Cross Sectional Imaging 2 Study Notes

Title: Cross Sectional Imaging 2, Lecture 2
Lecturer: Helen Adamson
Focus: Scanner operation and parameter selection in cross-sectional imaging modalities.

Describe and explain the physical principles that underpin the design and operation of cross-sectional imaging modalities.
Understand the impact of different parameter selection options on imaging outcomes.

Understand protocol selection for imaging.
Understand key imaging parameters and how they affect radiation dose and image quality.
Recognize the importance of the clinical question in protocol selection and determination of dose and image quality.

Factors influencing protocol design:
- Specific clinical needs and patient considerations (e.g., BUDO SIEMENS 00020 SOMATOM Definition AS).

Body Area: Different protocols are assigned based on the region of interest, such as chest, abdomen, neck, or head.
Field of View (FOV): This defines the area of interest, which can encompass whole organs.
Parameters:
- Ensure adequate image quality through appropriate radiation levels; sufficient photons must reach the detectors.
- More parameters exist in CT compared to conventional radiography (e.g., kV, mA) leading to diverse imaging capabilities.
- Timing considerations are critical for “capturing” contrast phases effectively.

Protocols must align with clinical queries (e.g., detecting a pulmonary embolism may necessitate an arterial thorax scan).
Radiation Dose Consideration: Always strive for the lowest possible radiation dose while ensuring effective imaging quality.

The protocol is selected by the radiologist/radiographer who will evaluate patient information and use CRIS/RIS or electronic systems to document the recommended protocol.
Common acronyms used include: NCUA (non-contrast unless abnormal), CAP (chest, abdomen, pelvis).
Each protocol includes parameters for body areas and imaging phases defined by clinical standards.

Upon receiving a new CT scanner, an applications specialist configures baseline protocols and guides fine-tuning based on the model.
Different manufacturers (e.g. Fujifilm, Siemens, Canon, GE, Philips) have unique operational mechanisms; even different models within the same brand may vary.
Collaboration among radiographers, leaders, and medical physicists ensures established protocols consistent across patients.

Successful protocol application can be hindered by inadequate clinical histories or conflicting clinical queries necessitating different protocols.

Four main types of image quality:
1. Spatial resolution
2. Contrast resolution
3. Longitudinal resolution
4. Temporal resolution

Spatial Resolution:
- Refers to the ability to visualize small, distinct objects that are significantly different from their background.
- Measured in line pairs per centimeter (lp/cm).
Contrast Resolution:
- Ability to differentiate small differences in density between objects and their backgrounds.
- Influenced by various imaging parameters.
Longitudinal Resolution:
- Pertains to the z-axis; each pixel represents attenuation within a 3D voxel (x, y, and z axis).
- Ideally, resolution should be isotropic, where it is equal in all three dimensions.
- An important consideration is slice width—the thinner the slice, the more accurate the data. However, thinner slices may result in higher radiation doses.
Temporal Resolution:
- Ability to resolve fast-moving objects, analogous to the shutter speed of a camera.
- Important for certain applications, particularly cardiac CT imaging.

Partial Scanning and ECG Gating:
- ECG gating facilitates scanning during heart rest (diastole) to minimize motion impact on images; can be done retrospectively or prospectively.
- Prospective gating generally results in lower radiation doses but requires a lower heart rate (<65 bpm).

Noise observed in CT images usually appears as grain or mottle, degrading image quality.

Kilovoltage (kV):
- Higher kV results in more penetrating photons, enhancing detail and spatial resolution, but increases radiation dosage.
Milliamperes per second (mAs):
- Defines the total number of photons; insufficient mAs can lead to noisy images.
- Increasing mAs improves contrast resolution but correlates to increased patient radiation exposure.
Pitch & Rotation Time:
- Pitch: Ratio of the distance the table advances per rotation to the slice thickness. A higher pitch results in faster scans and lower radiation doses but could compromise image quality by creating overlaps.
- Illustrated as:
  - $ext{Pitch} = rac{ ext{Table Distance per Rotation}}{ ext{Slice Thickness}}$
  - Higher pitch means larger travel distance per rotation and thus much quicker imaging with somewhat diminished dose.
Slice Thickness:
- The thinner the slice, the more accurate the data, but it also correlates with increased radiation exposure to the patient.
- Small structures necessitate thin slices for quality imaging.
CT Image Matrix:
- The matrix is a 2D grid of pixels used for composing images, usually at a size of 1024x1024 in modern scanners.
- The effective use of pixels dictates image resolution; utilizing the matrix properly is vital for spatial accuracy.
Kernels:
- Algorithms applied post-image acquisition to enhance characteristics like spatial resolution, soft tissue visibility or bone clarity.

Involves modifying parameters such as slice thickness and kernels to optimize viewing and noise management.
Window Level (WL): Set to the density of tissue being primarily focused on (e.g., Hounsfield Unit [HU]).
Window Width (WW): Determines the range of densities considered around the WL for optimal visualization.

Clinical questions directly inform which imaging protocol and parameters are chosen.
Imaging protocols are complex and contain various parameters impacting both image quality and radiation dose.
The understanding and management of resolution types (spatial, contrast, longitudinal, temporal) are essential for effective imaging outcomes.