Computed Tomography and Its Application to Nuclear Medicine
Computed Tomography and Its Application to Nuclear Medicine
Chapters 18-19 (Cherry), Chapter 11-12 (Gilmore)
Lecture Schedule
Breakout Sessions
Scheduled Breaks:
1:50
2:50
3:45
Objectives
Explain CT scanner imaging and scanner components
Describe CT quality control methods
Discuss SPECT/CT and PET/CT Hybrid Imaging
Explain additional quality control (QC) methods needed for hybrid scanners
Identify artifacts generated by hybrid scanning
Describe MRI imaging and the use of PET/MR technology
Basics of X-Ray and CT
Production of X-Rays:
X-ray tube involves a filament that releases electrons, which are accelerated toward a tungsten anode.
Electrons undergo ionization and Bremsstrahlung interactions in the anode.
Bremsstrahlung and Characteristic X-Rays:
These x-rays are directed toward the patient.
Operator Control:
Controls for current (mA) and potential (kVp).
X-Ray Tube (Figure 18-2)
Description:
An evacuated glass container housing a cathode (heated tungsten filament) that produces electrons.
Anode containing tungsten target where electrons interact via Bremsstrahlung and ionization interactions to generate x-rays.
Bremsstrahlung interactions yield many low-energy x-rays that do not contribute to imaging; filters are used to absorb these.
Collimators absorb wide-angle x-rays, creating a compact x-ray beam.
X-Ray Energy Spectrum
Effect of kVp on Spectrum:
A higher kVp generates a Bremsstrahlung spectrum that extends to higher energies.
X-Ray Production:
Tube current (mA) determines the number of electrons and thus the quantity of x-rays produced
(Figure 18-3).
CT Gantry (Figure 18-5)
Description:
Houses the x-ray tube and detector arrays opposite each other.
In helix CT, the gantry rotates continuously around a table that moves.
X-Ray Detection
Types of Detectors:
Scintillating or solid-state detectors.
Photodiodes convert x-rays into current signal.
Small detector arrays (0.5 mm dimensions).
Tissue Attenuation:
Different tissues (bone, soft tissue, air) absorb varying amounts of x-rays, resulting in different intensities passing through the object.
Image Reconstruction
Based on the attenuation equation and Radon’s concept of reconstructing 3D objects from multiple 2D projections.
Each small cube of tissue has its unique attenuation value ($ ext{µ}$).
Filtered Back Projection (FBP):
Techniques include using ramp and Shepp-Logan or Hamming filters for reconstruction.
Multidetector Helical CT
Helical Scan Motion:
The table moves through the gantry while the gantry rotates.
One rotation is divided into approximately 1000 views akin to 2D projections in SPECT.
The x-ray beam is cone-shaped to illuminate all detectors constantly.
Pitch in Multidetector Helical CT
Definition:
Ratio of table speed to gantry rotation speed.
Interpretation of Pitch (>1):
Indicates that not all tissues are directly exposed to x-rays.
Pitch Equation:
Interpolation of Helical Acquisitions
Creates a transverse slice from interpolation data of helical passes on either side.
Each point in the helical sinogram contributes based on proximity to slice location (Figure 18-7c).
Multiple slices can be created through interpolation using appropriate weights.
Cited Reference: Bushberg et al., The Essential Physics of Medical Imaging.
CT Image Acquisition
Procedure:
An IV catheter is placed, and the patient consumes two liters of dilute barium oral contrast.
A topogram is acquired to set scan limits for CT acquisition, performed while the x-ray tube is static above the patient.
IV contrast is administered via a high-pressure injector, and CT acquisition occurs as the bolus reaches the target area, typically requiring breath-hold.
CT Image Display
Attenuation Measurement:
Expressed relative to water using Hounsfield units (HU), with a range from -1000 (air) to +1000 (metal). Water is defined as 0.
Window/Level Combinations:
Selection of HU levels to manipulate gray scale display, with equations for windowing where
Image Quality in CT
Image Contrast:
Can visualize attenuation differences as low as 0.5%.
Most affected by mA (number of x-rays).
Spatial Resolution:
Typically around 0.1 mm, mainly determined by detector size and focal spot.
Image Noise:
Dependent on mA, kVp, and pitch.
Radiation Dosimetry
Radiation dose metrics include:
CT Dose Index (CTDI100):
Accumulated dose from multi-rotation scan of dosimetry phantom.
Dose-Length Product (DLP):
Total radiation measure for an acquisition.
Radiation Protection for Technologists:
Safety measures against both scattered radiation and leakage radiation from the x-ray tube.
Contributors to Radiation Dose
Key factors affecting radiation dose include:
Patient size and density
Detector size
Patient dose
Slice thickness
Image matrix size
Display field
Daily Quality Control for CT
Procedures include:
Warm-up of x-ray tube, verification of tube output, and detector response across various kVp and mA settings.
