304 7.8  Biomedical Physics Tools

7.7.4  TISSUE ACOUSTICS

The measure of resistance to acoustic propagation via phonon waves in biological tissue is

the acoustic impendence parameter. This is the complex ratio of the acoustic pressure to the

volume flow rate. The acoustic impendence in different animal tissues can vary by two orders

of magnitude, from the lungs at the low end (which obviously contain significant quantities of

air) and the bone at the high end, and is thus a useful physical metric for the discrimination of

different tissue types, especially useful at the boundary interface of different tissue types since

these often result in an acoustic mismatch that is manifested as a high acoustic reflectance,

whose reflection signal (i.e., echo) can thus be detected. For example, a muscle–​fat interface

has a typical reflectance of only ~1%; however, a bone–​fat interface is more like ~50%, and

any soft water-​based tissue with air has a reflectance of ~99.9%. This is utilized in various

forms of ultrasound imaging.

KEY POINT 7.7

Bulk tissue measurements do not allow fine levels of tissue heterogeneity to be

investigated, in that as an ensemble technique, their spatial precision is ultimately

limited by the relatively macroscopic length scale of the tissue sample and any inference

regarding heterogeneity in general is done indirectly through biophysical modeling;

however, they are often very affordable techniques and relatively easy to configure

experimentally and generate often very stable measurements for several different

ensemble physical quantities, many of which have biomedical applications and can

assist greatly in future experimental strategies of using more expensive and time-​

consuming techniques that are better optimized toward investigating heterogeneous

sample features.

7.8  BIOMEDICAL PHYSICS TOOLS

Many bulk tissue techniques have also led to developments in biomedically relevant bio­

physical technologies. Whole textbooks are dedicated to specific tools of medical physics,

and for expert insight of how to operate these technologies in a clinical context, I would

encourage the reader to explore the IPEM website (www.ipem.ac.uk), which gives profes­

sional and up-​to-​date guidance of publications and developments in this fast-​moving field.

However, the interface between medical physics, that is, that performed in a clinical envir­

onment specifically for medical applications, and biophysics, for example for researching

questions of relevance to biological matter using physics tools and techniques, is increas­

ingly blurred in the present day due primarily to many biophysics techniques having a

greater technical precision at longer length scales than previously, and similarly for medical

physics technologies experiencing significant technical developments in the other direc­

tion of smaller-​scale improvements in spatial resolution in particular, such that there is

now noticeable overlap between the length and time scale regimes for these technologies.

A summary of the principle of biophysical techniques relevant to biomedicine is therefore

included here.

7.8.1  MAGNETIC RESONANCE IMAGING

MRI is an example of radiology, which is a form of imaging used medically to assist in diag­

nosis. MRI uses a large, cooled, electromagnetic coil of diameter up to ~70 cm, which can

generate a high, stable magnetic field at the center of the coil in the range ~1–​7 T (which

compares with the Earth’s magnitude field strength of typical magnitude ~50 μT). The phys­

ical principles are the same as those of NMR in which the nuclei of atoms in a sample absorb

energy from the external magnetic field (see Chapter 5) and reemit electromagnetic radiation

at an energy equal to the difference in nuclei spin energy states, which is dependent on the

KEY BIOLOGICAL

APPLICATIONS: BULK

SAMPLE BIOPHYSICS TOOLS

Multiple simple, coarse but

robust mean ensemble average

measurements on a range of

different tissue samples.