Medical PhysicsBrain

The ear and hearingDescribe the basic structure of the human ear. The

structure should be limited to those featuresaffecting the physical operation of the ear.

State and explain how sound pressure variations in air are changed intolarger pressure variations in the cochlear fluid.

This can be dealt with in terms of the different areas of the eardrum and oval window, together with the lever action of the ossicles. Although the concept of impedance matching is not formally required, students should appreciate that, withouta mechanism for pressure transformation between media of different densities (air and fluid), most sound would be reflected, rather than transmitted into the cochlear fluid.

The eardrum is some fifteen times larger than the oval window of the inner ear, giving an amplification of about fifteen compared to a case where the sound pressure interacted with the oval window alone.

The ossicles can be thought of as a compound lever which achieves a multiplication of force. This lever action is thought to achieve an amplification by a factor of about three under optimum conditions, but can be adjusted by muscle action to actually attenuate the sound signal for protection against loud sounds.

With a long enough lever, you can lift a big rock with a small applied force on the other end of the lever. The amplification of force can be changed by shifting the pivot point.

State the range of audible frequencies experienced by a person with normal hearing.

i.e. 20Hz to 20kHz

State and explain that a change in observed loudness is the response of the ear to a change in intensity.

Source IntensityIntensity

Level# of Times

Greater Than TOH

Threshold of Hearing (TOH)

1*10-12 W/m2 0 dB 100

Rustling Leaves 1*10-11 W/m2 10 dB 101

Whisper 1*10-10 W/m2 20 dB 102

Normal Conversation 1*10-6 W/m2 60 dB 106

Busy Street Traffic 1*10-5 W/m2 70 dB 107

Vacuum Cleaner 1*10-4 W/m2 80 dB 108

Large Orchestra 6.3*10-3 W/m2 98 dB 109.8

Ipod at Maximum Level

1*10-2 W/m2 100 dB 1010

Front Rows of Rock Concert

1*10-1 W/m2 110 dB 1011

Threshold of Pain 1*101 W/m2 130 dB 1013

Military Jet Takeoff 1*102 W/m2 140 dB 1014

Instant Perforation of Eardrum

1*104 W/m2 160 dB 1016

State and explain that there is a logarithmic response of the ear to intensity.

Intensity level (dB)

1.Find the ratio of the intensities.

2.Express this as a power of 10.

3. Multiply this by 10 to give the intensity level in dB.

Intensity is the power of the sound wave reaching the eardrum. It is measured in Wm-2.

Define intensity and also intensity level (IL).

Intensity is the power of the sound wave reaching the eardrum. It is measured in Wm-2.

We define Loudness ≡ Intensity Level L which is proportional to the {base-10}logarithm of the intensity.i.e. it is defined by the formula in words.

N.B. to add 2 sounds, you must add the intensities, not the dB.

Find the resulting intensity level when a 70dB and an 80db sound are added.

1. A mosquito's buzz is often rated with a decibel rating of 40 dB. Normal conversation is often rated at 60 dB. How many times more intense is normal conversation compared to a mosquito's buzz?

2. The table at the right represents the decibel level for several sound sources. Use the table to make comparisons of the intensities of the following sounds.How many times more intense is the front row of a Smashin' Pumpkins concert than ...a. ... the 15th row of the same concert? b. ... the average factory?c. ... normal speech?d. ... the library after school?e. ... the sound which most humans can just barely hear?

3. On a good night, the front row of a concert would result in a 120 dB sound level. An IPod produces 100 dB. How many IPods would be needed to produce the same intensity as the front row of the concert?

1. Answer: C. 100 timesNormal conversation is 20 dB more intense. This 20 db difference corresponds to a 2-Bel difference. This difference is equivalent to a sound which is 102 more intense. Always raise 10 to a power which is equivalent to the difference in "Bels."

a. 10 X more intense b. 102 X more intensec. 105 X more intensed. 107 X more intensee. 1011 X more intense

2

3. Answer: 100 IPodsSince 120 db is 102 times or 100 times more intense than 100 dB. It is necessary to wear 100 IPods to produce the same sound level.

IB question

Answer

State the approximate magnitude of the intensity level at which discomfort is experienced by a person withnormal hearing.

Describe the effects on hearing of short-term and long-term exposure to noise.

How do we diagnose hearing problems?

