (SSD) are used in Multi-Slice CT
• High detection efficiency (~90%)
• High geometric efficiency (~80%)
• it's compact
• Ionization Chamber Detectors are used in Single-Slice CT
• Lower detection efficiency (~50%)
• High stability & sensitivity
• No longer used in modern multi-slice CT scanners
• 640-slice scanner is the latest CT Scan Slice
• Collimators:
• They ensure good image quality by reducing scatter radiation and minimizing unnecessary patient radiation exposure
• Filtration:
• Placed between X-ray source & patient
• Removes low-energy X-rays to reduce patient exposure and a bow-tie filter ensures uniform beam intensity

CT Image Acquisition Process

• Localizer (Scout/Topogram/Surview/Preview/Pilot Scan)
• It is uses to select the Display Field of View (DFOV) & correct image center
• Scans are in Anterior-Posterior (AP) & Lateral (LAT) views
• Methods of Data Acquisition:
• Step-and-Shoot Scanning (Axial Mode)
• The table moves to position, the X-ray tube rotates and then the image is captured
• Used in Single-Detector Row Systems & Multi-Detector CT (MDCT).
• Helical Scanning (Continuous Acquisition Mode)
• X-ray tube continuously rotates while the table moves
• Used for fast & uninterrupted data collection

CT Image Characteristics

• CT images are composed of voxels (3D volume elements), not pixels
• Each voxel has a depth equal to the slice thickness
• The CT number (Hounsfield Unit, HU) represents tissue attenuation
• Hounsfield Scale:
• Water = 0 HU
• Air = -1000 HU
• Bone = +1000 HU
• Windowing Adjustments in CT Imaging:
• Window Level (WL): Controls image brightness
• Window Width (WW): Controls image contrast
• Common CT Windows:
• Bone Window
• Soft Tissue Window
• Lung Window

Factors Affecting CT Image Quality

• Slice Thickness
• Field of View (FOV) → Scan FOV (SFOV) & Display FOV (DFOV)
• Pitch (Table movement per 360° gantry rotation)
• Focal Spot Size
• Patient Motion & Size
• Pixel Size
• Pitch Definition:
• Pitch = Table distance traveled per 360° gantry rotation + Total slice thickness

Introduction to CT Image Quality

• Image quality refers to the visibility of diagnostically important structures in the CT image
• Influenced by multiple technical parameters
• Five basic factors affecting CT image quality:
• Spatial Resolution
• Image Contrast

Spatial Resolution

• Definition:
• The ability of the CT scanner to differentiate two closely placed objects
• Higher resolution produces more detailed images
• Factors Affecting Spatial Resolution:
• Focal Spot Size: Smaller focal spot = Higher resolution
• Detector Width: Narrower width = Better resolution
• Number of Projections: More projections = Finer resolution
• Slice Thickness: Thinner slices = Sharper images
• Kernels: Sharp kernels = Better spatial resolution
• Pitch: Lower pitch = Higher spatial resolution
• Pixel Size: Smaller pixels = More detail
• Field of View (FOV): Smaller FOV = Finer resolution
• Patient Motion: Less motion = Higher spatial resolution
• Spatial Resolution Measurement:
• Measured in line-pairs per millimeter (lp/mm)

Contrast Resolution

• Definition:
• The ability of the CT scanner to differentiate objects with slight differences in density
• Depends on bit-depth of the system:
• 8-bit system = 256 gray values
• 12-bit system = 4096 gray values
• Higher bit-depth = Higher contrast resolution
• Factors Affecting Contrast Resolution:
• Higher mAs = Improves contrast resolution
• Smaller Pixel Size = Decreases contrast resolution
• Thicker Slices = Improves contrast resolution
• Larger FOV = Improves contrast resolution

Temporal Resolution

• Definition:
• The time required to acquire an image
• Important for imaging moving organs (e.g., heart in cardiac CT)
• Factors Affecting Temporal Resolution:
• Fast MDCT scanners = Better temporal resolution
• Better temporal resolution = Fewer motion artifacts
• Example - Cardiac CT:
• 3-second temporal resolution = Image acquired within 3 seconds of the cardiac cycle

