Ultrasound
Ultrasound
Unlike other imaging modalities, ultrasound resolution and penetration depend on the center frequency and type of transducer 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 used focal depth to aperture ratio is five, so that the half-power beam width is approximately two wavelengths at the center frequency; therefore, the transmitted lateral spatial resolution is about two wavelengths. For typical frequencies in use ranging from 1 to 15MHz, lateral resolution ranges from 3mm to 0.3mm and is the smallest in the focal region and varies elsewhere in a nonuniform 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 major factor in determining resolution is attenuation that limits penetration. Attenuation increases with higher center frequencies and depth; therefore, penetration decreases correspondingly so that fine resolution is difficult 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 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 to hundreds of frames per second. Diagnostic ultrasound is noninvasive. Ultrasound is also safe and does not have any cumulative biological side effects. Two other strengths of ultrasound imaging are its relatively low cost and portability. With the widespread availability of miniature portable ultrasound systems for screening and imaging, these two factors will continue to improve. A high skill level is needed 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 required to find relevant planes and targets of diagnostic significance and to optimize instrumentation. Furthermore, a great deal of experience is required to recognize, interpret, and measure images for diagnosis.
Related pages:
- Medical Imaging
- Electrocardiogram [ECG]
- Electroencephalogram [EEG]
- How does ultrasound work?
- Computed tomography [CT Imaging]
- Magnetic resonance imaging [MRI]
- Positron emission tomography [PET]
- Commonly Used Radionuclides in Nuclear Medicine