Air Conduction assesses the function of both the conduction (outer and middle ear) and sensorineural (cochlea and auditory nerve) components of the ear.

Bone Conduction (BC) assesses the function of the cochlea and auditory nerve only.

Analyse and give a simple interpretation of graphs where IL is plotted against the logarithm of

frequency for normal and for defective hearing.

Hyperlink

Conductive loss happens when there is a problem conducting sound waves through the outer ear, eardrum or middle ear (ossicles). This can be corrrected by surgery or a hearing aid. This is tested with an air conduction test.

Sensorineural loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear to the brain. Sensorineural hearing loss cannot be medically or surgically corrected. It is a permanent loss. This is diagnosed with a bone conduction test.

Conductive lossSensorineural loss

X = left ear air conduction

O = right ear air conduction

] - left, bone conduction [ - right, bone conduction

air conduction test

bone conduction test

If there is a problem in the external or middle ear (conductive hearing

loss) the AC threshold will be less than the BC threshold because the person will hear better by bone than air conduction

Conductive hearing loss

Sensorineural hearing loss If there is a problem with the cochlea or the auditory nerve, the AC and

BC thresholds will be the same (See diagram below).

Mixed hearing loss Mixed hearing loss is a reduction in hearing of both AC and BC, but

they are not the same. (See diagram below).

The bone conduction test is normal, but there are losses in the air conduction test. Therefore the problem is in the outer/middle ear, not the cochlea.

How do you diagnose a middle ear problem?

How do you diagnose a inner ear problem?

How do you diagnose a mixed problem?

You should consider which test to use and how the audiograms would look.

Tsokos

Page 698 Q’s 1-9.

Medical imaging

X-ray scan

Define the terms attenuationcoefficient and half-value thickness.

The intensity I is the amount of energy per unit area in the beam.

X is the distance travelled through the material.

The attenuation coefficient describes the extent to which the intensity of an energy beam is reduced as it passes through a specific material.

When used in the context of X-rays or Gamma-rays, it is represented using the symbol μ, and measured in cm-1.

Lead has a high attenuation coefficient.

The half-value thickness is the thickness of the material that reduces the intensity of the X-rays by 50%

Derive the relation betweenattenuation coefficient and half-value thickness.

Taking logs

As the intensity drops to 50%

Staring with

Solve problems using the equationI = I0 e− μx .

Describe X‑ray detection, recordingand display techniques.

Students should be aware of photographic film, enhancement, electronic detection and display.

Photographic plates are sensitive (the efficiency of X-Ray film to absorb x-ray photons is only » 1%) to X-rays, they provide a means of recording the image, but require a lot of exposure (to the patient), so intensifying screens were devised (scintilator and photomultiplier tube). They allow a lower dose to the patient, because the screens take the X-ray information and intensify it so that it can be recorded on film positioned next to the intensifying screen.

ScintillatorsSome materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier tube. These detectors are called “scintilators", filmscreens or “scintilation counters". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.

Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier tube. These detectors are called “scintilators", filmscreens or “scintilation counters". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.

Scintillators

Photostimulable phosphors (PSPs) (Electronic detection)

An common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. After the plate is x-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography (also referred to as digital radiography). The PSP plate can be reused, and existing X-ray equipment requires no modification to use them.

Image plates have the following advantages.1. dynamic range larger than 5 orders of magnitude in x-ray dose, 2. lower limit of useful dose compared to the x-ray film, 3. reusability, 4. no wet chemical processing, 5. images are available in digital form, 6. image processing and pattern recognition possible, and 7. simple data storage on optical or digital media.

Disadvantages of image plates compared to the conventional x-ray film are 1. a poorer spatial resolution due to light scattering at the storage phosphor

grains during the readout process.

How X Rays WorkHow X Rays Work

X Rays (continued)X Rays (continued)

X-rays: x-radiography

How do we obtain a clear x-ray image?

X rays are scattered or absorbed as they pass through the body.

If the energy (tube voltage) of the x rays is too high, then all the x – rays get through (penetration) and there is no contrast on the image.

If the x-ray energy is too low, then no x-rays reach the x-ray plate.

The choice of tube voltage (energy) depends upon the type of tissue and the thickness of the tissue.

