Title: InkPhysics / Medical physics
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Half-value thickness.
Back: The thickness that halves the beam intensity: x = ln 2/μ.

Front: DEFINITION
Define: Accommodation.
Back: The eye changing the power of its lens, by changing the lens shape, so that objects at different distances all form sharp images on a retina that cannot move.

Front: SPOT THE ERROR
Say what is wrong with this claim: The eye focuses on near and far objects by moving its lens, the way a camera does.
Back: The lens does not move: the retina is a fixed distance behind it, so v cannot change. Ciliary muscles change the shape of the lens instead, altering its focal length and so its power, and that change of power is accommodation.

Front: RECALL
State the change in intensity level when the intensity is multiplied by ten.
Back: The intensity level rises by 10 dB, because 10 log 10 = 10; doubling the intensity is a rise of about 3 dB.

Front: SPOT THE ERROR
Say what is wrong with this claim: A PET scanner works by sending radiation into the body.
Back: PET sends no radiation in: a positron-emitting tracer inside the body is the source, and the scanner detects the back-to-back 511 keV photons produced when positrons annihilate with electrons.

Front: RECALL
Explain why sound is measured on a logarithmic scale.
Back: The audible range spans about twelve powers of ten of intensity, and the ear judges by ratio, so equal multiples of intensity are heard as equal steps of loudness.

Front: RECALL
State what the angle θ is measured between in Doppler ultrasound.
Back: In Δf/f = 2v cos θ/c, θ is the angle between the ultrasound beam and the direction of blood flow, and c is the speed of ultrasound in tissue. A beam square across the vessel has θ = 90°, so cos θ is zero and no shift is measured however fast the blood is going. (OCR A only.)

Front: RECALL
State how a heart rate is read from an ECG trace.
Back: Measure the interval T between two successive R spikes; the rate is 60/T beats per minute.

Front: RECALL
Explain pair production as an attenuation mechanism.
Back: Above 1.022 MeV a photon can convert into an electron-positron pair near a nucleus; it matters for high-energy beams, not diagnostic X-rays.

Front: RECALL
State the energy of each PET annihilation photon.
Back: 511 keV, the rest energy of one electron through E = mc^2.

Front: DEFINITION
Define: Acoustic impedance.
Back: The product of a material's density and the speed of sound in it: Z = ρc.

Front: SPOT THE ERROR
Say what is wrong with this claim: An ECG is a recording of the heart moving, so the trace is a picture of the muscle contracting.
Back: The trace plots potential difference at the skin against time, not movement. What it records is the wave of depolarisation crossing the heart muscle, and the mechanical contraction of each chamber follows the electrical event that caused it.

Front: RECALL
Explain why cones resolve finer detail than rods.
Back: Each cone in the fovea has effectively its own nerve fibre, so neighbouring cones stay separate signals, while many rods share one fibre and their signals are combined.

Front: EQUATION · NOT ON THE DATA SHEET
Intensity of a wave from its power and area:
I = ?\(I = \, ?\)I = ?
Back: I = PA\(I = \frac{P}{A}\)I = P/A

Front: RECALL
Explain how the middle ear raises the pressure.
Back: The ossicles act as a lever, and the eardrum has roughly twenty times the area of the oval window, so the same force acts over a much smaller area.

Front: DEFINITION
Define: Depolarisation.
Back: The collapse and reversal of the p.d. across a cell membrane as ions cross it, which sweeps across the heart muscle as a wave and triggers contraction.

Front: RECALL
State why conducting gel is used under an ECG electrode.
Back: Dry skin makes a contact of tens of kilohms, and the gel lowers that resistance and keeps the reading steady as the patient moves.

Front: RECALL
Explain why the QRS complex is the largest deflection.
Back: The ventricles carry far more muscle than the atria, so far more charge moves at once and the p.d. at the skin is several times larger than during the P wave.

Front: RECALL
State what the QRS complex represents.
Back: The depolarisation of the ventricles, which then contract.

Front: RECALL
Explain the X-ray contrast of bone.
Back: Bone's larger attenuation coefficient means it absorbs X-rays more strongly, so it shows clearly against soft tissue.

Front: DEFINITION
Define: Piezoelectric effect.
Back: A pd applied across the crystal deforms it, and deforming the crystal produces a pd.

Front: DEFINITION
Define: Tracer.
Back: A radionuclide attached to a molecule the body processes, so detectors outside the body can map where it is taken up. A gamma camera uses a gamma emitter; PET uses a positron emitter, whose annihilation photons are what escape.

Front: RECALL
State what the dBA scale is for.
Back: It passes the sound through a standard A-weighting curve, which approximates the ear's response at moderate levels, before the level is computed, so the reading counts the frequencies the ear is sensitive to most heavily rather than treating all of them alike. It is an exposure metric, not a measurement of perceived loudness.

Front: RECALL
State what radionuclide imaging shows.
Back: Radionuclide imaging maps function, where the body's chemistry takes the tracer, rather than anatomical structure.

