C23 Past papers

Question 1

What is acoustic impedance in the context of ultrasound?

Answer 1

Acoustic impedance 𝑟=𝜌 /𝑐, where ρ is density and c is the speed of sound in the medium. It governs reflection/transmission at boundaries.

Question 2

What are the pressure reflection and transmission coefficients for a plane wave normally incident on an interface?

Answer 2

𝑅=𝑟2/𝑟1𝑟2+𝑟1, 𝑇=2 𝑟2/𝑟1+𝑟2

Question 3

Define the near field (Fresnel) and far field (Fraunhofer) zones of an unfocused ultrasound transducer.

Answer 3

Near field: Region close to the transducer where interference creates fluctuations in beam intensity, typically up to 𝑧≈𝑎2𝜆. Far field: Region beyond that distance where the beam spreads in a more predictable pattern.

Question 4

What is the primary mechanism of High-Intensity Focused Ultrasound (HIFU)-induced tissue damage?

Answer 4

Thermal ablation (rapid heating of tissue) and/or mechanical disruption (cavitation).

Question 5

What is cavitation in ultrasound therapy?

Answer 5

The formation, oscillation, and collapse of gas bubbles in a liquid under acoustic pressure, which can cause enhanced mechanical disruption of tissue or improved drug delivery.

Question 6

Why is the theoretical tensile strength of a liquid not typically the actual cavitation threshold in tissue?

Answer 6

Tissue contains gas nuclei (microbubbles), meaning cavitation can occur at much lower negative pressures than pure-liquid theoretical limits.

Question 7

What is the mechanical index (MI) on clinical ultrasound machines?

Answer 7

A safety metric approximating the likelihood of cavitation; MI≈𝑝negpeak𝑓.

Question 8

In the Rayleigh–Plesset equation, which physical effects primarily govern bubble expansion and collapse?

Answer 8

Liquid inertia, viscosity, and the driving acoustic pressure; surface tension and vapor pressure can also matter for small radii or high amplitudes.

Question 9

What does the bioheat equation describe?

Answer 9

The temperature evolution in tissue, accounting for conduction, local heat generation (e.g., from ultrasound absorption), and blood perfusion.

Question 10

Give the formula for cumulative equivalent minutes (CEM) at 43 °C.

Answer 10

CEM43=∑i 𝑡𝑖 𝑅(43−𝑇𝑖), where R=0.5 if 𝑇𝑖≥43∘C, R=0.25 if 𝑇𝑖<43∘C.

Question 11

Name two ways focused ultrasound can enhance drug delivery to tumours.

Answer 11

Hyperthermia: increases perfusion and vessel permeability. Cavitation: microbubble action increases vascular permeability.

Question 12

Define drained vs. undrained conditions in a poroelastic tissue.

Answer 12

Drained: Fluid can flow out; pore pressure = 0. Undrained: Fluid is trapped; pore pressure increases with loading.

Question 13

Why is oxygen diffusion a common limiting factor in 3D tissue-engineered constructs?

Answer 13

Cells deeper than ~200 μm from a nutrient source risk hypoxia because diffusion of oxygen through dense tissue is slow, causing necrotic cores.

Question 14

What is Fick’s first law of diffusion?

Answer 14

J=−D dx/dC, where J is flux, D is diffusivity, and C is concentration.

Question 15

Name four desirable properties of a scaffold for tissue engineering.

Answer 15

Biocompatibility, Interconnected porosity, Mechanical integrity, Controlled biodegradability.

Question 16

Which scaffold property is crucial for cell infiltration and nutrient transport?

Answer 16

Porosity (including pore size and interconnectivity).

Question 17

Why is mechanical stimulation sometimes applied in tissue-engineering bioreactors?

Answer 17

Many tissues (bone, cartilage, muscle) require mechanical cues for proper ECM deposition, growth, and differentiation.

Question 18

What is a rotating wall vessel (RWV) bioreactor used for?

Answer 18

Creating a low-shear, nearly microgravity environment to culture 3D cell aggregates with minimal shear-induced damage.

Question 19

Compare stirred-flask vs. perfusion bioreactors for tissue engineering.

Answer 19

Stirred-flask: simple mixing, potential shear stress on cells. Perfusion: direct fluid flow through scaffold pores, more uniform oxygen supply but more complex design.