X ray contrast• Structures in the body like bones are very dense and contain elements

such as calcium that have a high atomic number. This makes bone absorb a high proportion of the x-rays. Soft tissues like fat and muscle allow more x-rays to pass though. The body casts an x-ray shadow onto the film. Where the x-rays have passed though bone, the film is less exposed so it looks white; where they have not passed though anything the film is exposed and turns black; and where the x-rays have passed through soft tissues the film has different levels of grey.  

• In order to make some parts of the body show up better, contrast media with a high atomic number can be used. This can be a 'barium meal', where the patient drinks a liquid containing barium (atomic number 56) which makes the digestive tract show up clearly on x-rays, or the patient can have an injection of iodine (atomic number 53) which makes the blood vessels stand out (this is called angiography).

Explain standard X‑ray imagingtechniques used in medicine.

Students should appreciate the causes of loss of sharpness and of contrast in X‑ray imaging. They should be familiar with techniques for improvingsharpness and contrast.

The general relationship between scattering angle and size of the object is,

Angle = (object size)-1

This will cause the edge of

objects to lose sharpness

X-ray SharpnessA quantitative measure of the loss of edge detail which is due to geometric properties of the object and imaging system and not due to image noise or X-ray scatter. It is usually expressed as the width of the band of changing density or brightness arising from a sudden change in the intensity of the radiation incident on the film or fluorescent screen. From this definition it can be understood that unsharpness and resolution are different concepts. It is possible for an edge to be "spread" by one of many factors, and at the same time for two such edges to be resolved in the image. The factors which contribute to the total image unsharpness include geometric unsharpness, movement unsharpness, absorption unsharpness, image receptor unsharpness, and parallax unsharpness. The various unsharpness factors all contribute to the observed unsharpness of structures in an image. However, the quantitative manner in which the factors combine is in general complicated and is not completely understood. It is known from observation that the total unsharpness is not the direct sum of the contributing factors. In general, it appears that the total image unsharpness is dominated by the unsharpness of the largest individual factor.

X-ray Sharpness

• Unsharpness is introduced by

1.A wide source (parallax)

2.A moving patient

IB Questions

Questions 7,11, 19b,c from review pack.

(b) An X-ray beam that consists of photons of the same energy is used to image a possible bone fracture in the leg of a patient. At this photon energyattenuation coefficient of bone = 0.62 cm–1

attenuation coefficient of tissue = 0.12 cm–1.In passing through the leg, the X-rays effectively encounter a thickness of tissue equal to 14 cm and thickness of bone equal to 8.0 cm.Use the above data to explain why X-rays of this energy are suitable for imaging a possible leg fracture.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [4]

(b) intensity after passing through bone = I0e-0.62x8.0

= 7x10-3I0

intensity after passing through tissue =0.19I0

reduction by bone much greater than by tissue so good contrast between bone and tissue;

Outline the principles of computedtomography (CT).

A CT scan (sometimes called computed axial tomography, or a CAT scan) uses x-rays. In a CT scan the patient lies on a table and is moved though a doughnut-shaped machine. It creates images that are slices through the patient. It does this by moving the x-ray tube and detector in a circle taking x-ray images of the slice from all angles around the body. A computer then processes these images to produce a cross sectional image (a picture of a slice through the body). CT scans are useful as they can show a range of very different tissue types clearly: lung tissue, bone, soft tissue and blood vessels.

CT Scan

X-rays: Computed tomography image (CT scan)

Secondmetatarsal bone

(the bone thatDavid Beckham and

Wayne Rooney broke!)

X-rays: Computed tomography image (CT scan)

Ultrasound

Ultrasound scan

Ultrasound

Ultrasound imaging: What does it look like?

Ultrasound imaging: development of a pregnancy

8 weeks gestation (out of a 40 week pregnancy)

18 weeks

24 weeks

Describe the principles of the generation and the detection of ultrasound using piezoelectric

crystals.

When the ultrasound beam is generated, an electrical signal is sent to the crystal, and converted to mechanical vibrations.

When the reflected signal is received, the mechanical vibrations are converted back to electrical signals and sent to the computer for processing into an image.

Naturally-occurring crystals

Cane sugar, quartz, topaz, bone, enamel, dentine and tendons.

Define acoustic impedance (Z) as the product of the density (ρ) of a substance and the speed of

sound (c) in that substance.