Front: RECALL
State the unit of acoustic impedance.
Back: kg m^−2 s^−1, from the product of density and speed.

Front: RECALL
State the three parts of a prescription for astigmatism.
Back: The spherical power SPH in dioptres, the additional cylindrical power CYL in dioptres, and the AXIS in degrees from 0 to 180 giving the orientation at which the cylinder is set.

Front: RECALL
State the two limits on the detail a pulse-echo scan can resolve.
Back: Two boundaries closer than about half the length of the pulse return echoes that overlap into one, and no pulse can be shorter than about one cycle, so the resolution stops near the wavelength of the radiation used.

Front: EQUATION · NOT ON THE DATA SHEET
Maximum photon energy from the tube voltage:
E_max = ?\(E_{max} = \, ?\)E_max = ?
Back: E_max = eV\(E_{max} = eV\)E_max = eV

Front: RECALL
State the half-life of technetium-99m.
Back: Six hours, long enough to prepare and image but short enough to spare the patient a lasting dose.

Front: RECALL
Explain how an equal loudness curve is produced.
Back: A 1 kHz reference tone is set to a chosen intensity level, and at each other frequency a test tone is adjusted until the listener judges the two equally loud.

Front: RECALL
Explain where the p.d. an ECG measures comes from.
Back: Part of the heart is depolarised and part is not, so the heart acts as a changing electric dipole, and the conducting body fluids carry a p.d. of about 1 mV to the skin.

Front: DEFINITION
Define: Hypermetropia.
Back: Long sight: the eye has too little power for its length, so rays from a near object would focus behind the retina and the near point is further away than 0.25 m.

Front: RECALL
State the attenuation law for ultrasound in tissue.
Back: Intensity falls exponentially with the distance travelled through the tissue: I = I₀e^−μx, where μ is the attenuation coefficient. CIE prints the radioactive-decay exponential on its formula sheet and does not print this one, so it has to be recalled. (CIE only.)

Front: SPOT THE ERROR
Say what is wrong with this claim: Ultrasound scans photograph the inside of the body with sound.
Back: An ultrasound image is built from echo timings, not captured like a photograph: each boundary in the body reflects a pulse, and d = ct/2 converts each echo's return time into a depth.

Front: DEFINITION
Define: Myopia.
Back: Short sight: the eye is too powerful for its length, so parallel rays focus in front of the retina and the far point is at a finite distance instead of infinity.

Front: SPOT THE ERROR
Say what is wrong with this claim: A sound at 80 dB carries twice the power of one at 40 dB.
Back: 80 dB is 40 dB more, which is four factors of ten, so the intensity ratio is 10^4. Ten thousand times the power, not twice: the decibel scale is logarithmic precisely because the ear judges by ratio rather than by difference.

Front: DEFINITION
Define: Intensity.
Back: The power carried by a wave through unit area at right angles to its direction of travel: I = P/A, in W m^−2.

Front: RECALL
State what the P wave represents.
Back: The depolarisation of the atria, which then contract.

Front: DEFINITION
Define: Attenuation coefficient.
Back: The constant μ in I = I₀e^−μx, measuring how strongly a material absorbs and scatters X-rays.

Front: RECALL
State where the ear is most sensitive.
Back: Between about 2 kHz and 5 kHz, where every equal loudness curve dips lowest; sensitivity falls at both ends of the 20 Hz to 20 kHz range and worst at the bass end.

Front: EQUATION · NOT ON THE DATA SHEET
Relative intensity level of two sounds:
relative intensity level = ?\(\text{relative intensity level} = \, ?\)relative intensity level = ?
Back: relative intensity level = 10 logI_2I_1\(\text{relative intensity level} = 10\,\text{log}\frac{I_{2}}{I_{1}}\)relative intensity level = 10 log (I_2/I_1)

Front: RECALL
State how to find the effective half-life of a tracer.
Back: 1/T_E = 1/T_B + 1/T_P, always shorter than either half-life alone.

Front: RECALL
Explain why a PET scanner is combined with a CT scanner.
Back: PET measures function but shows anatomy poorly, so a bright spot alone does not say which organ it is in. CT gives the structure and no function, so the two images are overlaid: one says what is happening, the other says where.

Front: DEFINITION
Define: Equal loudness curve.
Back: A curve joining the combinations of frequency and intensity level that a listener judges equally loud, labelled by its own intensity level at the 1 kHz reference tone.

Front: DEFINITION
Define: Threshold of hearing.
Back: The minimum intensity a normal ear can detect at a frequency of 1 kHz, taken as 1.0 × 10^−12 W m^−2.

Front: DEFINITION
Define: Action potential.
Back: The brief pulse in which a triggered cell depolarises, its membrane p.d. reversing, and then repolarises as the resting state is restored.

