The Choice of Imaging Modality for the Patient with Breast Lump - Essay Example

? Imaging modalities Insert (s) Imaging modalities Introduction At the age of 35, the risk of breast cancer substantially increases among women and certain imaging modalities may not be safe to be used during the medical examination of the lumps. The choice and appropriateness of an imaging modality often depends on age of the patient, the optimal performance of the particular imaging technique, safety considerations as well as the medical circumstance of under examination (Berna, Nievas and Romero, 2001, p.6). Consequently some of the medical imaging techniques may expose the patient to high levels of radiation which not only may result in misdiagnosis of the condition but may also increase the risk of the diseases such as breast cancer on the patient. Medical imaging techniques employ various principles of medical physics to create human body images for purposes of diagnosing, revealing and medical examination purposes as well as during the normal physiology and anatomy studies. All the radiographic imaging modalities are however considered non-invasive in the sense that they do not produce the images of the internal aspects of the body through physical penetration of the skin. The imaging modalities such as ultrasound imaging, magnetic resonance imaging and computerized tomography are preferred as some of the best ways of screening and diagnosing breast cancer particularly in high risk women such as those over the age of 35. These techniques are designed to produce images of the internal aspects of the body through non invasive means. For instance ultrasound imaging techniques uses ultrasound pressure echoes and waves within the human tissues to reveal the internal structures of the body. Focusing on the physics of imaging, this paper critically analyzes the safety considerations and suitability of ultrasound, CT, MRI and mammography modalities of imaging as the best choices of imaging for a 35 year old woman with an equivocal breast lump. The physics of ultrasound imaging Ultrasound imaging techniques are based on the principle of pulse-echo and radar technologies. The use of these technologies in medical imaging became popular after World War II when a number of electronic pulse technologies were developed. The Ultrasound principles used in the contemporary medical imaging generally utilizes ultrasonic sound waves (sounds beyond human hearing capability) to generate electrical signals (Gent, 1997, p. 106). The human hearing generally detects sound waves of between 20 to 20,000 Hz but ultrasound frequency sound waves are mostly between 3 to 10 MHz. Before the signals are converted into an image they should be able to detect by an ultrasound transducer. The underlying principle behind an ultrasound transducer is based on the piezoelectric effect of the materials such as quartz that are often used to make the transducer (Bontrager and Lampignano, 2010, p.207). Such materials often have the ability to change their shape when an electric current is passed through them. Similarly a change in the shape of a piezoelectric material can also produce an electric current. During medical diagnostic imaging processes, a given amount of oscillating current is normally applied to the piezoelectric material in order to make it rapidly vibrate and produce ultrasound waves. The transducer emits ultrasonic waves which invasively penetrate the human tissues at an average velocity of nearly 1540m/s. When ultrasound waves detect a surface or object of a different acoustic nature or texture, they are usually reflected back and received inform of echoes by the apparatus. The echoes are then converted into electric current which are eventually used to produce 2 D images. In this regard, as many as 20 frames of images such as of the internal body aspects can be generated every second to give a smooth and real-time image. Some of the key properties of ultrasound waves include the fact that these waves are often attenuated through scattering and absorption. The waves may also be refracted or reflected just like light rays subjected to different media. In this regard, reflected ultrasound waves during the imaging process returns the signals to transducer apparatus and this often takes place at the boundary between the two different body surfaces. In ultrasound imaging, acoustic impedance is generally a measure of the rate at which ultrasound travels across the tissues such as the fat, calcified, fibroids or glandular tissues in human breasts. According to Edelman (1996, p.48), acoustic impedance is calculated as density of the tissues (p) X velocity of the ultrasound propagation (v). Hence Z=pv and is measured in kgm-2s-1. The basis of examination of different tissues in ultrasound is the use of echoes. For example bones or calcified tumors tend to have the highest acoustic impedance as compared to the soft tissues such as fats, fibroids and glandular tissues. On the other hand, refraction of the waves may take place at the oblique angles found between the different tissues of the body and this often results in errors of their depth estimation. Ultrasound imaging technique is preferred as one of the modalities which can be used in the medical examination of a 35 year old woman with unequivocal breast lumps because it is relatively safe and is particularly suitable for single field applications such as breast examination. The other potential advantages of this modality include the fact that it does not have ionizing radiation, inexpensive and is non invasive. The few disadvantages of ultrasound imaging however include poor image resolution and its limited use to only particular body sites. The physics of Mammography Mammography is another common imaging technique that can be effectively used for screening of possible case of breast cancer particularly in women beyond the age of 35 years. Currently traditional mammography which uses screen film is increasingly being replaced by digital mammograms which offer high quality images useful for diagnostic purposes. There are different principles that are used in building the X-ray detectors used in digital mammography. Generally all detectors have several layers including a conversion layer and a substrate layer (Berna and Romero,2001,p.7). The conversion layer of a mammogram is designed to transform the X-ray photons into visible light electrons and photons while the substrate layer is used to collect these light particles and covert them into images. The images obtained in mammography are generally absorbed at varying rates depending on the nature of the tissues such as muscles, bone or fat tissues. Some of the potential advantages of using mammography for cancer diagnostic purposes include high resolution images. The safety concern of this modality is however the fact that the radiation from the X-ray can result in tissue damage if the breasts are exposed to them for a longer