Application of Ultrasound, MRI and PET in Cardiac Disease Diagnosis

Transcription

1 Application of Ultrasound, MRI and PET in Cardiac Disease Diagnosis Zarina Che Amin Ultrasound is defined as sound above the hearing range of normal human and similar to audible sound. Ultrasound provides in its simplest form the means to obtain a limited amount of structural information without exotic technology and with good degree of patient and operator safety. The principle for ultrasound involves usage of a transducer usually called as probe which contains crystals that send out and receive the ultrasonic wave through the body at certain speed (m/s). The wave will be refracted and part of the wave will be reflected back and detected by the probe. The rest of the ultrasonic wave continues deeper into the body, and is reflected as an echo from the surface of tissues lying further inside the body. The quantity of reflected waves depends on the densities of the respective tissues and the speed of the sound wave as it passes through these tissues. Also, the time taken for the reflected wave to return indicates the depth of the tissue lies within the body. In this way, one can portrait the relative locations of the tissues in the body. Attenuation, absorption and scattering are crucial for determining the ultrasound frequency for particular application. Table 1 depicted the clinical application for a range of frequencies of ultrasound and operating depth. This is to ensure the appropriate specific depth is comply thus reduces the interference from moving interfaces outside the intended region. 1

2 Table 1: Ultrasound frequency range for specific clinical application Clinical Application Depth of focused field Ultrasound Frequency Obstetric mm 2MHz Cardiovascular mm 4MHz Peripheral Vascular 8-15 mm 8 MHz Opthalmic / Peripheral 7-10 mm 10 MHz The application of Ultrasonic imaging of the soft tissues of the body began in the early 1970s. The imaging technologies evolve and become available to capture and display the echoes backscattered by structures within the body as images, at first as static compound images and later as real-time moving images. The development followed much the same sequence that resembled similar terminology as in radar and sonar, from initial crude single-line-of-sight displays (Amode, amplitude mode) to recording these side by side to build up recordings over time to show motion (M-mode, motion mode), to finally sweeping the transducer either mechanically or electronically over many directions and building up two-dimensional views (B-mode, brightness mode or 2D). Figure 1 showed example of M mode imaging. Figure 1 (a) Diastole 2

3 Figure 1(b) Systole FIGURE : Example of M-mode imaging of a heart at two points during the cardiac cycle. (a) Upper panel shows heart during diastole (relaxation) (b) The lower panel shows the same heart during systole (contraction) A-mode and M-mode displays are able to demonstrate the motion of valves, thickening of heart chamber walls, relationships between heart motion and pressure, and other parameters that enabled diagnoses of heart problems that has not been made possible earlier. For some vascular diseases, the preferred display format for diagnosis is still the M-mode, on which the speed of valve motions can be measured and the relations of valve motions to the electrocardiogram (ECG) are easily observed. Additionally, when 2D displays became available, ultrasound was used to a greater extent in imaging of the soft abdominal organs and in obstetrics. In this format, organ dimensions and structural relations are observed without difficulty and since the images are now available in real time, motions of organs such as the heart are still well appreciated. Real time 3D (RT3D) is the latest 3D cardiac imaging modality based on matrix-array (rather than linear-array) technology. It offers advantages over conventional 2D ultrasound imaging in that 3D volumes can be acquired rapidly on line ( real-time ) minimizing acquisition artifacts due to respiration and transducer movement and over other 3D cardiac imaging techniques in that RT3D s systems are portable, and acquire 3D data at instance. 3

4 Advantages and disadvantages Ultrasonic imaging has many economic advantages over other imaging modalities where the imaging systems are typically non-ionising and much less expensive than those used for other modalities and do not require any special preparations of facilities such as shielding for x-rays or uniformity of magnetic field for MRI. Most ultrasonic imaging systems easily rolled from one location to another, so one system can be shared among technicians or examining rooms or located to patient s room for critically ill patients. There are minimal expendables used in ultrasonic examinations, mostly the coupling gel is used to couple the transducer to the skin and videotape or film for recording. Transducers are reusable and amortized over many examinations. These low costs make ultrasonic imaging one of the least expensive modalities, far preferred over others when indicated. The disadvantages of ultrasound are the inability to penetrate gas and bone. On the other hand, the quality of the image depends on the sonographer and thus increases chances for artifacts. The operator must be able to think cross section ally for 2D ultrasound. Magnetic Resonance Imaging (MRI) Imaging The next prevalent imaging is the MRI. Principle MRI is an imaging technique that takes advantage of the property of certain atomic nuclei to vibrate or resonate when exposed to burst of magnetic energy. When the hydrogen nuclei resonate in response to changes in a magnetic field, they emit radiofrequency energy. The MRI machine detects this emitted energy, and converts it to an image. Hydrogen nuclei are used because hydrogen atoms are present in water molecules (H2O), that makes it suitable to be used by body tissues. 4