Z = ρc

The acoustic impedance of the eardrum, for instance, corresponds well with that of the auditory canal, guaranteeing maximum efficiency of energy transfer, but it does not correspond well with air. The pinna may be described as an impedance matching device between the air and the auditory canal. Likewise, the flared end of a trumpet results in less energy being lost by being reflected back down the tube of the instrument.

Solve problems involving acousticimpedance.

Students should understand the use of a gel on the surface of the skin.

Z = ρc

Gel is generally necessary because the acoustic impedance mismatch between air and the body is large. Without gel, nearly all of the energy is reflected and very little is transmitted into the body.

Z2

Z1

Outline the differences betweenA‑scans and B‑scans.

An A-scan, is routine type of diagnostic test used in ophthalmology. The A-scan provides data on the length of the eye. (i.e. is 1 dimensional)

A B-scan, is a diagnostic test used in ophthalmology to produce a two-dimensional, cross-sectional view of the eye.

B-scan A-scan

Ultrasound imaging: A-scan How does it work?

• An ultrasound element acts like a bat.

• Emit ultrasound and detect echoes

• Map out boundary of object

Ultrasound imaging: B-scan How does it work?

• Now put many elements together to make a probe and create an image

A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen. M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.

How do the scans work?

Identify factors that affect the choice of diagnostic frequency.

Students should appreciate that attenuation and resolution are dependent on frequency.

Low frequencies are less absorbed (attenuated), therefore can penetrate deeply. But have lower resolution.

High frequency gives you high resolution but high attenuation i.e. limited depth penetration.

Tsokos

Page 710 Q’s 1-8.

IB Q 2 from the review pack.

NMR and lasers

Hyperlink to video

MRI Scan

Hyperlink

Outline the basic principles of nuclearmagnetic resonance (NMR) imaging.

Students need only give a simple qualitative description of the principle, including the use of a non-uniform magnetic field in conjunction with thelarge uniform field.

Magnetic Field When a person is lying in the magnetic field of the MRI scanner the nuclei of the hydrogen atoms in their body line up, like compass needles in the Earth's magnetic field, either pointing in the direction of the field or opposite to it. The hydrogen nuclei (protons) don’t stay still though, but move like a spinning top around the direction of the magnetic field.

How MRI works

1.The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field.

2. A second radio frequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy. When this field is turned off the protons release this energy at a radio frequency which can be detected by the scanner.

3. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.

Proton nuclear magnetic resonance (NMR) detects the presence of hydrogen nuclei (protons) by subjecting them to a large magnetic field to align the nuclear spins, then exciting the spins with properly tuned radio frequency (RF) radiation, and then detecting weak radio frequency radiation from them as they “relax" from this magnetic interaction. The frequency of this proton "signal" is proportional to the magnetic field to which they are subjected during this relaxation process. In the medical application known as Magnetic Resonance Imaging (MRI), an image of a cross-section of tissue can be made by producing a well-calibrated magnetic field gradient across the tissue so that a certain value of magnetic field can be associated with a given location in the tissue. Since the proton signal frequency is proportional to that magnetic field, a given proton signal frequency can be assigned to a location in the tissue. This provides the information to map the tissue in terms of the protons present there. Since the proton density varies with the type of tissue, a certain amount of contrast is achieved to image the organs and other tissue variations in the subject tissue.

Since the MRI uses proton NMR, it images the concentration of protons. Many of those protons are the protons in water, so MRI is particularly well suited for the imaging of soft tissue,

It is estimated that about 80% of the body's atoms are hydrogen atoms, so most parts of the body have an abundance of sources for the hydrogen NMR signals which make up the magnetic resonance image.

Describe examples of the use of lasers in clinical diagnosis and therapy.

Applications such as the use in pulse oximetry and in endoscopes should be discussed. Students should be familiar with the use of a laser as a scalpeland as a coagulator.

Infrared: Pulse oximetry

Heart rate:81 bpm

Blood oxygenation: 99%

Pulse oximetry is a simple non-invasive method of monitoring the percentage of haemoglobin (Hb) which is saturated with oxygen. The pulse oximeter consists of a probe attached to the patient's finger or ear lobe which is linked to a computerised unit.

The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated haemoglobin. Oxygenated haemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) haemoglobin absorbs more red light and allows more infrared light to pass through.

