For certain types of clinical cases, imaging techniques can be used that do not require the use of ionizing radiation. For example, high-frequency sound waves are used in Ultrasound scans. In magnetic resonance imaging (MRI), radio waves are used together with strong magnetic fields.
Ultrasound, sometimes known as sonography, is used to view soft tissues such as muscles, internal organs and blood flow especially in real-time, while MRI delivers excellent soft tissue contrast with high resolution.
In Ultrasound imaging, computerized images are produced by sound waves reflected by organs and other interior body parts in real-time.
Magnetic resonance imaging (MRI) is a sophisticated medical imaging technique that uses magnetic fields and radiofrequency to visualize the body’s internal structures.
Ultrasound imaging (sonography) is a diagnostic medical procedure that uses high-frequency sound waves to produce dynamic visual images of organs, tissues or blood flow inside the body. The sound waves are transmitted to the area to be examined and the returning echoes are captured to provide the physician with a ‘live’ image of the area. Ultrasound does not require the use of ionizing radiation, nor the injection of nephrotoxic contrast agents.
Ultrasound has several advantages which make it ideal in numerous situations, in particular, studies of the function of moving structures in real-time. It can be used to examine many parts of the body, such as the abdomen, heart and blood vessels, breasts, muscles, carotid arteries, and female reproductive system including pregnancy and prenatal diagnostics (Figure 1)1. Because of its non-ionizing nature, it is a good choice for imaging when radiation-sensitivity is a concern, such as in pediatrics or in women of child-bearing age.
Fig. 1: Prenatal 3D ultasound showing baby's head, arms and hand.
Ultrasound is based on sonar, and uses a machine with a computer processor to create Ultrasound images. A transducer is placed on the area of the body to be visualized. Returning sound waves, or echoes, are processed through the computer and converted into images.
The specific form of Ultrasound that is used to examine the heart is known as echocardiography and allows the heart valves and blood flow to be visualized.
MRI systems use a powerful magnetic field and radiofrequency pulses to produce detailed images of the body’s internal structures as cross-sectional images or slices (Figure 1). Without exposing the patient or staff to ionizing radiation (X-rays), MRI provides high quality images with excellent contrast detail of soft tissue and anatomic structures such as gray and white matter in the brain. It’s used in a wide range of examinations from brain tumors and inflammation of the spine to slipped discs, assessing blood flow and functioning of the heart. MRI does not emit any ionizing radiation.
Fig. 1: MRI provides excellent high contrast definition, as shown in this image of the thorax.
Many diseases, such as certain brain tumors, can be visualized using MRI because of a high contrast definition, which does not always require contrast agents to produce detailed images of blood vessels. MRI scanners can image a wide range of body parts including injuries of the joints, the blood vessels, the breast, as well as abdominal and pelvic organs such as the liver or reproductive organs.
The MRI system consists of a very powerful superconducting magnet that creates a static magnetic field, smaller ‘gradient’ magnets that allow the magnetic field to be very precisely altered and designated coils for specific body parts that emit radiowaves.
During the examination, the gradient coils are used to ‘focus’ the magnetic field on the part of the body to be scanned. The radio signal is turned on and off and the energy absorbed by different atoms is reflected back out of the body. The coil measures these radio waves and the computer then calculates the way they have been absorbed or reflected to compile the cross-sectional images. The tapping sound heard during the examination is caused when gradient magnets are switched on and off.
MRI scanners can acquire direct views of the body in almost any orientation and without exposing the patient or staff to ionizing radiation (X-rays). Care must be taken, however, in patients with implants, because they might be affected by the strong magnetic field. Patients with common pacemakers, for example, can generally not have an MRI scan. There is also a slight risk of allergic reaction by some patients to contrast agents, if used.