How the three imaging modalities compare? We examine each of the three diagnostic imaging methods as discussed in the following sections:
Ultrasound Imaging
Unlike other imaging modalities, ultrasound resolution and penetration depends on the center frequency and type of frequency selected. The resolution is spatially variant and depends on both the size of the active aperture and the center frequency (and bandwidth) of the transducer and the selected transmit focal depth.
A commonly employed focal depth to aperture ratio is five, so the half-power beam width is approximately two wavelengths at the center frequency, and therefore the transmitted lateral spatial resolution is about two wavelengths. For normal frequencies in use ranging from 1 to 15 MHz, lateral resolution ranges from 3 mm to 0.3 mm and is the smallest in the focal region and varies elsewhere in a non-uniform way because of diffraction effects caused by apertures on the order of a few to tens of wavelengths. For a short pulse, axial resolution is approximately two wavelengths.
Another key factor in determining resolution is attenuation that limits penetration. Attenuation increases with higher center frequencies and depth; therefore, penetration decreases correspondingly, therefore fine resolution is hard to achieve at deeper depths.
Ultrasound images are highly detailed and geometrically correct to first-order maps of the mechanical structures of the body according to their “acoustic properties” such as the differences in characteristic impedance that depend on stiffness or elasticity and density. The dynamic motion of organs such as the heart can be revealed by ultrasound operating at up hundreds of frames per second.
Diagnostic ultrasound is non-intrusive. Ultrasound is also safe and doesn’t have any cumulative biological side effects. Other key strengths of ultrasound imaging are relatively low cost and portability.
A high skill level is required to obtain good images with ultrasound. This expertise is necessary because of the number of access windows, differences in anatomy, the many possible planes of view, and the experience needed to find the relevant planes and targets of diagnostic significance and to optimize instrumentation. Additionally, a great deal of experience is needed to recognize, interpret, and measure images for diagnosis.
Related: Ultrasound Scanning Techniques
Computed Tomography (CT) Imaging
Computed Tomography (CT) (also termed to as computed axial tomography or CAT) scanning involves x-rays. As the x-rays pass through the body, they are absorbed by tissue so an overall “mean attenuation” image results along the ray path. Spatial resolution is not determined by wavelength but by focal spot size of the x-ray tube and scatter from tissue; a typical resolution is about 1 mm.
Radioactive contrast agents can be ingested or injected to improve visualization of vessels. Though exposures are short, x-rays are form of ionizing radiation, so dosage effects can be cumulative and extra precautions are required for sensitive organs such as eyes and for pregnancies.
CT equipment is large and stationery so a person can fit inside, and as a result, it is relatively expensive to operate. Consecutive pictures of a moving heart are now achievable through the synchronization to ECG signals. The resolution of CT images is typically 1 mm. CT scanning creates excellent images of the brain, bone, lungs, and soft tissue, making it complementary to ultrasound.
Even though, the taking of CT images requires training, it is not difficult. The interpretation of CT cross-sectional images demands considerable experience for a definitive diagnosis.
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Related: What is CAT scan?
Magnetic Resonance Imaging (MRI)
For magnetic resonance imaging, the patient is placed in a strong magnetic field created by a large enclosing electromagnet. The resolution is mainly determined by the gradient or shape of the magnetic field, and is typically 1 mm. Images are calculated by reconstruction algorithms based on the sensed voltages proportional to the relaxation times. Tomographic images of cross-sectional portions of the body are computed. The imaging process is fast and reasonably safe, since no ionizing radiation is employed. Care must be taken to keep ferromagnetic materials away from the power magnetics used and there are limits to the strength of the applied magnetic fields and how quickly they are switched. Because the equipment required to make the images are expensive, medical exams using this imaging method are costly.
MRI equipment has several degrees of freedom such as the timing, orientation, and frequency of magnetic fields; hence a high level of skill is necessary to acquire diagnostically useful images. Diagnostic interpretation of images involves both a thorough knowledge of the settings of the system and experience.
Related: Magnetic Resonance Imaging (MRI)
