CT Acquisition and Parameters

Projections

  • Definition: A view from the anatomical cross-section along the path of the x-ray beam from only one vantage point is defined as a projection.

  • CT Scan Formation: A CT scan is created from numerous projections in order to provide a comprehensive view of the object.

  • Perspective Requirement: To ensure accurate representation of an object's structure, it is essential to observe it from all angles.

Topogram (Localizer) Scanning

  • Functionality: Provides a single projection and does not create a cross-sectional image.

  • Purpose: Used to position the Field of View (FOV) where cross-sectional CT and PET images will be captured.

  • Operation: Utilizes a stationary x-ray tube paired with a moving table.

Topogram Lengths for the Biograph Systems

Sequence Scanning

  • Description: A cross-sectional imaging method that allows for retrospective reconstruction.

  • Movement: Patient table advances incrementally after each x-ray tube rotation.

  • Rotation Details: The tube and detector assembly rotate 360° in one direction and back for each slice increment.

  • I.S.D: Inter Scan Delay must be noted.

Scanning Methods Details

Standard Sequence Scanning

  • Characteristics: Longer I.S.D. but instant data display.

Dynamic Sequence Scanning

  • Characteristics: Faster acquisition with delayed image display.

Downfalls of Sequence Scanning

  • Issues Identified:

    • Respiratory-induced slice misregistration.

    • Data reconstruction is limited to scanned areas; no retrospective slice overlap is available.

    • Potentially long inter-scan delays.

    • Higher volumes of contrast material may be necessary.

Single Slice vs. Multi-Slice System

  • Single Slice System: Only one slice per rotation can be acquired.

  • Multi-Slice System: Multiple slices can be acquired per rotation.

Spiral (Helical) Scanning

  • Structure: The x-ray beam follows a spiral or “corkscrew” path around the patient.

  • Path Determination: The trajectory of the x-ray beam is based on:

    • Thickness of the x-ray beam, correlated with pre-patient collimation and slice thickness.

    • The speed of the patient table movement.

  • Construction: X-ray tube and detectors rotate around the patient simultaneous with table movement, yielding complete volume data acquisition.

The 4 “C”s of Spiral CT Imaging

  • Continuous rotating tube/detector system.

  • Continuous radiation.

  • Continuous data acquisition.

  • Continuous table feed.

Advantages of Spiral Scanning

  • Benefits:

    • Faster acquisition times.

    • More anatomical coverage with each breath-hold, enhancing detection of small lesions.

    • Reduced contrast material is required.

    • Arbitrary slice positioning is possible, ideal for various post-processing techniques.

Preparation of Spiral Images for Reconstruction

  • Data Characteristics: Raw data collected does not correspond to specific slice positions since it is acquired in a “corkscrew” format. Mathematical operations are essential for data reconstruction, deriving attenuation information from planar single slice designs.

Factors that Affect CT Image Quality

  • Key Parameters:

    • mAs

    • kVp

    • Slice Thickness

    • Pitch

    • Isocentering

mAs (Milliampere-Seconds)

  • Components:

    • mA (tube current): Controls the photon quantity generated by the x-ray tube.

    • s (seconds): Rotation time, denoting the interval needed for a full 360° rotation.

  • Impact on Scan: Affects total scan time and coverage along the Z-axis.

mAs Definition and Effects

  • Definition: mAs is the product of tube current (mA) and rotation time (s). It dictates the amount of radiation the patient is subjected to.

  • Visual: Conceptualize mAs as a rainstorm of photons, where mA is the intensity of the rain, and s is the pace of movement through the downpour.

  • Effects of Increasing mAs:

    • Decreases image noise.

    • Enhances image detail.

  • Clinical Scenarios:

    • For soft tissue diagnostics, higher mAs is required due to closely similar CT values between tissues.

    • For structures like bones or lungs, lower mAs is appropriate since CT values differ significantly.

kVp (Kilovoltage Peak)

  • Definition: Voltage applied to the x-ray anode and cathode, necessary for facilitating tube current flow.

  • Energy Interpretation: Expressed in kilo-electron volts (keV), representing the maximal energy of x-ray photons.

  • Impact on Image: Controls image contrast—higher kVp enhances penetration while lower kVp reduces it.

Slice Thickness

  • Concept: Measure of millimeters of anatomical detail covered by the x-ray beam; affects image properties directly.

  • Trade-offs: Increasing slice thickness provides greater anatomical coverage but may reduce resolution and increase partial volume effects. Slice thickness represents a balance between edge definition and noise levels.

Table Increment (Sequence Mode)

  • Definition: The millimeter advancement of the patient table between successive sequence slices dictates the overlap and gaps.

  • Visualization Examples:

    • Increment equal to slice thickness = no gaps.

    • Increment less than the slice = overlap.

    • Increment greater than slice thickness = gaps.

  • Overlap Benefits: Enhances fine detail visibility but increases radiation dosage.

Pitch

  • Definition: Describes the stretch of x-ray data during a spiral scan.

  • Calculation for Multi-Slice Systems:
    ext{Pitch} = rac{ ext{Table Feed}}{ ext{Rotation} imes ext{Collimator Size} imes ext{# of Collimators}}

  • Changes due to Pitch Adjustment: Increasing pitch beyond 2 can lead to anatomical skip but allows faster scanning, minimizing patient doses and expediting emergency procedures.

Isocentering

  • Definition: Proper positioning to ensure the x-ray beam accurately displays anatomy across all tube positions.

  • Importance of Isocentering: Non-centering can lead to image artifacts, compromising interpretation accuracy.

Review Questions

  1. Scannings Setting: The scanning mode used to set the field of view is:

    • 1. Sequence

    • 2. Spiral

    • 3. Topogram

    • 4. Single slice

  2. Downfalls of Sequence Scanning: Which are downfalls?

    • I. Larger amounts of contrast required.

    • II. Respiratory induced mis-registration.

    • III. No inter-scan delay.

    • 1. I and II

    • 2. II and III

    • 3. I and III

    • 4. I, II and III

  3. Spiral Scanning Characteristics: Which are true?

    • I. Longer acquisition times compared to sequence scanning.

    • II. Tube/detector system rotates around the patient in a cork-screw structure.

    • III. Complete volume of data is acquired.

    • 1. I and II

    • 2. II and III

    • 3. I and III

    • 4. I, II and III

  4. Factors Affecting Photon Exposure: Which parameter affects photon exposure?

    • 1. kVp

    • 2. Pitch

    • 3. mAs

  5. Max Energy of Photons: What is the maximum energy called?

    • 1. mAs

    • 2. Pitch

    • 3. kVp

  6. Reasons to Increase Pitch: Reasons include:

    • I. Greater anatomical coverage in less time.

    • II. Faster scanning for trauma patients.

    • III. Increase patient dose.

    • a. I only

    • b. I and II

    • c. II and III

    • d. I, II and III

  • Final Note: All images are credited to Siemens Training and Development Center as per the transcript.