Image Noise

• Definition:
• Grainy appearance in CT images
• Higher noise = Lower image quality
• Caused by low photon count in an image
• Measured by:
• Signal-to-Noise Ratio (SNR) = Higher SNR = Less noise
• Causes of Noise:
• Quantum Mottle – Caused by insufficient photons detected
• Factors Affecting CT Noise:
• Smaller Pixel Size = Increases noise
• Higher mAs = Decreases noise
• Larger Patients = Absorb more radiation, reducing SNR

Patient Dose in CT Scanning

• CT scanners generate high radiation doses
• Radiation exposure can alter tissues and produce free radicals, increasing cancer risk
• Factors Affecting CT Dose:
• Higher mAs increases patient dose
• Higher kVp (without decreasing mAs) increases dose
• Higher Image Quality increases dose
• Thinner Slice Thickness increases dose
• Methods to Reduce Patient Dose:
• Reduce tube current (mA)
• Increase table pitch
• Adjust mA settings according to patient weight
• Reduce multiple scan sequences

Effective Radiation Doses in CT Procedures

• Estimated doses for common CT scans:
• CT Abdomen & Pelvis = 10 mSv
• CT Chest = 7 mSv
• CT Angiography = 12 mSv
• CT Head (without contrast) = 2 mSv
• International Commission on Radiological Protection (ICRP) Dose Guidelines:
• Abdominal Region
• CT Abdomen & Pelvis is 7.7 mSv, equivalent to 2.6 years of natural background radiation
• CT Abdomen & Pelvis (with & without contrast) is 15.4 mSv which is the equivalent of 5.1 years
• A CT Colonography is 6 mSv which is two years of natural background radiation
• Intravenous Urography (IVU) is 3 mSv, equivalent to 1 year of natural background radiation
• A Barium Enema gives 6 mSv which is two years of natural background radiation
• Chest Region
• CT Chest is 6.1 mSv, equivalent to 2 years of natural background radiation
• CT Lung Cancer Screening gives 1.5 mSv, which is 6 months of radiation
• Chest X-ray gives 0.1 mSv which is equivalent to 10 days of radiation Brain & Spine
• A CT Brain produces 1.6 mSv with the equivalent of 7 months of radiation
• A CT Brain (with & without contrast) produces radiation of CT 3.2 mSv, the equivalent of 13 months A CT Head & Neck scan produces 1.2 mSv, which is equivalent to 5 months of radiation A CT Spine scan produces 8.8 mSv, the equivalent of 3 years of natural background radiation.
• CT Image Quality is influenced by multiple factors, including spatial resolution, contrast resolution, temporal resolution, noise, and artifacts
• Patient dose should be carefully managed to optimize image quality while minimizing radiation exposure
• Following radiation protection guidelines is essential in clinical practice

Introduction to CT Artifacts

• CT artifacts are distortions or errors in the image that do not correspond to the actual anatomy of the patient
• Artifacts can reduce image quality and affect diagnosis
• Types of CT Artifacts:
• Physics-Based Artifacts
• Patient-Based Artifacts
• Scanner-Based Artifacts

Physics-Based Artifacts

• Beam Hardening Artifacts:
• Caused by low-energy photons being absorbed, leaving behind higher-energy photons, which creates distortions
• Streaks or dark bands between dense structures (e.g., skull base) are the appearance
• Prevention: Use of beam-hardening correction algorithms and proper patient positioning
• Photon Starvation Artifacts:
• Insufficient X-ray photons reach the detector, occurring in high-attenuation areas
• Streaking artifacts in areas like the shoulders or pelvis appear
• Prevention: Increase mA to ensure enough photons reach the detector and/or apply adaptive filtering techniques in modern CT scanners
• Partial Volume Artifacts:
• Different tissue densities are averaged together in a single voxel
• Blurring of tissue boundaries, especially in small objects, will appear
• Prevented by using thinner slice thickness for scanning and reconstruct with high-resolution algorithms
• Scatter Artifacts:
• Caused when X-ray photons are deflected before reaching the detector, leading to image fogging
• Low contrast and hazy images will appear
• Prevented by using anti-scatter grids and software-based scatter correction

