1423

Question 1

State the Beer-Lambert law.

Question 2

On what does the linear attenuation coefficient depend on?

Answer 2

• Atomic number of tissue, i.e. for bone which contains calcium (Z=20) has a large value of μ.

Question 3

What are the 2 types of damage caused by x-rays?

Answer 3

• Energy is deposited into tissues by ionisation.
• Ions go on to damage DNA.
• Somatic damage - Damage happening to the individual (i.e. cancer)
• Genetic damage - Damage happeing to the sex cells which are passed on.
• Damage can be repaired at times- there may exist a threshold below which no damage occurs.

Question 4

What is the ALARA principle?

Answer 4

• Radiation doses must be ‘As Low As Reasonably Achievable’.
• Doses should be kept to a minimum to ensure the safety of others.

Question 5

What measures can be taken to ensure safety against radiation?

Answer 5

• Workers should be kept in protective clothing.
• Alternative work should be found for pregnant workers.
• Personal film badge should be worn.

Question 6

Which medical imaging technique should be used in finding the location of metal fragments?

Answer 6

CT - Computed Tomography.

It generates a cross-sectional (transverse) slice image by rotating an x-ray source and detectors around the patient.

Shows depth unlike normal x-ray images.

Question 7

Describe the method of back-projection behind CT.

Answer 7

• Measured values of attenuation are assigned to a series of columns in a 2D array. These columns are summed by rotating each array so that it corresponds to the anlge used in the projection. The resulting image is contaminated by a ‘star artefact’. This can be mathematically removed by filtering the projections before performing back-projection. The resulting CT image provides a map of attenuating properties of the tissues across the body.

Question 8

What is sound?

Answer 8

Sound is a pressure wave that causes particles in a medium to oscillate back and forth.

Question 9

What are the advantages and disadvantages of using high frequency ultrasound?

Answer 9

Speed of sound in tissue i independent of frequency, so using higher frequencies means that the wavelength nis smaller. To see smaller details, the wavelegths used must be equal to or smaller than the size if the object.

Higher frequencies provided greater spatial resolution, but are attenuated more in tissue.

Question 10

Describe the pulse-echo principle used in ultrasound imaging.

Answer 10

When the sound waves reach a boundary between media of different acoustic impedances, reflection and transmission occurs. The time it takes to receive an echo from when a pulse was emitted is measured.The distance covered in this time is twice the distance to the boundary from the surface - d = c ∆t / 2.

Question 11

Give the the reflection intensity.

Question 12

Why are sound waves reflected off bone and air, in the case of air, what can we do to prevent this?

Answer 12

Bone has a much higher acoustic imedance compared to the surrounding soft tissue and the reflection intensity is approximately 1.

Air has much lower acoustic impedance compared to the surrounding tissue so R is also close to 1. A couping gel is used to avoid trapping air between ultrasound probe and skin.

Question 13

How are ultrasound waves generated in the probe?

Answer 13

Transducers are used which use piezoelectric crystals. These are aligned elongated molecules which have a net negative chanrge and positive charge on either end. When a voltage is applied, the molecules rotate and the crystal is deformed. f an alternating voltage is applied, the crystals will vibrate producing ulrasound. (If the crystal is subject to mechanical stress, it will produce a voltage).

Question 14

How is an NMR signal produced in an MRI?

Answer 14

Protons and Neutrons in nucle have angular momentum, i.e. spin. Thus can be interpreted as a charge orbiting the spin axis. We know that spinning charges generate magnetic fields. The particles cannot, however, align with external magnetic fields according the quantum mechanics but occupy spin “up” and “down” states, of which the down state has ess energy. More nuclei therefore occupy this state. Magnets unable to align with external fields precess about the magnetic axis of the feild at the Larmor frequency ω_o = γ B_o. If another magnetic field is applied perpendicular to the external filed but also spinning about it at the larmour frequency, the nuclei switch spin states which causes current to be induced in the coil. This is the NMR signal.

Question 15

How are MRI images prouduced based on NMR signals?