Front: SPOT THE ERROR
Say what is wrong with this claim: A CT scanner is just a sharper X-ray photograph.
Back: A CT scan is not a single sharper image: the scanner records many X-ray projections from different angles and reconstructs a cross-sectional slice, locating structures in depth as one shadow image cannot.

Front: DEFINITION
Define: Coupling gel.
Back: The gel between transducer and skin that displaces the air, so the impedances are matched and the pulse enters the body.

Front: RECALL
Explain the frequency trade-off in ultrasound.
Back: Higher frequency resolves finer detail but is attenuated more strongly, so it cannot reach deep structures.

Front: RECALL
State what the T wave represents.
Back: The repolarisation of the ventricles as they recover and relax.

Front: RECALL
State how to find the depth of a boundary from a pulse-echo time.
Back: d = ct/2, the factor of two because the pulse travels to the boundary and back.

Front: RECALL
State the three requirements of an ECG amplifier.
Back: High gain, to lift a millivolt to a usable size; high input impedance, so the large skin contact resistance does not swallow the signal; and low noise, to reject 50 Hz mains hum.

Front: DEFINITION
Define: Coincidence detection.
Back: Registering two 511 keV photons arriving at opposite detectors at effectively the same instant.

Front: RECALL
State the nature of the image on the retina.
Back: Real, inverted and diminished.

Front: RECALL
Explain why the Doppler shift from flowing blood is doubled.
Back: The blood is moving relative to the transmitter, so it meets an already shifted frequency; it then re-radiates that wave while still moving relative to the receiver, so the frequency shifts a second time. (OCR A only.)

Front: DEFINITION
Define: Intensity level.
Back: A logarithmic measure of a sound against the threshold of hearing, 10 log(I/I_0), quoted in decibels.

Front: RECALL
State what determines the fraction of a pulse reflected at a boundary.
Back: The fraction of an ultrasound pulse reflected at a boundary grows with the mismatch in acoustic impedance across it.

Front: DEFINITION
Define: Resting potential.
Back: The potential difference held across the membrane of a resting muscle or nerve cell by the unequal distribution of ions, about 90 mV with the inside negative.

Front: DEFINITION
Define: Fovea.
Back: The small patch of retina on the axis of the eye, packed with cones and carrying no rods, where the sharpest vision is formed.

Front: RECALL
State the maximum X-ray photon energy.
Back: One accelerated electron caps one photon: E_max = eV for tube pd V.

Front: DEFINITION
Define: Near point.
Back: The closest object an eye can bring into sharp focus, taken as 0.25 m for a normal adult eye; also called the least distance of distinct vision.

Front: RECALL
Explain what noise damage or age does to the equal loudness curves.
Back: The threshold curve lifts, most at the high-frequency end, so a greater intensity is needed to hear anything and the top of the range is lost first.

Front: RECALL
Explain why annihilation photons leave back to back.
Back: The electron-positron pair is nearly at rest, so momentum conservation demands equal and opposite photon momenta.

Front: RECALL
State what a coincidence pair locates.
Back: The annihilation that produced the pair lies on the straight line between the two detectors that fired.

Front: EQUATION · NOT ON THE DATA SHEET
Depth of a boundary from its echo time:
d = ?\(d = \, ?\)d = ?
Back: d = ct2\(d = \frac{ct}{2}\)d = ct/2

Front: DEFINITION
Define: Astigmatism.
Back: A cornea or lens whose curvature differs between two perpendicular directions, so its power is not the same along both and a point object cannot be focused to a point.

Front: RECALL
State what PET scanning is used to diagnose.
Back: Fluorine-18 on a glucose analogue maps how fast tissue is burning sugar. Tumours burn it fastest and show as bright spots, so PET finds and stages cancers; reduced uptake in regions of cortex marks dementia, an over-active patch locates the origin of epileptic seizures, and heart muscle still taking up the tracer after a heart attack is muscle still alive.

Front: RECALL
Explain why the T wave is broader and lower than the QRS complex.
Back: Repolarisation is slower than depolarisation and less well synchronised across the muscle, so the same recovery is spread over more time.

Front: DEFINITION
Define: Far point.
Back: The furthest object an eye can bring into sharp focus, at infinity for a normal eye, and the distance the relaxed eye is focused on.

Front: RECALL
Explain the different effects of tube voltage and tube current.
Back: Raising the voltage raises the photon energies; raising the current only makes more photons.

Front: RECALL
State the power of the lens that corrects myopia.
Back: The lens must image an object at infinity at the far point, so P = −1/(far point in metres): negative, and therefore diverging.

Front: RECALL
State why the middle ear must raise the pressure.
Back: The cochlea beyond the oval window is fluid-filled, and without the gain almost all of the sound would reflect at the air-to-fluid boundary instead of entering.

Front: RECALL
State the wavelength the eye is most sensitive to in bright light.
Back: About 555 nm, in the yellow-green; the rods peak nearer 500 nm, and the response has gone by roughly 400 nm and 700 nm.