Pulse oximetry

Visible: Endoscopy

Visible: Endoscopy

Visible: Endoscopy

ParasiticWorm!

The stomach wall hasrelapsed back

into the oesophagus.This is a hernia.

This is the endoscope comingout of the oesophagus

Radiation in medicine

State the meanings of the terms exposure, absorbed dose, quality factor (relative

biological effectiveness) and dose equivalent as used in radiation dosimetry.

Students should be able to discuss the significance of these quantities in radiation dosimetry.

Exposure

Intensity of Radiation (Exposure)The roentgen (R) is a measure of radiation intensity of x-rays or gamma rays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It is one of the standard units for radiation dosimetry, but is not applicable to alpha, beta, or other particle emission and does not accurately predict the tissue effects of gamma rays of extremely high energies. The roentgen has mainly been used for calibration of x-ray machines.

X = Q/mCoulombs per Kg

Absorbed dose (J/kg)The charge deposited per Kg of air does not take into account the type of tissue and therefore the energy deposition.

The absorbed dose is equal to the energy deposited per unit mass of the medium, and so has the unit J/kg, which is given the special name Gray (Gy).

Note that the absorbed dose is not a good indicator of the likely biological effect.

The dose equivalent is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionising radiation. Equivalent dose is therefore a less fundamental quantity than radiation absorbed dose, but is more biologically significant. Equivalent dose has units of sieverts.

    Radiation type Quality factor

       X -rays            1

      gamma            1

       beta            1

       alpha          20

    slow neutrons             5

     fast neutrons           10

        protons              5

Discuss the precautions taken in situations involving different types of radiation.

Students should consider shielding, distance and time-of-exposure factors. They should be familiar with the film badge.

TOK: They should appreciate that current practice is determined from a gradual increase in available data.

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 aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.

Alpha Particles (2n, 2p)

Beta Particles (e-or+)

Photons (hv)

(x or gamma rays)

Paper Concrete

Penetrative power

Minimizing Radiation Minimizing Radiation ExposureExposure

Basic ConceptsBasic Concepts

• TimeTime

• DistanceDistance

• ShieldingShielding

Minimizing Exposure - TimeMinimizing Exposure - Time

• Minimize the Minimize the amount of time amount of time spent near spent near sources of sources of radiation.radiation.

Minimize Exposure by Minimize Exposure by Maximizing DistanceMaximizing Distance

As the distance from a radioactive source doubles, the exposure rate

decreases by a factor of four.

Minimizing Exposure By Minimizing Exposure By Using ShieldingUsing Shielding

Lead shielding Lead shielding around radiation around radiation sourcessources

Surry Power Station

The type of shielding required depends on the type of radiation present.

Loss of Life ExpectancyLoss of Life ExpectancyActivity or Behavior LLE (DAYS)__________________

Recreational swimming 40 Being 15 percent overweight 900 Smoking 20 cigarettes per day 1,600 Using pesticides at home 12 Being exposed to radon in a home 35

Living within 10 miles of a nuclear power plant 0.4 Riding a bicycle 6 Driving a car 200

Skydiving 25 Consuming alcohol (U.S. average) 230

Discuss the concept of balanced risk.

Aim 8, Int, TOK: Students should appreciate that codes of practice have been developed for conduct involving the use of radiations.

Alara principle

Distinguish between physical half‑life, biological half-life and effective half‑life.

Students should be able to calculate the effective half-life from the physical half-life and the biological half-life.

Radiation DoseRadiation Dose Dose or radiation dose is a generic term for a measure of radiation exposure. In radiation protection, dose is expressed in millirem.

X-Ray Machine Image

(film)Subject is not radioactive but has been exposed to a radiation dose (single chest x ray = 5-10 mrem).

After

External Dose

ContaminationContamination Contamination is the presence of a radioactive Contamination is the presence of a radioactive

material in any place where it is not desired, material in any place where it is not desired, and especially in any place where and especially in any place where

its presence could be harmful.its presence could be harmful.

Yuck!

Solve problems involving radiationdosimetry.

1. This question is about radiation used in medicine.(a) Define the terms exposure and absorbed dose. (2)Exposure: .........................................................................................................

.........................................................................................................Absorbed dose: .........................................................................................................

