OVERVIEW OF DIAGNOSTIC RADIOLOGY METHODS:
Dr. Ünal KURTOĞLU Bursa Şevket Yılmaz EAH. Radiology Specialist.
The character of “Diagnostic Radiology”, which was born with the discovery of X-ray by Wilhelm Conrad Röntgen in 1885, has developed and changed significantly in the last 20-30 years. In fact, in the last 10-15 years, radiological treatment methods have been developed besides radiotherapy and “Interventional Radiology” was born. Knowing the formation mechanism of the image is indispensable for the accuracy of its interpretation. In this article, it is aimed to briefly repeat the imaging mechanisms and usage areas of radiology methods.
Diagnostic radiology methods can be grouped into 5 main groups (Table-1). While X-ray, which is ionizing radiation, is used for image formation in X-ray and computed tomography, sound waves are used in ultrasonography, radiofrequency in MRI and gamma rays, which are ionizing radiation in radionuclide imaging.
Table-1:Diagnostic Radiology Methods
1-Conventional Radiography (X-Ray):
a-Gray scale ultrasonography
3-Computed Tomography (CT)
4-Magnetic Resonance Imaging (MRI)
5-Radionuclides Imaging (Nuclear Medicine)
1-Conventional Radiography (X-Ray):
In this method, a two-dimensional image of the tissues is formed as a result of the transmission (transmission) of the X-ray through the tissues, that is, there is a superposition of the tissues. Due to the difference between the densities, thicknesses and atomic weights of the tissues through which X-rays pass, the amount of radiation that can or cannot pass (absorbed) through each tissue is different. These differences are detected with x-ray film or with detectors (in digital x-ray) and an image is created. 4 basic shadows or densities can be defined in x-rays. These are from the most dense (white-radiopaque) to the less dense (black-radiocente); bone, fluid (soft tissue, blood), fat and air (Picture -1). Metals and contrast agents also appear white.
Picture.1: Density differences seen in X-ray: In the first X-ray taken by WC X-ray, bone and metal appear white, soft tissues appear in gray tones, and the air appears black. Anatomical structures that are not normally visible can be visualized with the use of contrast agents.
Radiopaque or radiolucent contrast agents can be used in X-ray. While radiopaque contrast agents are heavy metal salts such as barium sulfate or organic iodine compounds, gases such as air, oxygen and carbon dioxide are used as radiolucent contrast agents. Gastrointestinal system with orally administered contrast agents, vessels with IV or intraarterial contrast agents, kidneys and urinary system as in IVP can be visualized. In mammography, it is aimed to display higher soft tissue details by using low-dose, homogeneous X-rays. With mammography, it is aimed to detect small breast tumors early and increase patient survival. As the woman ages, the breast parenchyma tissue decreases and is replaced by adipose tissue. Therefore, as age progresses, dense tumor tissue in radiolucent adipose tissue is more easily recognized. According to the recommendation of the American Cancer Society, basal mammography screening is required for asymptomatic women at the age of 35-39, every 1-2 years at the age of 40-49, and annually at the age of 50 and older.
Angiography is currently performed digitally and is called Digital Subtraction Angiography (DSA). In this technique, the non-contrast image memorized is subtracted (subtraction) from the image created by the contrast agent administered from the vein, and only the image of the contrast agent, that is, the image of the vessels, is obtained.
2- Ultrasonography (US):
In ultrasonography, sound beams with a frequency higher than 2 Mhz are sent to the tissue and an image is formed by evaluating the intensity and return time of echoes reflected from the tissue. As the frequency of the sound beam increases, the resolution increases, but as the absorption of the sound increases, the penetration decreases. In other words, a low frequency range is used to image deeper tissues, but the image resolution is also lower. Ultrasound technology does not use ionizing radiation and is less expensive than other techniques.
It is an indispensable modality of obstetric imaging because it does not use radiation. Real-time ultrasound images are like sequential video images, so they play a very large role in imaging moving organs such as the heart. While there are no or very few echoes in fluid-filled tissues and cysts (black), highly homogeneous small echoes occur in organs such as the liver due to the fibrous interstitial internal structure (shades of gray). Calcifications, oil, and air produce high-intensity (white) echoes. Since the transmission of sound waves is less in the air than in solids and liquids, the gas in the organs (especially the intestines) significantly prevents the formation of images. On the other hand, in dense tissues such as bone, the penetration of the sound beam will decrease, so image formation is prevented.