5 The images obtained by MRI scanning are remarkably precise and detailed. With current MRI machines, these images are generated as 3-D projections. Once a 3-D MRI image is obtained it can be sliced and examined in detail in any plane. Also, slight differences in the hydrogen atoms between various parts of a tissue differences caused, for instance, by differences in blood flow or in the viability of the tissue emit different amounts of energy. These energy differences show up as different shades of grey on the MRI image. Thus, the MRI offers a potential means of detecting areas of cardiac tissue that have poor blood flow (as in coronary artery disease) or that has been damaged (as in a heart attack). However, there are several technical drawbacks in imaging moving structures for instance imaging of the heart with MRI. Movement of the heart during scanning will distort the image significantly. Magnetic resonance imaging (MRI) is a clinically important medical imaging modality due to its exceptional soft-tissue contrast. MRI has proven very useful in evaluating patients with aortic dissection prior to surgery. The detailed images offered by MRI informed the surgeon precisely location of the tear in the wall of the aorta begins and the full extent of the dissection. MRI can also locate and characterise the rare cardiac tumour. MRI has the potential to diagnose heart attacks in patients presenting with chest pain and can detect myocardial infarction immediately thus can reduce the time it takes to begin definitive treatment. A new MRI processing technique called black-blood MRI seems to be able to distinguish significantly between normal and atherosclerotic coronary arteries. While further refinements are necessary, such techniques will be the potential in the near future where MRI will be able to replace cardiac catheterisation for diagnosing coronary artery disease. 5

6 MRI has already proven useful in the research setting for identifying restenosis after angioplasty. MRI might thus prove an accurate, non-invasive means of following patients after angioplasty. Furthermore, MRI has the potential of detecting changes in the tiny blood vessels of the heart that are completely missed by cardiac catheterisation. Detecting such changes seem to be useful in predicting the outcome of patients after a heart attack and may prove to be useful in assessing patients with cardiac syndrome, diabetes and certain other conditions. Ultimately, MRI may replace the x-ray tube in both diagnostic and therapeutic situations. Advantage and disadvantage MRI has major advantages against other imaging modalities where these advantages are largely potential advantages and will only be realised until technology currently being tested becomes more refined and widespread. MRI has the potential of replacing 4 other cardiac tests which are echocardiogram, the MUGA (Multiple Gated Acquisition) scan, the thallium scan and diagnostic cardiac catheterisation. This is because, MRI does not expose the patient to ionising radiation and the images generated by MRI are remarkably complete, detailed and precise far more so than other cardiac imaging tests. The disadvantages of cardiac MRI are during scanning process that requires a patient to be placed in the MRI scanner can induce significant claustrophobia. Patients with medical devices such as pacemakers and implantable defibrillators as well as some artificial heart valves could not utilize MRI imaging. This is because the MRI image will become distorted by metal when there is metal object such as surgical clips or stents inside the patient body. Normally, a metal detector is used to check on patient as a safety procedure prior to enter the MRI room to avoid any complication to the device or the patient. The MRI imaging device is extremely complex and expensive, thus it needs a well trained technician. 6

7 Positron Emission Tomography (PET) The other imaging method is PET. Principle PET is a major diagnostic imaging modality used predominantly in determining the presence and severity of cancers, neurological conditions and cardiovascular disease. A PET scan is a painless test that will inspect and examine different parts of the body. With PET imaging, the amount of blood flowing to an area of the body and how well the tissues in that area are using nutrients such as oxygen will be made known. Also, PET imaging is capable to provide relevant information regarding medication and chemical content inside human body. Generally, a PET scanner consists of an array of detectors that surround the patient. Using the gamma ray signals given off by the injected radionuclide PET measures the amount of metabolic activity at a specific site of the body the signals are resembled as signals. For instance, cancer cells have higher metabolic rates than normal cells and represented as denser areas on a PET scan. PET is useful in diagnosing certain cardiovascular and neurological diseases because it highlights areas with increased diminished or no metabolic activity, thereby pinpointing problems. Most modern PET scanners are multilayered with 15 to 47 levels or trans-axial layers to be reconstruct. Application A positron emission tomography (PET) scan is a unique non-invasive nuclear imaging technique that produces three-dimensional images of the living heart, brain or other organs. By measuring both blood flow (perfusion) and metabolic rate within the heart, physicians using PET scans can pinpoint areas of decreased blood flow, such as those with blockages and differentiate living muscle from damaged muscle, which has inadequate blood flow. This information is particularly important in patients with previous myocardial infarction or patient that has undergo angioplasty procedure or 7