Patient-Based Artifacts

• Motion Artifacts:
• Are caused by patient movement during the scan
• Streaking, blurring, or double contours will appear
• They are prevented with immobilization devices, breath-hold coaching for patients, and use of fast scan modes (e.g., spiral/helical CT)
• Metal Artifacts:
• Caused by metallic implants (e.g., dental fillings, pacemakers, prosthetics) which cause extreme attenuation, leading to streaks
• Bright streaks and dark bands appear across the image
• Prevent with metal artifact reduction (MAR) algorithms, SEMAR (Single Energy Metal Artifact Reduction), and dual-energy CT scanning
• Out-of-Field Artifacts:
• Caused when part of the patient's body is outside the scan field of view (FOV)
• Streaking artifacts will appear from structures outside the FOV
• Prevent with ensuring the entire anatomy of interest is within FOV

Scanner-Based Artifacts

• Ring Artifacts:
• Occur due to malfunctioning or miscalibrated detectors
• Concentric rings are centered around the scan axis
• Prevented with detector recalibration and regular maintenance
• Helical Artifacts (Windmill Artifacts):
• Caused by interpolation errors in helical scanning
• Spiral distortions will occur around high-density objects
• Prevented with optimal pitch settings and image reconstruction techniques
• Higher mA settings reduce photon starvation
• Use MAR algorithms for metal implants
• Reduce slice thickness to avoid partial volume effects
• Proper patient positioning and immobilization -Regular scanner calibration & detector maintenance
• CT artifacts can distort images and affect diagnosis
• Understanding and minimizing artifacts is essential for high-quality imaging
• Modern CT scanners use advanced algorithms to reduce artifacts

Functions of CT Scan in Radiotherapy

• CT Simulation for Treatment Planning:
• Allows greater precision in dose distribution, dose optimization, and patient positioning
• 3D dose calculation improves visualization of the tumor and normal tissues
• Radiation dose calculation is optimized to ensure the best dose distribution in the tumor and minimization of radiation to surrounding normal tissues
• CT enables the creation of Digitally Reconstructed Radiographs (DRRs) for patient position verification using a linear accelerator (LINAC)
• Uses of CT in Planning:
• CT is the only 3D imaging method fully accepted for treatment planning
• Most treatment-planning algorithms are developed for CT
• CT provides better geometric fidelity than MRI
• CT has shorter acquisition times compared to MRI or PET
• CT allows real-time organ/tumor motion assessment
• CT enables precise dose calculation due to its ability to identify attenuation characteristics for high-energy photons (X-rays and gamma rays)
• Limitations of CT in Planning:
• Suboptimal soft tissue contrast
• Lack of functional imaging
• Inability to detect microscopic cancer cell clusters outside the gross tumor
• Solution: CT Image Fusion to enhance tumor visualization
• CT Image Fusion:
• Enhances tumor volume definition, reduces dose to organs at risk, and maintains low recurrence rates
• Types of Image Fusion:
• CT + MRI Fusion combines anatomical images with high soft-tissue contrast from MRI
• CT + PET Fusion combines anatomical images from CT with metabolic imaging from PET
• Advantages of Image Fusion in Radiotherapy:
• MRI provides superior soft-tissue contrast to help distinguish between tumors and healthy tissues
• PET identifies metabolically active tumor areas to enable dose escalation to the most aggressive tumor regions
• CT-MRI Image Fusion example includes CT-MRI fusion with delineation of anatomical structures: Falx cerebri (violet line), Cornu anterius ventriculi lateralis (green line), an Astrocytoma Grade II (cyan blue line)
• CT-PET Image Fusion example is that PET-CT enables precise metabolic imaging of tumors
• Verification Using CBCT (Cone Beam CT):
• Serves as an effective tool for Image-Guided Radiotherapy (IGRT) to verify patient position before treatment
• Used for Adaptive Radiotherapy (ART) to track anatomical changes in the tumor, tumor regression during treatment, and changes in normal tissue density (e.g., lung tissue density alterations)
• Advantages of CBCT in Radiotherapy:
• Fast image acquisition & isotropic spatial resolution
• Reduces treatment setup time
• Allows a 50% reduction in Clinical Target Volume (CTV) to Planning Target Volume (PTV) margins which enables higher dose escalation & reduced toxicity
• Provides accurate patient positioning before irradiation. Is integrated into modern linear accelerators (LINAC) for real-time verification
• First Prototype CBCT-Guided LINAC developed by D. Jaffray et al. (2002)
• Varian Trilogy™ is a currently available CBCT system
• Siemens Artiste ™ is a currently available CBCT system
• Elekta Synergy is a currently available CBCT system
• CT plays a vital role in radiotherapy by providing accurate tumor localization, dose calculation, CT image fusion with MRI and PET enhances tumor definition and functional assessment
• CBCT serves as an essential IGRT tool, improving patient positioning and adaptive treatment planning

