Medical Physics Review

HearingThe ear

Outer Middle Inner

The functions

• Ear canal: channel sound wave to ear drum

• Eardrum: vibrate in resonance with sound wave to drive the ossicles

• Eustachian tube: equalize pressure on both sides of eardrum

• Ossicles: vibrate the oval window causing fluid in cochlea to vibrate

• Cochlea: vibration of cochlear fluid causes hairs (cilia) to vibrate which creates electrical impulse different hairs resonate with different frequencies transform vibrations of fluid into electrical impulses

• auditory nerves: send electrical impulses to brain to allow sound to be interpreted

The mechanism

• Sound Pressure is multiplied in two ways

• a) Lever action of ossicles – mechanical advantage that turns a small force at tympanic membrane into a larger force at oval window (1.5:1)

• fD = Fd

• b) Different areas – small area of eardrum and smaller area of oval window (20:1)

• AT>AO

• Why middle ear? The impedance of air and liquid is different, most sound wave will be reflected without the middle ear.

Pitch

• A normal human ear can hear sounds in the range 20 Hz up to 20000 Hz (20 kHz)

• The most sensitive frequency is 3000Hz for human ear

Intensity and Loudness

• SOUND INTENSITY

• The sound intensity is the amount of energy that a sound wave brings to a unit area every second. The units of sound intensity are W m-2.

• Intensity (amplitude)2

• LOUDNESS

• Intensity is a measurable quantity whereas loudness is subjective and depends on the listener.

• Different people can describe different intensity sounds as appearing to have the same loudness – the frequency of the sound is an important factor.

Decibel

Hearing tests

• Air conduction / bone conduction

• Hearing loss:

Air conduction

Bone conduction

Result Cause

OK OK Normal Hearing

-

Loss OK Conductive loss

blockages - build up of wax or fluid.accidents - the eardrum can be ruptured or the middle ear could be damaged.diseases - the bones in the middle ear (and the oval window) can be prevented from moving.age - with increasing age, the bones in the middle ear(and the oval window) tend to become solidified

Loss Loss Sensory loss Sensory loss can be caused by ageing or the exposure to excessive noise over periods of time.

Other types of loss

• SELECTIVE FREQUENCY LOSS

• This could lead to loss in speech discrimination.

X-ray

Attenuation and HVT

• If the energy of the beam is absorbed ， then it is said to be attenuated. If there is nothing in the way of an X-ray beam, it will still be attenuated as the beam spreads out. Two processes of attenuation by matter, simple scattering and the photoelectric effect are the dominant ones for low-energy X-rays.

• both attenuation processes result in a near exponential transmission of radiation as shown in the diagram below.

• For a given energy of X-rays and given material there will be a certain thickness that reduces the intensity of the X-ray by 50%. This is known as the half-value thickness.

• Relationship:

X‑ray detection, recordingand display techniques

• The basic principle of X-ray imaging is that some body parts (for example bones) will attenuate the X-ray beam much more than other body parts (for example skin and tissue). Photographic film darkens when a beam of X-rays is shone on them so bones show up as white areas on an X-ray picture.

• Since X-rays cause ionizations, they are dangerous. This means that the intensity used needs to be kept to an absolute minimum. This can be done by introducing something to intensify (to enhance) the image. There are two simple techniques of enhancement:

• When X-rays strike an intensifying screen the energy is re-radiated as visible light. The photographic film can absorb this extra light. The overall effect is to darken the image in the areas where X-rays are still present.

• In an image-intensifier tube ， the X-rays strike a fluorescent screen and produce light. This light causes electrons to be emitted from a photocathode. These electrons are then accelerated towards an anode where they strike another fluorescent screen and give off light to produce an image.

Computed Tomography

• X-ray image of target taken at different angles (many different directions)

• computer produces detailed image of slice (these images are combined using computers to form a two-dimensional image of section)

• images of many sections/slices can be obtained

• combined to build up a 3D image so image can be rotated for viewing from any angle

Comparison

• X-ray imaging

• 1) uses ionizing radiation (X-rays)

• 2) short duration of exposure

• 3) smaller dose of radiation

• 4) two-dimensional shadow image

• CT scanning

• 1) uses ionizing radiation (X-rays)

• 2) long duration of exposure (hard for kids)

• 3) larger dose of radiation

• 4) three-dimensional image

Barium

• All tissues in the abdominal cavity have approximately the same attenuation coefficient so there is little to no contrast on photographic film.

• The attenuation coefficient for barium is greater than for the tissues in the abdominal cavity.

• Barium meal lines the stomach

Ultrasound

Principles

• High frequency sound waves are transmitted from a probe into the patient’s body and are reflected at each boundary between different types of tissue and bone. The same probe both transmits and receives the ultrasound waves. By measuring the time between transmission and reception, the distance to each boundary can be calculated using the speed of sound and thus the location and surface of each organ can be mapped.