Ultrasonographic imaging; is limited to the parameters set up on the device, the practitioner, and the patient’s field of view. Correct interpretation of images will not be possible if proper parameters are not set on the device. Doppler shift; The frequency increases as a fixed frequency sound source approaches, and decreases as it moves away. By using this physics rule, the flow direction, velocity and shape of the moving blood cells can be determined. While evaluating blood flow with Doppler US, the basic principle is to determine the change in the frequency of the sound beam sent to the vessel at a certain angle, according to the direction and velocity of the flow.
3- Computed Tomography (CT):
In computed tomography; As the X-ray beam rotating around the patient passes through the patient, the weakening of the tissues according to the absorption differences is detected by the detectors and images are created via the computer. New generation multidetector devices can take 64-128 or even 256 sections in one exposure with spiral or helical technique. A large body part can be viewed simultaneously on CT. For example, in the examination of the thorax, all tissues, from the skin and subcutaneous tissues to the lung parenchyma, major vascular structures and spinal column, are visualized in their actual position without superposition. Since the data is in computer memory (digital), it can be created in 3D or reformat images. By changing the windowing intervals, tissue layers of different densities can also be examined. In CT, as in x-ray, intravascular and other anatomical structures can be more clearly distinguished by the use of contrast material. With the development of multidetector multislice CTs, the vessels of moving organs such as the heart (CT coronary angiography) can be evaluated by non-invasive methods.
Density differences in CT are similar to those in X-rays. Dense tissues such as metal and bone are coded as white (hyperdense), soft tissues as light grey, fat as dark gray and air as black (hypodense). One advantage of CT is that it can show the X-ray absorption of a given area. This is evaluated using the “Hounsfield unit” and the density of the water is considered “0”. With this method, it can be determined whether the lesion is solid or cystic. The most important handicap of the method is the use of 10-100 times more radiation in CT compared to X-rays. With the developing technology, the amount of radiation is significantly reduced compared to previous generations.
4-Magnetic Resonance Imaging (MRI):
Hydrogen atom nuclei (protons) in a strong magnetic field absorb energy when excited by a radio wave of the appropriate frequency, located at the low energy end of the electromagnetic spectrum. After the radio wave is interrupted, they emit the energy they receive as an alternating current signal. These signals are collected and processed in computers to create a cross-sectional image as in CT. Since the signals emitted from the patient are collected, MRI is a method dependent on the “emission” event and does not use ionizing radiation. MRI is the diagnostic method with the highest soft tissue contrast resolution. Very good visualization of soft tissues close to the bones, visualization of vessels without contrast, and the fact that it does not use radiation are its important advantages over CT. Although T1 and T2 weighted images are the two main sequences of the method, there are many more MRI techniques. Roughly, normal anatomy can be evaluated on T1-weighted images, and pathologies can be better evaluated on T2-weighted images. Fat is white (hyperintense) and water (CSF) is black (hypointense) on T1-weighted images. On T2-weighted images, fat is coded black (hypointense), edema, blood and CSF white (hyperintense). Unfortunately, bone and calcium are difficult to visualize on MRI. The disadvantages of MRI include sensitivity to motion, artifact formation due to the proximity of ferrous objects to the magnetic field, and expensiveness.
5-Radionuclides Imaging (Nuclear Medicine):
In radionuclide imaging; A short-lived radioactive substance that emits gamma rays is given to the patient and its distribution in the organism is determined by detectors. Radionuclide imaging is an “emission” method that uses ionizing radiation. Gamma rays coming from the examined organ form a two-dimensional shape of the organ consisting of dots, and the radioactivity in the third dimension is superposed. To eliminate this superposition, CT was added to the system and it was named “Single Photon Emission Computed Tomography” (SPECT). The tomographic scintigraphy method using positron source radionuclides is called “Positron Emission Tomography” (PET). The radiation dose is related to the type and amount of radioactive material administered to the patient. Besides providing morphological information, the main advantage of Radionuclide Imaging is that it can show physiological function. Physiological evaluation such as renal function, myocardial perfusion, or regional lung aeration cannot be performed with any of the other imaging modalities.
As a result; Although the presence, localization, spread, change and character of the lesions can be defined by radiological diagnosis methods, histopathological diagnosis cannot be made. Although they have several advantages over each other, radiological diagnostic methods often give similar information. For this reason, the working principles, advantages and limitations of the methods should be well known. In fact, it is the most accurate method to decide which method will be used in specific cases together with the radiologist.
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