Answer 15

To form images, the signal must be different at different locations in the body. For this we use magnetic field gradients. The Larmor frequency now depends on spatial location.The small field B₁ is rotated at a range of frequencies causing many signals. The signals obtained tell us about the density of protons and their environments at different spatial locations.

Question 16

Describe how SPECT imagng works.

Answer 16

•A radioactive drug is administered, and emitted radiation (gamma ray photons) are detected using one or more “gamma cameras” rotated around the body. Multiple views enable cross-section images to be reconstructed. •Images reveal distribution of drug in brain or other organ (proportional to blood flow). •Temporal resolution of SPECT is low. –One image requires about 30 minutes. –Limited use for study of brain activity.

Question 17

Describe how PET imaging works.

Answer 17

• Uses radioisotope tracer which emits positrons.
• Positrons annihilate with electrons to give two identical photons of 511 keV (gamma rays), which are emitted in opposite directions. Must be detected smultaneously.

Reconstruction algorithm used to determine point of annihilation.

•The tracer is attached to a biologically active molecule. A common molecule is fluorodeoxyglucose (FDG), so that concentrations of tracer imaged reveal metabolic activity (regional glucose uptake).

Question 18

Describe how fMRI images are produced.

Answer 18

fMRI allows observation of the brains response to tasks, whereas MRI only gives anatomical images. Haemoglobin, is diamagnetic (sets up a field in opposition to an applied field) when oxygenated but paramagnetic (produces a field in line with the applied field) when deoxygenated. CHhange in the oxygenation of haemoglobin causes change to the magnetic field in its immediate vicinity. MRI different depending on the level of oxygenation. Changes in signal can be detected during MRI scanning, using BOLD (blood-oxygen-level dependent) contrast. Two sets of data are needed, rest data and when brain is performing a task. The difference in the signal produced shows changes.

Question 19

Describe x-ray angiography.

Answer 19

The x-ray linear attenuation coefficients of blood and surrounding soft tissues are almost identical - blood does not show up in conventional x-ray images. Can be overcome by injecting a high-attenuating liquid into the blood - Iodine (Z = 53). X-ray images are recorded before and during the injection of the contrast agent. For most tissues except the heart, x-ray images are recorded at a rate of two or three per per second, which allows the radiologist to evaluate the flow of the blood through a vessel. An image recorded before injecting the contrast agent is subtracted from the subsequent images, so only the vessels filled with contrast agent can be seen. Images of the heart must be recorded at much faster rates (15-30 per second) and without using a subtraction technique (because surrounding tissues move too much).

Question 20

Describe the method of Doppler Ultrasound.

Answer 20

Doppler effect: change in perceived frequency due to relative motion of source and receiver. For medical applications we are concerned with moving reflectors (blood flow).

Doppler frequency = difference between frequency of emitted wave and frequency of reflected wave:

The Doppler effect also occurs when ultrasound
waves are reflected (back-scattered) by moving blood cells in tissue. If the blood cell is moving
towards the transducer, the cell “sees” a higher frequency, since the waves appear to arrive more
rapidly. Similarly, a lower frequency is “seen” by the cell if it is moving away from the transducer. Once reflected by moving blood cells, the returning echoes undergo a further shift in frequency, since
the wavelength is stretched (when the cells are moving away from the transducer) or compressed (when moving towards). For a transducer emitting ultrasound waves of frequency f, the total change in frequency (known as the Doppler frequency) is given by: .

Question 21

What are continuous wave and pulsed doppler ultrasound?

Answer 21

In Continuous Wave Doppler ultrasound, a transducer emits a wave continuously while another
transducer detects the returning wave continuously. The difference between the frequencies (the
Doppler frequency) provides a direct measurement of the blood flow within the entire region of tissue
exposed by the beam. In Pulsed Doppler instruments, Doppler measurements are combined with
ultrasound imaging. Blood flow at specific locations can be determined by examining successive
echoes. The phase of the echoes, relative to a fixed reference signal, changes at a rate equal to the
Doppler frequency. Blood flow information is normally displayed in colour on top of the conventional
greyscale ultrasound image.