Untitled Quiz

CT Imaging System Components

• The CT scanner has several components:
  • Imaging system
  • Computer system
  • Image display, recording, storage, and communication system

Gantry

• The gantry is the ring-shaped part of the CT scanner that houses many components necessary to produce and detect X-rays.
• It is mounted in the scanning room and has a rotating scan frame.
• The gantry includes:
  • X-ray tube
  • Filter
  • Slip ring
  • High-voltage generator
  • Data acquisition system (collimator, detectors)

Outside Components

• Display panel: shows the reading of the gantry tilt, table height, and position of landmark.
• Control panel: incorporates several buttons to control the movements of the gantry and table.
• Emergency button: stops every mechanical movement and X-ray emission.
• Positioning lights: red laser lights used for positioning the patient.
• Aperture: the opening through which the patient passes.

Slip Rings

• Early CT scanners used recoiling system cables to rotate the gantry frame, which limited the scan method to the step-and-shoot mode.
• Alternative to cabling system: slip-ring, which uses a brush to provide continuous electrical power and electronic communication across a rotating surface.
• Slip rings serve three functions:
  • Supply of high-voltage electricity to the X-ray tube.
  • Supply of low-voltage electricity to various control mechanisms.
  • Enable the transmission of data from detector to computer system.

X-ray Generator

• CT scanners use high-frequency generators, which are small, compact, and more efficient than conventional generators.
• The generator produces high kV (generally 120-140 kV) to increase the intensity of the beam, which increases the penetrating ability.

Cooling Systems

• Cooling mechanisms are included in the gantry to prevent temperature fluctuations from affecting imaging components.

X-ray Tube

• The X-ray tube is responsible for X-ray production.
• Rotating anode X-ray tubes have become common in CT due to the demand for increased output.
• These tubes produce a heterogeneous X-ray beam.

Filtration

• Filtration serves a dual purpose:
  • Remove long wavelength X-rays that do not play a role in CT image formation but contribute to patient dose.
  • Create a more uniform beam intensity, improving the CT image by reducing artifacts.
• Two commonly used filters are:
  • Flat filter
  • Bowtie filter

Collimation

• Collimation is used to restrict the beam to the slice thickness and reduce scatter radiation.
• There are two types of collimators:
  • Pre-patient collimators
  • Pre-detector collimators

Detector Characteristics

• The detector must exhibit several characteristics essential for CT image production:
  • Efficiency: refers to the ability to capture, absorb, and convert X-ray photons to electrical signals.
  • Stability: refers to the steadiness of detector response.
  • Response time: refers to the speed with which the detector can detect an X-ray event and recover to detect another event.
  • Dynamic range: detectors must be sensitive enough to measure a broad range of X-ray transmission data.

Types of Detectors

• Scintillation detector: converts X-ray to light and then light into electrical signals.
• Gas ionization detector: converts X-ray energy directly to electrical energy.

Detector Characteristics

• Scintillation detectors:
  • Solid-state detectors made of a scintillation crystal and a photomultiplier tube.
  • When X-ray falls onto the crystal, flashes of light or scintillation are produced.
  • Light from the crystal strikes the photocathode of the photomultiplier tube, which then releases electrons.
• Gas detectors:
  • When X-rays fall onto the xenon gas, ionization of the gas results and produces positive and negative ions.
  • Positive ions migrate to negatively charged plate, whereas negative ions are attracted to positively charged plate.
  • This migration of ions causes a small signal current that is related directly to the number of photons absorbed.

Single (SSCT) and Multiple (MSCT) Detector

• The main difference between SSCT and MSCT is the detector arrays.
• SSCT uses detectors that form a one-dimensional array.
• MSCT is divided into several smaller detectors along the z-axis, forming a two-dimensional array.
• The CT scanners currently used in hospitals have at least 64 rows of detectors that provide a very high resolution.