.........................................................................................................(b) Explain, with reference to α and γ radiation, the distinction between absorbed dose and dose equivalent..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (3) (c) Explain why, when using radioactive tracer elements in the treatment of cancer, it is better to use radioactive isotopes that have a long physical half-life and a short biological half-life..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (2)(Total 7 marks)

1. (a)exposure:total ionised charge produced in unit mass of airby a particular radiation;

absorbed dose: energy absorbed per unit mass; 2 (b) dose equivalent is the amount of energy absorbed;but a quality factor is introduced to describe the effects of differenttypes of radiation;α is absorbed more that γ radiation and so has a much higher Q factor; 3Do not look for this precise wording but look for the understanding. (c) if the biological half-life is long then the tracer can do a lot of damageto healthy cells;with a short biological half-life and long physical half-life the tracerwill have a high activity during the time it is in the body; 2Again do not look for this precise wording but look for the understanding.[7]

2. This question is about dosimetry.(a) Describe what is meant by the term relative biological effectiveness (quality factor)........................................................................................................................................................................................................................................................................... (2)

The whole body of a person of mass 70 kg is exposed to monochromatic X-rays of energy 200 keV. As a result of this exposure, the person receives a dose equivalent of 500 μSv in 2.0 minutes.(b) Deduce that the person absorbs about 1010 X-ray photons per second............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (4)(Total 6 marks)

2.(a) a factor that compares (the effectiveness) of different types of radiationto that of X-rays;some detail e.g. because different radiations (of the same intensity)produce different amounts of ionization / cause different amounts of damage; 2Second marking point can be implied in first point. (b) D = 5 × 10–4 J kg–1 (Q = 1);E = m × D = 70 × 5 × 10–4 J;1 photon has energy 200 × 103 × 1.6 × 10–19 J;number of photons in 1 second = » 1010; 4[6]

Outline the basis of radiation therapy for cancer.

Radiation TherapyRadiation TherapyUsed for treating cancer. Why does it work? Used for treating cancer. Why does it work?

External Beam Brachytherapy (implants)

Image courtesy of

Photo by Karen Sheehan

X-rays: RadiotherapyThis should include the differential effects on normal and malignant cells, as well as a description of the types of sources available.

Cancer occurs when cells divide too quickly. The dividing cells become a tumour which can damage surrounding tissue or spread to the rest of the body

X-rays: Radiotherapy

• X-rays or other radiation can damage the DNA in cells and kill them

• This is why radiation can be dangerous

• But cells which are dividing rapidly are more likely to be killed

• So we use x-rays to kill the rapidly-dividing cancer cells

• We must still ensure that healthy tissue is undamaged

X-rays: Radiotherapy

A linear accelerator generates x-rays. It rotates around the body, irradiating the tumour

from all directions

X-rays: Radiotherapy

X-ray CT scan of chest shows lungs, heart and tumour (red)

X-rays: Radiotherapy

A medical physicist decides which angles to shine x-rays fromto destroy tumour and minimise damage to other tissue

X-rays: Radiotherapy

The treatment plan lists the directions the x-rays will come from and calculates the radiation dose to the tumour (in purple) and rest of body (grey)

X-rays: Radiotherapy

Solve problems involving the choice of radio‑isotope suitable for a particular diagnostic or

therapeutic application.

Students should be familiar with a variety of techniques. Where reference is made to a specific technique, sufficient description will be given forthe student to be able to answer any questions on that technique.

Things to consider;

Type of radiation. How far do you want it to travel in the body?

Half life. How long do you want the substance to be radioactive for?

Solve problems involving particulardiagnostic applications.

For example, assessment of total blood volume. Where reference is made to a specific technique, sufficient description will be given for the studentto be able to answer any questions on that technique.

Nuclear MedicineNuclear MedicineDiagnostic ProceduresDiagnostic Procedures

• Radioactive injectionRadioactive injection• Short half-life Short half-life

radionuclide radionuclide • Pictures taken with Pictures taken with

special gamma cameraspecial gamma camera• Many different studies:Many different studies:

ThyroidThyroidLungLungCardiac Cardiac White Blood CellWhite Blood Cell Photo by Karen Sheehan

Bone ScansBone Scans

Image courtesy of

Tsokos

Page 717 Q’s 1-9.

IB review pack Q’s 4,9,17,20,25,30.