Assessments for tomographic uniformity, CT number accuracy, image noise, and table increment accuracy.
Accuracy evaluations via CT dose index (CTDI), slice localization, gantry alignment, and slice thickness.
Image Quality Factors in CT
Include:
Contrast resolution
Linearity
Noise
Uniformity
Spatial resolution
Temporal resolution
Improving Contrast Resolution
Achieved through:
Smaller field of view
Thicker slices
Smaller matrix size
Higher mAs values
Utilizing low pass filters
Factors Influencing Quality Control
Determined by technologist control:
Selectable scan parameters
Viewing conditions
Beam geometry
Image receptor type
Slice Thickness and Spacing
Thinner slices yield more noise and cover less anatomy but enhance image quality.
Overlapped slices provide the best image quality but increase patient radiation exposure.
Artifacts in CT
Common artifacts include:
Partial volume averaging
Motion artifacts
Metal artifacts
Equipment artifacts
Ring artifacts
Cone beam artifacts
Edge gradient artifacts
Out of field artifacts
Beam hardening artifacts
Streak Artifact:
Influenced by:
mAs and kVp settings
Patient size
Use of CT contrast agents
Hybrid Imaging (Chapter 19, Cherry)
Defining Hybrid/Fusion Imaging:
A process where patients are scanned using CT, MRI, and either SPECT or PET modalities.
Spatial resolutions:
CT: 5-10 mm
MRI: Approximately 1 mm
SPECT: Greater than 10 mm
Dedicated PET: 5-10 mm
Accurate image registration is essential for avoiding artifacts in images combined from different modalities.
Fusion increases accuracy for surgeons and radiologists in image interpretation.
Steps for Image Fusion
Transfer data to a single computer system.
Ensure registration of 3D or tomographic datasets to a common coordinate system (e.g., DICOM compatibility).
Transform each dataset into the shared coordinate system with consistent spatial resolution and alignment along X, Y, and Z axes.
Display and evaluate combined registered images using DICOM-compatible software.
Methods of Image Registration
Visual Method:
The viewer aligns images manually, typically within an error margin of 4 mm or 3 degrees.
Interactive Landmark Method:
At least three landmarks are identified for each modality to line up data sets.
Automated Registration:
Utilizes algorithms to improve accuracy by matching corresponding surfaces across datasets through matrix multiplications for transformation.
SPECT/CT Details
SPECT/CT systems consist of:
Acquisition computer
CT component
Patient bed
Detector heads, including:
Collimator
Scintillator
Photomultiplier tubes and associated electronics
Quality Control for SPECT/CT
Combined QC Procedures for SPECT/CT include:
Water phantom QA
Tube warm-up protocols
Air calibration for fast QA procedures
Water phantom checks for slice thickness, accuracy, and positioning.
Frequency of assessments:
Daily for most tests
Monthly for advanced verification processes
Advantages of Combined PET/CT and PET/MR
Benefits include:
Co-registered functional and anatomical information from PET combined with CT/MR scans.
Reduction in examination time by using available CT/MR data in place of lengthy separate PET transmission images.
This increases accuracy and limits noise propagation in attenuation correction.
Fusion of PET with CT/MRI
PET Technology:
Identifies early biochemical markers of disease, detecting metabolic changes such as increased glucose consumption in tumors even before structural changes manifest.
CT and MR Capabilities:
Reveal structural changes—fusion of PET images with CT and MR enables comprehensive diagnosis.
PET/CT Gantry and Issues
Gantry Design:
Incorporates both PET and CT tomographs, ensuring coregistration.
Acquisition Process:
Generally involves performing a PET scan following a CT scan, with the earlier CT providing an attenuation map for reconstructing the PET data and anatomical localization.
Challenges:
Balancing between diagnostic and low-dose CT scans while converting Hounsfield units to 511-keV attenuation during PET scans.
Addressing misregistration and CT truncation artifacts.
Ensuring proper certification of technologists and physicians for standard operation and interpretation of PET/CT studies.
Image Display in CT and PET
Maximum Intensity Projections (MIP):
Generated from stacked image slices showing the brightest pixel along a defined path, enhancing visualization.
Contrast in Imaging:
CT images offer a gray scale contrast of approximately 4000, whereas nuclear images provide a contrast scale of just a few hundred.
Allow for generation of transaxial slices that can be repositioned to create coronal and sagittal views.
Misregistration and Interleaved Sampling
Misregistration Causes:
Incorrect reconstruction arises when CT and PET images misalign due to patient movement, predominantly respiratory.
Artifacts may also occur from differences in CT-based attenuation correction and the handling of overlapping data sets.
