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 medical 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.

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