Chapter 7 – Complementary Experimental Tools  307

(7.19)

k

k

t

2

1

2

2

=

∆

Thus, coincidence detection can result in a substantial reduction in random detection error.

If the true signal rate from coincidence detection is kS, then the effective single-​to-​noise ratio

(SNR) is

(7.20)

SNR

s

=

+

k

k

nk2

Here, n =​ 2 for delayed-​coincidence methods, which are the standard coincidence detection

methods for PET involving one of the detector signals being held for several sampling time

windows (up to ~10⁻⁷ s in total), while the signal in the other detector is then checked. Recent

improvements to this method involve parallel detector acquisition (i.e., no imposed delay) for

which n =​ 1. For both methods, the kS is much higher than k2 and so the SNR scales roughly

as √kS, whereas for SPECT, this scales more as kS/​√k1, which in general is <√kS. Also, the signal

rate for a single radionuclide atom is proportional to the reciprocal of its half-​life, which is

greater for PET than for SPECT radionuclides. These factors combined result in PET having

a typical SNR that is greater than that of SPECT often by more than two orders of magnitude.

PET can also be combined with CAT/​CT and MRI in some research development scanning

systems, called PET-​CT and PET-​MRI, which have enormous future diagnostic potential in

being able to overlay images from the same tissue obtained using the different techniques, but

the cost of the equipment at present is prohibitive.

7.8.4  ULTRASOUND TECHNIQUES

The measurement of acoustic impedances using an ultrasound probe in direct acoustical con­

tact with the skin is now commonplace as a diagnostic tool, for example, in monitoring the

development of a fetus in the womb, detecting abnormalities in the heart (called an echocar­

diogram), diagnosing abnormal widening (aneurysms) of major blood vessels, and probing

for tissue defects in various organs such as the liver, kidneys, testes, ovaries, pancreas, and

breast. Deep tissue ultrasound scanning can also be facilitated by using an extension to enable

the sound emitter/​probe to get physically closer to the tissue under investigation.

A variant to this technique is Doppler ultrasound. This involves combined ultrasound

acoustic impedance measurement with the Doppler effect. This results in the increase or

decrease of the wavelength of the ultrasound depending on the relative movement of the

propagation medium and so is an ideal biophysical tool for the investigation of the flow of

blood through different chambers in the heart.

Photoacoustic imaging is another modification of standard ultrasound, using the

photoacoustic effect. Here, absorbed light in a sample results in local heating that in turn can

generate acoustical phonons through thermal expansion. The tissues of relevance absorb

light strongly and have included investigations of skin disorders via probing the pigment

melanin, as well as blood oxygenation monitoring since the oxygenated heme group in

the hemoglobin molecule has a different absorption spectrum to the deoxygenated form.

The technique can also be extended to RF electromagnetic wave absorption, referred to as

thermoacoustic imaging.

7.8.5  ELECTRICAL SIGNAL DETECTION

The biophysical technique of using dynamic electrical signals from the electrical stimuli of

heart muscle tissue, ideally from using up to 10 skin-​contact electrodes both in the vicinity of

the heart and at the peripheries of the body at the wrists and ankles, to generate an electro­

cardiogram (EKG or ECG) is a standard, cost-​effective, and noninvasive clinical tool capable