Artifacts Caused by CT Attenuation Correction
Artifacts may arise due to:
Resolution mismatch when combining datasets from different modalities leading to tissue boundary issues.
Patient motion between CT and SPECT or PET scans.
High-Z materials' presence and use of iodinated IV contrast agents can exacerbate these artifacts.
Understanding PET and CT Scans
Distinctions between imaging modalities include:
PET scans reflect cellular activity, while CT scans show structural details in organs and bones.
The combination of PET/CT offers precise localization of increased cellular activity in the body.
Magnetic Resonance Imaging (MRI)
Concept of Magnetic Resonance:
Involves transferring electrical energy to atoms with unpaired protons under a magnetic field.
MRI employs hydrogen atoms and captures the signals emitted as they return to equilibrium post resonance interaction.
Net Magnetization in MRI
Magnetic Susceptibility:
Hydrogen protons align with or against a strong magnetic field, generating a net magnetic moment pointing in the direction of the field.
Precession of these protons results in a magnetic moment primarily aligned in the Z-axis.
Axes and Vectors in MRI (Figure 19-2)
Expressing Magnetization:
Net magnetization is illustrated by vectors, where MZ represents longitudinal magnetization, and MXY represents transverse magnetic vector.
Application of a second magnetic field causes the vector to move into the transverse plane.
Precession and the Larmor Equation
H protons precess in phase when exposed to an RF pulse at their Larmor or resonance frequency.
The frequency of precession is proportional to the magnetic field strength, aiding in creating a magnetic moment in the transverse plane.
Tissue Disturbance and Relaxation
Upon removal of the RF pulse, hydrogen atoms relax to random precession (relaxation process).
Relaxation components include:
T1 Relaxation: Growth of MZ vector
T2 Relaxation: Loss of MXY vector
The MRI tomograph measures the changing magnetic moment in the transverse plane as the free induction decay signal.
The Pulse Sequence
A series of RF pulses combined with magnetic field gradients facilitate the collection of signals that reflect differing relaxation properties.
Spin Echo Sequence:
An acquisition technique using 90° and 180° RF pulses, generating detectable signals as an 'echo' of the initial magnetic responses.
Forming the Magnetic Resonance Image
Susceptible MR signals are encoded by applying magnetic gradients reflecting their 3D body location.
Gradient Types include:
Slice Selection Gradient: In the Z direction selects a specific transverse slice.
Frequency Encoding Gradient: Determines resonance frequency per slice column in the X direction.
Phase Encoding Gradient: Creates phase shifts across different tissue rows along the Y direction, altering the gradient with each desired pixel row.
Data Storage and Image Reconstruction
Data undergoes collection in a k-space map; dimensions can be specified and may vary.
Each slice corresponds to its own k-space matrix:
One axis for frequency information and another for phase information, reconstructed using Fourier transformation.
MR Instrumentation
Key components include:
Superconducting magnet (requires liquid helium cooling),
Gradient coils constructed from loops of current-carrying wire,
RF coils for pulse transmission/receipt,
Computers for master control, pulse programming, and processing.
Tissue Characteristics in MRI
Adjustments made to repetition and echo times (TR and TE):
For T1-weighted images (short TE, short TR): Fat appears bright.
For T2-weighted images (long TE, long TR): Water/CSF appears bright, with fat remaining dark.
Proton Density-Weighted Images:
Achieved with long TR and short TE, showing hydrogen atom density with less contrast than T1 or T2 images.
Safety Issues in MRI
Potential Hazards:
Magnetic fields pose risks of localized heating and nerve stimulation.
Screening for metal implants is imperative to avoid accidents.
Objects susceptible to magnetic fields can become projectiles.
Noise from gradient coils can provoke discomfort among patients.
Use of Gadolinium:
Administered for contrast with a safety profile akin to CT contrast agents.
PET/MRI Technologies
Commercial Approaches:
Systems like GE Healthcare offer PET/CT and MRI tomographs connected via table tracks.
Alternative companies (Philips, Siemens) develop integrated systems optimizing PET-MRI synergy.
Technical Challenges:
PMTs need placement outside the MR tomograph or using fiber optics/avalanche photodiodes to protect against interference.
Ensure PET tomographs do not disturb MR signal for accurate imaging.
Clinical Benefits:
MRI's superior tissue contrast enriches anatomical correlation and clinical effectiveness.
True simultaneous imaging through combined modalities allows advanced evaluations for diagnostics.
Summary of MRI Techniques
MRI relies on the magnetic properties of protons in hydrogen, aligning protons in a magnetic field, and modifying that alignment with RF pulses.
Signals correspond to protons’ return to equilibrium and are measured to create MR images.
The integration into three-dimensional imaging harnesses precise control and technical nuances reflected in instrumentation, safety considerations, and procedural steps.