Gamma Camera Summary

• It's also known as a Scintillation or Anger Camera
• A device used to image gamma radiation-emitting radioisotopes
• Invented by Hal Anger
• Imaging Chain:
  • Patient -> Collimator -> Scintillator -> Photomultiplier Tubes (PMT) -> Computer
• Components:
• Collimator
• Made from lead, consisting of holes in plate. Selects the direction of gamma rays falling on crystal Only rays perpendicular to lead plate surface pass through to the crystal
• Types of Collimators:
  • Parallel-Hole (most common) has multiple holes running parallel to each other
  • Diverging has multiple holes that fan away from the center, providing a minified image in whole-body imaging
  • Pinhole has a single hole with a single aperture, providing a magnified & inverted image with superior spatial resolution but lower sensitivit when imaging small structures
• Converging has multiple holes converge onto a central point, providing a magnified image with improved spatial resolution in small structures
• Scintillator
• Scintillator converts gamma ray photons into visible light photons
• Crystals used: Sodium Iodide with Thallium (NaI:TI)
• Photomultiplier Tubes (PMT):
• Converts light photons to electrical signals
• Amplifies electrons produced by photocathode
• Amplified signal is converted to a digital pulse train using an Analog-to-Digital Converter (ADC)
• Computer (ADC): Processes projection data and converts it into a readable image
• Gantry:
• The mount that holds and moves the gamma camera head
• Capable of precisely moving head weighing 200-300 kg
• Image Quality:
• Inherent Properties Affecting Image Quality:
• Spatial Resolution
• Energy Resolution
• Non-Uniformity
• Distortions
• Factors Affecting Image Quality:
• Patient's Size – Larger patients increase influence of scattered photons
• Organ Localization – Deep-seated organs may be overlapped by other tissues, increasing background registrations
• Patient & Organ Movements – Motion artifacts reduce image clarity
• Detector-to-Patient Distance – Should be as short as possible to reduce resolution loss
• Examination Time & Matrix Size – Must be optimized to reduce noise.
• Image Artifacts:
• Deposition of tracer due to inadvertent spray during injection or urinary contamination
• Swallowed activity in esophagus may cause artifacts that mimic structural lesions
• Photopenic (Photon-Deficient) Defects are caused by attenuation from metallic or dense objects, and may be mistaken for osteolytic lesions or obscure important scan details
• Example Cases of Artifacts:
• Lactating Breast Uptake
• Post-Therapy Whole-Body Tc-99m Scintiscan found in a 27-year-old woman with papillary thyroid carcinoma & nodal metastases, post-total thyroidectomy
• Images showed I-131 uptake in chest due to lactating breast uptake
• Extravasation of Radiopharmaceutical
  • The injection site produces Focal intense activity seen in right antecubital region & right lateral abdominal wall on Tc-99m MDP scan
  • Scatter of photons from forearm to abdominal wall due to injection site is another effect
• The gamma camera is essential in nuclear medicine imaging for detecting gamma radiation from radiotracers
• Collimators, scintillators, PMTs, and computers work together to generate diagnostic images
• Image quality is influenced by spatial resolution, energy resolution, and patient factors
• Artifacts must be recognised and minimised for accurate diagnosis

Questions about the different generations of CT scanners and their technological advancements. Covers the evolution of CT technology, including innovations in data acquisition and detector configurations.