medical imaging

• CT scans or computed tomography scans provide a comprehensive 3D representation of the inner structures of the body using X-rays and computer reconstruction.
• 1. rays are produced when charged particles are decelerated, transforming their kinetic energy rapidly into high-frequency photons
• 1. rays used in medical imaging are often referred to as soft X-rays, having energies generally lower than gamma rays
• In X-ray tubes, a vacuum tube is used to prevent electrons from colliding with air molecules before emitting X-rays
• An external power supply creates a potential difference between the cathode and anode, allowing electrons to gain kinetic energy of up to 200keV
• Upon collision, electrons decelerate rapidly, emitting about 1% of their kinetic energy as X-rays, with the rest lost to thermal energy in the anode
• The X-rays emitted in all directions from the anode are directed into a collimator to further collimate the beam
• 1. rays of various energies are produced, with lower energy X-rays contributing to the overall X-ray dose received by the patient
• Modern X-ray systems use digital detectors instead of photographic films for easier processing, storage, and transmission using computers
• Attenuation refers to the gradual decrease in the intensity of X-rays as they pass through matter
• Radiographers aim to reduce patient exposure to harmful X-rays and improve image contrast for clearer visualization of different tissues
• Intensifier screens contain phosphor material that emits visible light when absorbing X-ray photons, reducing patient exposure by a factor of 100 to 500
• Image intensifiers in digital systems convert incoming X-rays into visible light photons, which can be electronically stored and viewed
• Image intensifiers are useful in fluoroscopy, providing real-time images without exposing the patient to dangerous levels of X-rays
• A contrast medium, such as iodine or barium, enhances image contrast by being a good absorber of X-rays, making tissues more visible
• 1. ray imaging is limited as it provides a two-dimensional shadow image, showing structures at different depths superimposed on each other
• Computerised axial tomography (CAT or CT scanner) uses a computer to control scanning motion and data manipulation to produce slice images through the patient
• Many CT exams require patients to hold their breath to eliminate blurring caused by breathing or other motion
• One technique for generating ultrasound waves involves the piezoelectric effect exhibited by crystals like quartz
• Crystals like quartz produce mechanical deformation when a potential difference is applied across them
• Conversely, when the crystal is mechanically caused to expand or contract, a potential difference is produced across it
• The device used to produce ultrasound waves is called an ultrasonic transducer
• When an alternating potential difference is applied to the transducer, periodic changes in stress occur, leading to forced vibrations with the same frequency as the applied potential difference
• The crystal then emits a pulse of ultrasound waves into the medium it is coupled with
• If ultrasound waves hit the crystal face, the crystal becomes deformed, converting some sound energy into electrical energy and producing a potential difference across the crystal
• The potential difference is amplified and displayed on the cathode ray tube
• Clear distinct echoes are obtained by using short pulses, which require vibrations to be damped out quickly by bonding a damping material (epoxy resin) to the back face of the crystal
• The function of the transducer is two-fold: to generate the pulse of ultrasound and then to act as a detector of the returning echoes
• The frequency of ultrasound is crucial because:

  • Higher frequency leads to higher resolution and the ability to distinguish smaller structures
• Ultrasound provides two main pieces of information about the boundary:

  • Depth: the time delay between transmission and receipt of the pulse
  • Nature: amount of transmitted intensity received, which varies depending on the type of tissue
• 1. scan:

  • Simplest type of ultrasound scan
  • A pulse of ultrasound is sent into the body and the reflected 'echoes' are detected and displayed on an oscilloscope or computer screen as a voltage-time graph
  • A pulse generator controls the ultrasound transducer and triggers a pulse of ultrasound that travels into the patient, starting a trace on the screen
  • Each partial reflection of the ultrasound appears as a spike on the screen
  • Information about the depth of reflecting tissues can be obtained from the positions of the spikes along the time axis; their relative amplitudes can indicate the nature of the reflecting surfaces
  • Used for straightforward procedures like measuring the thickness of the eye lens
• Equation to determine the thickness of a material:

  (Speed X Δt )/2

  • Ultrasound is attenuated as it passes into the body, and the energy of the ultrasound is absorbed, so the reflections must be amplified
• Problem:

  In a particular A-scan, the time interval between pulses 1 and 2 is 12 μs.

  The speed of ultrasound in bone is about 4000 m/s. Determine the thickness of the bone
• 1. scan:

  • Detailed image of a cross-section through the patient is built up from many A-scans
  • Ultrasound transducer is moved across the patient's body in the area of interest, with its position and orientation determined by small sensors attached to it
  • Each reflected pulse is analyzed to determine the depth of the reflecting surface
  • A two-dimensional image is built up on a screen by positioning dots to represent the position of the reflecting surfaces, with brightness determined by the intensity of the reflection
• In a B-scan, dots are produced on the screen rather than pulses as in the A-scan. By moving the transducer, a series of dots on the screen traces out the shape of the organ being examined
• Issues with B-scan:

  • It takes several seconds for the scanner to move across the body, leading to problems if the organs of interest are moving, resulting in a blurred image
• Positron Emission Tomography (PET) is used for investigating, diagnosing, and monitoring the treatment of various conditions such as cancers, heart disease, gastrointestinal disorders, brain function, Alzheimer's disease, and other forms of dementia
• PET involves injecting a small amount of tracer, known as a radiotracer, into a vein. This tracer travels through the body, is absorbed by organs and tissues, and the radiation emitted is used to produce images
• PET scanners require a radioactive isotope that decays by β+ emission
• When a positron is emitted from a tracer in the body, it travels a short distance before colliding with an electron. The positron and electron annihilate, converting their mass into pure energy in the form of two gamma rays moving in opposite directions