time. Physics of Magnetic Resonance Imaging (MRI) Magnetic resonance imaging modality uses the properties of nuclear spin to produce detailed tissue and organ images. As opposed to the use of ionizing radiation, MRI requires the use of strong magnetic field known as a magnetic moment which is often created by spinning protons, electrons and neurons around a central axis (McRobbie, Moore, Graves and Prince, 2007, p.34). The changing magnetic field and a computer are used to create the images of internal body aspects during medical examination of tissues and organs for potential abnormal conditions, injury or diseases. During the MRI imaging process, tunnel shaped MR scanners are normally used. The powerful magnetic field (between 1.5 and 3 Telsa) found in the scanners work by aligning the protons found in the body tissues. The radio waves eventually make these proton particles to produce signals that are used by the scanner to create tissue images. According to Horowitz (1992, p.76), the hydrogen atoms are used in the magnetic filed produce a precession effect on the nuclei making it to continuously spin in harmony. On the other hand, when the electromagnetic field is switched off, the nuclei regain their original precession and this often involves the two important processes known as T1 and T2 relaxation. T1 relaxation generally refers to the process by which the magnetic moments are regained towards the initial direction while T2 relaxation is the process of losing nuclear precession after the removal of the electromagnetic filed. The principle through which MRI scans are used to detect potential tumors in tissues is by measuring the proton density of the tissues. For example cancerous tissues often have high cellular activity and increased fluid content. This consequently increases the proton density of the areas as compared to the other normal tissues. Tissue T1 and T2 relaxation times refers to the processes through which the nuclei is able to return back to random precession. In the tissues, the Magnetic resonance signals are achieved only when the tissues spin in the free water pool (Buxton, 2001, p.77). With regard to the patient safety, MRI imaging examination is generally painless and does not result in any tissue damage as compared to the other medical imaging modalities. As a result MRI has increasingly become the preferred imaging technique for the examination of various diseases and abnormal conditions such as equivocal breast lumps. This is particularly with regard to the fact that MRI can create pictures that effectively shows the differences between unhealthy and healthy tissues. Some of the body parts that can be effectively examined by this imaging modality include breasts, pelvic region, blood vessels and brain among other tissues and body parts. The other known justifications for using MRI technology include its higher resolution, absence of ionizing radiation and excellent contrast between the various soft tissues. The physics of computerized tomography (CT) scanning CT scanning is one of the revolutionary diagnostic modalities of medical imaging that generated anatomical images using computerized synthesis of X-ray data from various directions. This modality uses mathematical technique commonly known as algebraic reconstruction to obtain digital images from the X-ray transmission data which are clearer than the normal X-rays (Tubiana, 2008, p.852). Computerized tomography scanning also reveals both the bones as well as soft tissues such as muscles, tumors and organs. When a computerized tomography scan is displayed there are often two significant numbers which are often seen on the screen. The first number is known as the window width (W) and is generally used to describe the Hounsfield unit ranges that are on display. Generally the maximum window width is often about 2000. Unaided human eye is however incapable of detecting this huge number of shades and therefore the window width is usually divided into 16 group’s shades representing Hounsfield values. The highest Hounsfield number in the window range is depicted as white while the lowest is generally shown as black. Another important Hounsfield number is known as the window level (L). This is the Hounsfield unit that is usually found towards the center of the window width. Finally the main advantage of CT imaging technique is that the tone of the images can be adjusted to reveal the tissues which have similar density. On the other hand, when graphic computer programs are used, the data from a number of cross sections can be combined to form a 3D image. CT scanning may be the preferred imaging modality for breast cancer examinations particularly in women over 35 years since it provides 3D accurate data and attenuation information. In X-rays, dose is the amount photons that are passed through the tissue while attenuation refers to reducing intensity of the beams as they traverse across the tissues. This is particularly caused by deflection as well as absorptions of the photons of the beam depending on the nature of tissues. Attenuation coefficient is therefore the value of attenuation radiation that passes through a particular thickness of tissues. Generally in low kV, attenuation is higher and this results in increased contrast resolution. On the other hand, attenuation is comparatively lower in high kV. Conclusion In conclusion, there are a number of considerations particularly with regard to patient safety which can influence and justify the choice of imaging modality for a 35 year old woman with a suspicious breast lump. This is because most of the modalities have some safety concerns related to the radiation and electromagnetic techniques that are used. Consequently our choice of the MRI, CT, ultrasound as well as mammography modalities over other imaging techniques is based on a critical analysis of the advantages as well as the safety concerns of the principles of medical physics used on these modalities. References Berna J.D, Nievas F.J, Romero T. 2001. A multi-modality approach to the diagnosis of breast hamartomas with atypical mammographic appearance. Breast Journal, (7), p.2-7. Bontrager, K.L & Lampignano, J.P. 2010. Textbook of radiographic positioning and related anatomy. (7th ed.). St. Louis: Mosby Elsevier. Buxton R.B.  2001. An Introduction to Functional Magnetic Resonance Imaging: Principles and Techniques. London: Cambridge University Press. Edelman, S. K. 1996. Understanding Ultrasound Physics. Houston: Armstrong and Company publishers. Gent, R. 1997. Applied physics and technology of diagnostic ultrasound. Australia: Milner Publishing. Horowitz, A.L. 1992. MRI physics for radiologists. A visual approach. (2nd ed.). New York: Springer-Verlage. McRobbie, D.W., Moore, E.A., Graves, M.J. & Prince, M, R. 2007. MRI from picture to proton. (2nd ed.). Cambridge: Cambridge University Press. Tubiana M. 2008. Comment on Computed Tomography and Radiation Exposure. N. Engl. J.Med, 358 (8), p.852–853. Read More