Question 20

Why do stem cells play a big role in tissue engineering?

Answer 20

They can self-renew and differentiate into multiple lineages, enabling regeneration of various tissues when seeded onto scaffolds.

Question 21

Distinguish between embryonic stem cells and adult stem cells in tissue engineering.

Answer 21

Embryonic: pluripotent, can become nearly any cell type, but ethical issues. Adult: multipotent, limited differentiation range, fewer ethical concerns.

Question 22

How do iPS cells differ from embryonic stem cells?

Answer 22

Induced pluripotent stem (iPS) cells are derived by reprogramming adult somatic cells to a pluripotent state, bypassing embryo use.

Question 23

What is the main advantage of thermosensitive liposomes in drug delivery?

Answer 23

They release their drug payload at hyperthermic temperatures (~ 40–45 °C), enabling targeted release in heated tumour regions.

Question 24

What factor typically limits the frequency chosen for HIFU treatments?

Answer 24

Trade-off between focal spot size (smaller at higher frequency) vs. attenuation (higher frequency = more energy lost).

Question 25

In laser-based thermal ablation, why is wavelength important?

Answer 25

Tissue absorption depends strongly on wavelength; choosing a wavelength that matches tissue absorption ensures efficient heating.

Question 26

Give an example of how non-invasive therapies can reduce surgical complications.

Answer 26

They target tumours or tissues without large incisions, reducing infection risk, hospital stay, and damage to healthy structures.

Question 27

What is zeroth-order vs. first-order clearance in drug pharmacokinetics?

Answer 27

Zeroth-order: drug cleared at a constant rate (mg/h). First-order: drug cleared at a rate proportional to its concentration (exponential decay).

Question 28

Why might you see a two-compartment model in drug distribution for tumour therapy?

Answer 28

Because distribution occurs first in plasma (central compartment) before slower equilibration into peripheral/tumour compartments.

Question 29

What is the rationale for using ultrasound contrast agents (microbubbles) in therapy?

Answer 29

Microbubbles can enhance ultrasound scattering (imaging) and, when excited, cause cavitation that improves local drug uptake or ablation.

Question 30

In thermal therapies, how can blood perfusion impede tissue heating?

Answer 30

Blood flow carries away heat, reducing the local temperature rise and possibly preventing reaching lethal temperatures for ablation.

Question 31

Why is vascularization a major challenge for engineering large tissues?

Answer 31

Cells deeper in thick constructs need blood vessels for O2 and nutrient delivery; purely diffusive transport is insufficient beyond a few hundred microns.

Question 32

Which bioreactor might you select for bone tissue engineering and why?

Answer 32

A perfusion bioreactor applying fluid flow and mechanical loading, mimicking in vivo stimuli crucial for bone formation and mineralization.

Question 33

What is the limiting nutrient for most tissue-engineered constructs?

Answer 33

Oxygen is often the limiting reagent because of its low solubility and relatively high cellular demand.

Question 34

Define pore pressure in a poroelastic tissue.

Answer 34

The pressure of the interstitial fluid within the pores; it influences load distribution between solid matrix and fluid phase.

Question 35

Why is collagen a common scaffold material?

Answer 35

It is the main ECM protein in many tissues, naturally biocompatible, and supports cell adhesion and growth.

Question 36

Give two mechanisms by which ultrasound hyperthermia can improve local chemotherapy effectiveness.

Answer 36

Increased tumour blood flow (better drug penetration). Triggered release from temperature-sensitive drug carriers.

Question 37

What does apoptosis mean in the context of tissue therapies?

Answer 37

Programmed cell death—often a desired mechanism in cancer treatment, as it eliminates cells cleanly without inflammatory necrosis.

Question 38

State a key advantage and disadvantage of using a stirred-tank bioreactor for 3D tissue cultures.

Answer 38

Advantage: Simple operation, improved mixing. Disadvantage: Possible excessive shear on cells, damaging delicate constructs.

Question 39

What is the usual range of frequency for HIFU treatments?

Answer 39

Typically 0.5–3 MHz, balancing penetration depth, focusing tightness, and attenuation.

Question 40

Why might 3D printing be useful for scaffold fabrication?

Answer 40

It enables precise control of pore geometry, architecture, and shape, tailored to patient-specific needs or complex organ shapes.