Principles (continued)

• Piezoelectric crystal （压电晶体） : a quartz crystal that changes shape when a potential difference is applied across it

• Production: apply an AC voltage to generate a vibration at desired frequency

• Detection: received sound wave causes it to vibrate and generate an AC voltage that can be measured

Impedance

• product of the density of a substance and the speed of sound in that substance

• Z = ρc

• Unit: Kg/(m2s) or Rayl

• The greater the difference of impedance between the two media, the more reflection occurs.

• So gel is used.

Choice on Frequency

Item Higher Frequency

Lower Frequency

Resolution Good Bad

Attenuation Bad Good

Type of scans

• A-scan: amplitude-modulated scan – one-dimensional – not usually used

• B-scan: brightness-modulated scan – two-dimensional – most frequent

Type of scans (continued)

• A-scan: Strength vs. Time graph.

• Depth can be calculated by reflectional time:D=0.5*(t1*v1+t2*v2+t3*v3+...)

Type of scans (continued)

• B-scan uses the signal strength to affect the brightness of a dot on the screen many B-scans are combined to give an image of the internal organs/baby

NMR （核磁共振） / MRI （核磁共振成像）

• NMR: nuclear magnetic resonance

• MRI: magnetic resonance imaging

Principles

• a) Large constant uniform magnetic field causes hydrogen atoms to line up (align their spin axes) – act like tiny magnets

• b) Small non-uniform magnetic field is superimposed on top of larger field – localized magnetic field – weak oscillating field in the form of pulses of radio waves

• c) If frequency of radio waves matches that of the hydrogen atoms (resonance) then the smaller field makes some hydrogen atoms realign

• d) When small non-uniform field is removed, atoms relax back to original alignment

• e) As they relax they emit radio-waves

• f) Time it takes to relax is measured

• g) Frequency of emitted radio waves and relaxation times are processed to produce the NMR image

Radiation in medicine

TermsTerm Symbol Formula Unit

Exposure X (eXposure) X = q/m C/kg

Exposure – ratio of total charge of air ionized by radiation to mass of air.Significance: attempt to measure total amount of ionization produced by radiation – limited application since doesn’t measure ability of body tissue to absorb radiation

Absorbed Dose D (Dose) D = E/m J/kg

Absorbed Dose – total energy absorbed per unit mass of tissueSignificance: difficult to measure directly – attempts to measure amount of radiation tissue absorbsFactors: Type of radiation Intensity of radiation Exposure time

Quality Factor (Relative Biological Effectiveness)

RBE (Relative Biological Effectiveness)Q (Quality)

- - (arbitrary)

Quality Factor (Relative Biological Effectiveness) – for the same absorbed dose, this measures the relative effectiveness of different radiations in destroying cellsSignificance: other radiations measured against X-rays– alpha particles are 10 times more damaging per dose than X rays

Precautions for Different Radiation

• Shielding: lead, leaded glass• Purpose:

• a) material absorbs energy before it reaches worker,

• b) prevents energy from going to other body parts of patient other than target area

• Distance: maximize distance from source – eg. Controls remote from source• Purpose: reduces intensity of energy worker receives

• Time-of-Exposure: minimize time of exposure to radiation• Purpose: reduces intensity of energy worker or patient receives

Precautions for Different Radiation

• Film badge• There is a light-proof packet of photographic film inside the

badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminum, copper, lead-tin alloy and plastic. There is also an open area at the center of the badge.

Balanced Risk

• If the dose is too high, too many healthy cells are killed. If the dose if too low, cancer is not destroyed. The dose needs to be as high as possible in the region of the cancer and as low as possible everywhere else.

• In general all additional exposure needs to be as low as can be reasonably achieved (ALARA) and needs to show a positive overall benefit to the patient.

Physical Half-life, Biological Half-life And Effective Half-life.

• Physical Half-life (TP) (same as T1/2) –

• a. the time taken for ½ the number of radioactive nuclei in sample to decay

• b. the time taken for the activity of a sample to decrease to ½ its initial value

• Biological Half-life (TB) – time taken for half the number of ingested radioactive nuclei in the body to be removed by natural bodily (chemical) processes

• Effective Half-life (TE) – time taken for the number of radioactive nuclei present in the body to halve (combined effort of Tp and Tb)

Basis Of Radiation Therapy For Cancer

• Aim of radiotherapy:

• To target malignant cells in preference to normal healthy cells. Malignant cells are slightly more susceptible to damage from radiation than healthy cells.

• Types of sources:

• 1) A radioactive element

• 2) High energy X-rays, gamma rays or protons from particle accelerators

• Example of therapy technique using X-rays to kill cancer cells in a tumor:

• a) X-rays are much higher intensity than those used to take chest X-rays

• b) Cancer cells are targeted to receive a high dose by irradiating region from different angles with tumor in overlap region

• c) Aim is to minimize danger to other healthy cells while killing cancerous ones since malignant cells are preferentially susceptible to x-rays.

Choice Of Radio‑isotope