In X-rays, kinetic energy of fast moving electrons = Energy of x-ray photons.
So in the x-ray tube, x-ray photons of a range of energies (f) are produced.
Energy gained by electron = Maximum energy of x-ray photon moving through a potential difference (V).
For patients safety, low frequencies are filtered out using Aluminium.
Decrease in intensity of (parallel) x-ray beams = Attenuation. Attenuation cannot be calculated for non-parallel beams as intensity is not constant.
X-rays are ionising. To keep dosage minimum:
Intensifier screens (phosphor sheets)
Image intensifiers (digital system)
For tissues - to improve low frequency absorption --> Use contrast medium.
CAT (Computerised Axial Tomography)
Images of slices through the patient --> Computer --> gathers data --> produces 3D image.
* Can distinguish tissues with similar attenuation coefficient.
Disadvantage: Exposure to radiations.
ULTRASOUND: Echo sounding technique. Waves are partially reflected at boundaries between tissues. Reflected waves used to construct image. High intensity (frequency) waves give more details.
Advantage: No ionisation/harmful side-effects.
To calculate intensity of reflected wave, we use Acoustic Impedence Z = density x speed of sound.
Ir/Io = [(z2 - z1) / (z2 + z1)] ^2
If the difference between Z1 and Z2 is significant, Ir/Io will increase; at the air-skin boundary, this value is 99.95%. To overcome this, the transducer is coupled to the skin using a gel whose impedence matches the skins impedence.
Optimum thickness of 1/2 x lambda of ultrasound wave piezo-electric crystal.
A-Scan: Same as ultrasound. Reflected waves are displayed on oscilloscope. Thickness of bone = distance / 2.
B-Scan: Multiple A-Scans to produce dots on screen = 2D image.
PET Scanning: Radioactive substances are attached to a natural chemical called fluorodeoxyglucose. When injected into the patient's body as a radiotracer, it emits a positron. This positron combines with an electron in the tissue, leading to annihilation and producing two gamma rays that move apart in opposite directions.
The formation of gamma ray photons is plotted and displayed on screen. This helps detect cancer cells (they absorb glucose at a higher rate).
Q. Explain how X-rays are produced?
Electrons are emitted from a heated cathode filament. A very high potential difference exists between the cathode and the anode. This potential difference accelerates the electrons to high speeds. When the electrons strike the anode, they are suddenly decelerated, causing their kinetic energy to be converted into 1% x-rays and 99% heat.
* The emitted X-ray has energy (E = fh). A greater potential difference means a higher frequency means greater penetration of the beam. Therefore, greater hardness of the beam.* To increase the number of electrons emitted, increase the current in the cathode filament. More current means more heat produced. More electrons are emitted.
PET Scan
A tracer is a substance introduced into the body. It is absorbed by the tissue being studied.
Process:
- Tracer emits positrons.
- Positron interacts with an electron in tissue.
- When a positron and an electron meet, they annihilate each other.
- Before annihilation, the total momentum of the positron and electron pair is extremely small. (They have negligible mass, and they are almost at rest relative to each other when they collide.) Total momentum before collision ~ 0.
- On annihilation, 2 gamma photons are produced. Each gamma photon has momentum. So, to conserve momentum, the 2 photons must travel in opposite directions - exactly 180 degrees apart. Then the total momentum after the collision is equal to 0.
- The PET scanner calculates that the annihilation event occurred along the line joining the 2 detectors of gamma photons that went 180 degrees apart.
Q. Explain how x-rays are produced for use in medical diagnosis.
Electrons are accelerated by an applied potential difference. Electrons hit target. X-rays are produced when electrons are decelerated.
Q. State why x-ray images are taken of multiple sections of the body during computed tomography (CT) scanning.
Images of multiple sections are combined to create a 3D image.
Q. State what is meant by the contrast of an x-ray image.
Difference in degrees of blackening between structures.
Q. Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.
- When a potential difference is applied to a piezoelectric crystal, it generates ultrasound pulses with frequencies ranging from 1 to 15 MHz.
- A gel (or coupling medium) is used to fill the air gap between the probe (the piezoelectric crystal) and the skin. This is important because air would reflect nearly 100% of the ultrasound; the acoustic impedance of air is significantly lower than that of skin. Acoustic impedance (Z) is defined as the product of the material's density and the speed of sound within that material. The greater the difference in acoustic impedance values between two media, the less transmission occurs, resulting in almost all ultrasound being reflected.
- The ultrasound pulse then travels into the internal tissues, such as muscles. It reflects back at the boundaries of different media due to variations in acoustic impedance.
- The reflected pulse is received by the ultrasound transmitter, which displays and processes the signal.
- The intensity of the reflected pulse provides information about the characteristics of the boundaries.
- The time delay between sending the pulse and receiving the echo gives information about the depth of these boundaries.
Q. State what is meant by hardness of an x-ray beam.
Penetration of beam. Greater hardness = higher photon energy = higher frequency = greater penetration of beam.
Q. State how the hardness of an x-ray beam from an x-ray tube is increased.
Greater accelerating potential difference.
Q. Outline briefly the principles of CT scanning.
In CT scanning, x-ray image of one section is taken. Similarly images of the section are taken from different angles. One section's image gives a 2D image. It is repeated for many slices to build up a 3D image of the whole body or structure.
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