Medical imaging systems provide physicians with a tissue-specific visual representation of otherwise concealed clinical information within the body and hence enable them to perceive and implement unambiguous diagnostic procedures.
This report investigates the clinical applications of Ultrasound and Radioisotopes, explicates the rudimentary physical principles of the techniques and outlines the diagnostic and therapeutic limitations as well as the relevant patient safety aspects.
Unlike other medical imaging techniques that involve electromagnetic radiation, ultrasound uses elastic, high frequency acoustic waves partially reflected from targeted soft tissue to produce an image (ultrasonogram). This non-invasive process unifies acoustic wave propagation and signal processing procedures (Wolbarst, 1999). High frequency (~1-10 MHz) electrical pulses are produced from a transmitter, which are converted into mechanical oscillations using a piezoelectric transducer. A gel is applied between the skin and the transducer to provide for the requisite acoustical coupling. These mechanical vibrations traverse through the skin with negligible reflectance and propagate deep inside the body where they are reflected, albeit much attenuated, from the interfaces between the heterogeneous soft tissues and body fluids back to the transducer, which senses, collects and converts them to electrical signals. Attenuation depends on density and elasticity of the tissues concerned (Szabo, 2004;Bambler, 1986). The objective is to convert the temporal high frequency pulse information into spatial information using the wave propagation speeds in tissue. The reflected intensity depends on the heterogeneous tissue irregularities, which appear transparent if the dimensions of irregularities are comparable to the wavelength of sound in the tissue (Wolbarst, 1999). With a frequency dependant high spatial resolution (0.3-3mm) and penetration (3-25cm) (Szabo, 2004), a distinct advantage of ultrasound is its ability to reveal real time information.
Perhaps the most widespread application of Ultrasound since its induction into medicine has been investigation of the foetus in diagnostic clinical obstetric imaging, which includes visualising the premature foetus and determining its location and size during early pregnancy in addition to assessing the chances of pregnancy being sustained (Woo, 2006). Doppler effect, which accounts for the apparent increase in fixed ultrasound frequency due to the blood moving nearer, allows the blood flow speeds to be measured in the limbs, breast and eyes (Wolbarst, 1999). This is crucial in evaluating early pregnancy conditions (Abramowicz & Sheiner, 2007) although (Eyding et.al, 2002) pointed out insufficiency of the Doppler method to measure low blood flow velocities in essential organs. Another application is early diagnosis of stroke where cerebral vessel images are formed employing echo-enhancing agents (ECA) (Ringelstein, 1998) although ECAs can be destroyed by ultrasound (Eyding et al, 2002). Ultrasound heating therapy is used for soft tissue and joint ailments (Wolbarst, 1999).
Ultrasound can potentially cause a health hazard in the three ways (Wolbarst, 1999) listed below which dictate its clinical limitations.
2. Medium-powered ultrasound can cause considerable local heating in tissue.
3. Even at diagnostic levels, ultrasound can exert shear and torsional forces on suspended particles.
The ECURS* has laid down specific guidelines for tissue specific clinical applications in addition to output conditions and exposure time with regard to safe clinical practice (ECURS, 1995).
Diagnosis and therapy using nuclear medicine has witnessed impressive developments over the last decade and involves a choice between vast numbers of carefully selected, target specific and highly sensitive radioisotopes. The objective is either to image the diseased organ (diagnosis) or destroy unhealthy tissue sites while sparing normal tissue (radiation therapy) where absolute cure is not conceivable (Wechalekar & Cook, 2005).
*European Committee for Ultrasound Radiation Safety
Nuclear instability (radioactivity) is the process by which unstable nuclei (isotopes) of stable atoms spontaneously emit ionising radiations to achieve a lower energy state. This nuclear instability occurs in nuclei where the repulsive electrostatic interaction between protons exceeds the strong forces between nucleons, a phenomenon directly proportional to the mass number of the element concerned. The process also occurs in nuclei with uneven number of nucleons and in nuclei with neutron deficiency or overabundance. The three principal forms of ionising radiation emission include ?2+-particles (doubly charged Helium nuclei), ?+–particles (positrons/electrons) and ?-radiation (electromagnetic), each form detected by an explicit detector. However, nuclear medicine applications entail ?-radiation only, to avoid the introduction of potentially toxic particulate matter within the body (Billington, Jayson & Maltby, 1992). Radioisotopes attach to the target site and emit radiation, which can be detected. Detectors for radioisotopes in medical imaging include the traditional gamma cameras, Single Photon Emission Computed Tomography and Positron Emission Tomography (Billington, Jayson & Maltby, 1992).
Targeting specificity for radioisotopes can be brought about using seven distinct methods by injecting or inhaling the sample (Billington, Jayson & Maltby, 1992). Diagnostic applications of radioisotopes include static and dynamic imaging, both preoperative and postoperative, which utilise the technique’s ability to reveal operational rather than anatomical information of physiological abnormalities in organs (Billington, Jayson & Maltby, 1992). These include non-invasive detection of cardiovascular abnormalities, identification of cerebrovascular seizures, malignant growth in the breast, prostrate glands, bones, lungs and urinary and gastrointestinal tracts employing tumour specific radioisotope tracers (Wechalekar & Cook, 2005). Radioisotopes are also used to detect sub-radiologi¬cal fractures otherwise invisible to X-Rays (Cook, 2005). Therapeutical applications are site specific and generally involve endocrine system for detecting abnormalities in the thyroid glands such as hyperthyroidism and thyroid carcinoma; methods to address liver cancer and a complicated radioimmunotherapy for various haematological malignancies have recently been developed (Brans et.al, 2006).
The difference in affinity of the radioisotope to attach to an unhealthy tissue site and neighbouring healthy site limits the resolution of the image generated which is a diagnostic limitation of the technology. Administering radiation therapy involves side effects which depend on the treatment, dosage, organ and the patient and ranges from minor skin damage, hair loss and infertility to even secondary malignant growth, which limits non-malignant use of radiation therapy (Nieder et al., 2000).
Dedicated research is in progress to improve and make Ultrasound and Radioisotope treatment safer.