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Topic 24 : Medical Physics
TOPIC 24 MEDICAL PHYSICS.pdfForward Piezoelectric Effect: When an alternating current (AC) electric signal is applied to the piezoelectric material, it causes the material to deform (expand or contract). This is because the electric field from the AC signal causes a shift in the internal charges of the material, resulting in mechanical strain. This is the principle used to create ultrasound waves.
Reverse Piezoelectric Effect: When a mechanical force (such as compression or tension) is applied to the piezoelectric material, it causes the material to generate an electric signal (EMF). The mechanical deformation causes a redistribution of charges within the material, generating an electrical response. This is used in ultrasound to detect the sound waves reflected from tissue or objects.
The process of image generation from the detected sound waves in ultrasound involves several steps:
Sound Wave Emission: The piezoelectric transducer emits high-frequency sound waves (ultrasound) into the body or material being examined.
Wave Reflection: As the ultrasound waves travel through the medium, they encounter different tissues or structures with varying densities. These cause the sound waves to reflect back to the transducer.
Detection of Echoes: The same piezoelectric transducer detects the reflected sound waves (echoes) as they return. The time it takes for the sound waves to travel to and from the tissue gives information about the distance to the reflecting surfaces.
Signal Conversion: The reflected sound waves cause the transducer to vibrate, generating an electric signal (via the reverse piezoelectric effect). The strength and time delay of the echoes are measured.
Image Construction: The electric signals are then processed by a computer, which calculates the depth and location of the reflecting surfaces based on the speed of sound in the medium. These signals are used to create a visual image of the internal structures, with different tissues appearing in varying shades of grey or color depending on the density and composition of the tissue.
Summary:
The image is formed by processing the time delay and intensity of the reflected sound waves, with denser materials reflecting more strongly and at different speeds, allowing for a detailed internal view.
X-rays are produced when high-speed electrons collide with a metal target. Here’s a simplified breakdown:
1. Electron Acceleration:
- Cathode: A filament (similar to a light bulb filament) is heated, releasing electrons through a process called thermionic emission.
- Voltage: A high voltage is applied between the cathode (negative) and the anode (positive). This accelerates the electrons towards the anode at extremely high speeds.
2. Collision and X-ray Production:
- Anode: The accelerated electrons collide with a metal target (usually tungsten) on the anode.
- Two primary mechanisms:
- Bremsstrahlung Radiation:
- Most common.
- Electrons are slowed down or deflected by the strong electric field of the target’s nucleus. This sudden deceleration results in the release of energy in the form of X-rays.
- Characteristic Radiation:
- Less common.
- An incoming electron collides with an inner-shell electron of a target atom, knocking it out.
- An electron from a higher energy level fills the vacancy, releasing energy as an X-ray photon with a specific energy characteristic of the target material.
- Bremsstrahlung Radiation:
3. X-ray Beam:
- The resulting X-rays are emitted from the target in all directions.
- A window in the X-ray tube allows a specific beam of X-rays to pass through and be directed towards the patient or object being imaged.
Key Points:
- Voltage (kVp): Controls the energy (penetrating power) of the X-rays. Higher kVp results in higher energy X-rays.
- Current (mA): Controls the number of electrons emitted from the cathode, thus influencing the intensity of the X-ray beam.
In summary: X-ray production involves accelerating electrons to high speeds and then abruptly stopping them. This interaction results in the release of energy in the form of X-rays, which can be used for medical imaging and other applications.
