Computed Tomography (CT) | Medical Science Optional Notes for UPSC PDF Download

Introduction

CT utilizes ionizing radiation, specifically x-rays, in conjunction with an electronic detector array to capture a density pattern and generate an image of a tissue "slice" or "cut." The x-ray beam revolves around the object within the scanner, allowing multiple x-ray projections to pass through it. The X-ray tube and detector bank complete a 360-degree rotation around the patient, acquiring, for instance, 720 projections at 0.5-degree intervals.

X-rays that are not impeded by the body are detected by sensitive x-ray detectors positioned 180 degrees opposite the X-ray tube.

• Areas with higher X-ray attenuation (e.g., bone) exhibit heightened density on the resulting image.
• Conversely, areas with lower X-ray attenuation (e.g., soft tissue) manifest as regions of lower density.

CT-HU Scale

The linear attenuation is expressed in terms of HU (Hounsfield Unit) values, spanning from -1000 to +1000:

• Air (-1000 HU)
• Fat (-120 to -200 HU)
• Water (0 HU)
• Soft tissues (20-40 HU)
• Blood (50-60 HU)
• Bone (800-1000 HU)

CT Variants

• Axial CT vs. Helical (Spiral) CT
  In axial CT, the couch motion is characterized by stepping, whereas helical CT involves the continuous progression of the patient through the scanner beam.
• Multidetector Computed Tomography (MDCT) - Multislice CT (Helical MDCT is the current standard)
  Instead of the linear array of detector elements employed in conventional and helical CT scanners, multidetector computed tomography utilizes a two-dimensional array of detector elements. This configuration enables CT scanners to capture multiple slices or sections simultaneously, significantly enhancing the speed of CT image acquisition.

Cardiac CT and limitations

Principle: Electron beam CT relies on a fixed detector array and radiation source. X-rays are generated by an electron beam continuously sweeping across the target anode ring. Employing electrocardiographic triggering, often accompanied by a brief breath-hold of 10-15 seconds, allows the acquisition of both static and cine images.

Clinical Applications: Preferred Investigation - Provides 100% negative predictive value for excluding coronary artery disease (CAD) in low to intermediate-risk patients.

• Assessment of cardiac masses and thrombus
• Evaluation of the status of bypass grafts and stents
• Calcium scoring and CT angiography

Limitations:

• Involves ionizing radiation
• Requires iodinated contrast (poses challenges for patients with renal insufficiency or contrast allergies)

Image courtesy-Author Jto410 (GNU Free documentation license)