8 coronary artery bypass surgery. A PET scan will provide accurate information because it actually shows your heart at work. Advantage and disadvantage The major advantage of PET scans is the images are less likely to be distorted by parts of the patient s body such as large breasts, obesity etc. Secondly, PET is capable to examine blood flow to the heart muscle and provide information of heart disease signs and with PET imaging, medical practitioners can determine if heart muscle is malfunction or otherwise thus PET provides high level of accuracy for measurement of myocardial perfusion and viability. This procedure enables the clinician to determine the value and course of treatment for patients with heart disease. The risks associated with PET scan are minimal due to the reactions to the radioactive "tracer" injected. The level of tracer given to a patient is very tiny and the chances of a reaction occurring are very low. The disadvantage of PET scans can sometimes lead to misdetection as cancer lesion due to areas of high activity. Inflammatory conditions like rheumatoid arthritis or tuberculosis absorb excessive tracing process and may cause confusing results. A PET scan is less accurate in certain situations such as less active tumours may not absorb much tracer, small tumours (less than 7mm) may not be detectable and high levels of blood sugar can cause the cells to absorb this normal sugar rather than the radioactive. Patients will normally need to fast for up to 4 hours before a PET scan procedure and blood sugar levels will be measured to lower the chances of misdiagnosed. The radioactive substance has a very short decay and therefore appointments must run on schedule. PET scans are a very expensive form of imaging and are not readily available. They often accompany other scans such as CT and MRI in order to be diagnostically effective. 8

9 Integration of the review on Hypertrophic Cardiomyopathy (HCM) HCM is the most common cause of cardiac death among young people and affects one in every 500 individuals, including professional athletes. It is characterised by an enlargement of the left ventricle, the heart's main pumping chamber. This enlargement results in a thickening of the walls of the heart, which then prevents the heart from functioning properly. 1 st scenario: Ultrasound imaging technique is used as inexpensive method to diagnose HCM, without the risk of an invasive and an ionising procedure. A real-time ultrasound is used to analyse the movement of heart muscle moves during contraction and relaxation thus identify the thickened of the heart. Such an early warning system could allow physicians to intervene with appropriate therapies and thus prevent problems that could lead to heart attack or heart failure. 2 nd scenario: MRI imaging technique is considered an expensive method to diagnose HCM, in addition to being non-invasive (not requiring an incision) and riskfree, with MRI imaging technique a large portion of the body, such as the chest, in one session can be done. This allows MRI to display abnormalities in cardiac chamber contraction and to show abnormal patterns of blood flow in the heart and great vessels. Using MRI, physicians can obtain images of the chest and cardiovascular system from many angles. This allows better assessment of complex anatomic abnormalities than with other imaging techniques. 3 rd scenario: PET imaging technique is also an expensive method to diagnose HCM. Patient is requested to consume [18] F-fluorodeoxyglucose and [11] C-acetate. This technique is incapable to provide significant result for the physician to make decision. As a conclusion, ultrasound method is the most appropriate choice and MRI will be the second choice in failing to obtain a good image via ultrasound. 9

10 Reference [1] Röntgen WC., (1896), "On a new kind of rays"(english translation), Nature, 53: [2] Eisenberg RL., (1992), Radiology: An Illustrated History. [3] Beth W.O., (2007), Cardiac Imaging Radiology Today Vol 8 No.5 P.24 [4] Journal of Applied Physiology (2005) [Online] JApplPhysiol [5] Wolf KJ, Fobbe F, eds (1995) Color Duplex Sonography. Principles and Clinical Applications [6] Krebs CA., Giyanami VS., Eisenberg RL.,(1999) Ultrasound Atlas of Vascular Diseases [7] Nanda NC., Schlief R, Goldberg BB, (1997) Advances in Echo Imaging using Contrast Enhancement 2nd Edition. [9] Forsberg F., Liu JB., Burns PN., Merton DA., Goldberg BB., (1994) Artifacts in ultrasonic contrast agent studies J Ultrasound Med 1994; 13: [10] Fürst G., Saleh A., Wenserski A.,(1999) Reliabiltiy and validity of noninvasive imaging of internal carotid pseudo-occlusion [11] Hennerici M., Neuerburg-Heusler D.,(1998) Vascular diagnosis with ultrasound 10