Types of Medical Imaging
X-ray, Ultrasound, Magnetic, and Nuclear Modalities
Among the wide array of technologies in modern medicine, medical imaging is a major advancement. The ability to take pictures of the human body has many useful clinical applications. Different modalities of medical imaging have emerged over the years, each with their own advantages and disadvantages. For any patient who needs his or her body visualized, doctors select the most appropriate imaging studies in order to deliver the best care.
X-ray Imaging
X-rays are waves that have a relatively high frequency along the electromagnetic spectrum. They are absorbed or transmitted by different body tissues in varying amounts, producing different shades of black and white on an x-ray image. In general, bone appears white, soft tissue appears gray, and air appears black.
The basic type of x-ray imaging is plain radiography. This involves an x-ray machine aimed at the patient's body with a recording plate positioned behind the region of interest. Once the machine delivers its radiation, the image is captured on the plate. This allows a physician to assess the bones for fractures, the abdomen for bowel obstruction, and the breasts for signs of cancer (mammography), among other applications.
Certain x-ray studies involve contrast dye to enhance the image, such as visualization of certain arteries (angiography) and the spinal cord (myelography). In such cases, x-ray is delivered as fluoroscopy, constantly delivering radiation to produce a real-time set of images similar to a video. This ensures that an optimal image can be taken.
X-rays are also used in computed tomography (CT). This type of study involves a patient who lies on a sliding table of a CT scanner and moves slowly through a circular rim where x-rays are delivered in multiple directions. A nearby computer combines the data of each x-ray shot and forms two-dimensional slices of the patient's body. CT scans allow clinicians to assess parts of the body that are otherwise not visible on plain radiography.
Ultrasound Imaging
Ultrasound waves have a frequency just beyond that of audible sound. Similar to sonar used by submarines, these waves are emitted and bounce back once they strike an object. As a clinical tool, ultrasound imaging (ultrasonography) can detect differences between solid and liquid material in the body.
Ultrasound studies use a portable machine with a computer and transducer probe. The probe is placed on the region of the patient's body to be visualized. A gel is placed between the probe and patient's body to maximize clarity of the image. The computer takes ultrasonic data and coverts it into visual form. This is useful for noninvasive real-time assessment, such as with fetal development in pregnant women and abdominal organs to rule out trauma or tumors.
Some ultrasound studies involve the additional component of Doppler ultrasound, which visualizes movement of blood cells. The motion is usually represented by different colors on top of the basic grayscale ultrasound image. For example, ultrasound of the heart (echocardiography) allows one to visualize the heart muscle and valves as grayscale images and an overlying mixture of red and blue to represent direction of blood flow.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI), unlike x-ray imaging, does not use radiation. Instead, MRI works based on magnetic waves and the spin of protons. Data is processed by a computer to form the images that clinicians use.
Compared with CT, images on MRI have more detail, which can be useful for analyzing soft tissue that is harder to look at on CT. However, MRI places the patient in a long narrow tube, which can be claustrophobic for some individuals. Furthermore, MRI is not safe for patients with metallic devices or implanted electrical devices, such as pacemakers.
Nuclear Imaging
Nuclear imaging is unique in that the means of visualization are not external waves delivered to the body. Rather, electromagnetic waves are emitted from within. In general, a radioactive marker is introduced into the patient's bloodstream and accumulates in the organ of interest. The marker gives off gamma rays that are detected by a special camera and processed into images by a computer.
This type of imaging is also unique in that nuclear medicine physicians, not radiologists, are the ones who interpret nuclear imaging studies. Applications of nuclear imaging include perfusion of the heart, activity of the thyroid gland, and late-stage cancer.
References
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