Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Nuclear medicine images (planar images, SPECT, and PET) are maps of thespatial distribution of radioisotopes in the patients. Thus, contrast in nuclear imagesdepends on the tissue's ability to concentrate the radioactive material. The uptakeof a radiopharmaceutical administered to the patient is dependent on the pharmacologicinteraction of the agent with the body. PET and SPECT have much bettercontrast than planar nuclear imaging because, like CT, the images are not obscuredby out-of-slice structures.Contrast in MRI is related primarily to the proton density and to relaxationphenomena (i.e., how fast a group of protons gives up its absorbed energy). Protondensity is influenced by the mass density (g/cm 3 ), so MRI can produce images thatlook somewhat like CT images. Proton density differs among tissue types, and inparticular adipose tissues have a higher proportion of protons than other tissues,due to the high concentration of hydrogen in fat [CH3(CH2)nCOOH]. Two differentrelaxation mechanisms (spinllattice and spin/spin) are present in tissue, andthe dominance of one over the other can be manipulated by the timing of theradiofrequency (RF) pulse sequence and magnetic field variations in the MRI system.Through the clever application of different pulse sequences, blood flow can bedetected using MRI techniques, giving rise to the field of MR angiography. Contrastmechanisms in MRI are complex, and thus provide for the flexibility and utilityof MR as a diagnostic tool.Contrast in ultrasound imaging is largely determined by the acoustic propertiesof the tissues being imaged. The difference between the acoustic impedances (tissuedensity X speed of sound in tissue) of two adjacent tissues or other substances affectsthe amplitude of the returning ultrasound signal. Hence, contrast is quite apparentat tissue interfaces where the differences in acoustic impedance are large. Thus,ultrasound images display unique information about patient anatomy not providedby other imaging modalities. Doppler ultrasound imaging shows the amplitude anddirection of blood flow by analyzing the frequency shift in the reflected signal, andthus motion is the source of contrast.Spatial ResolutionJust as each modality has different mechanisms for providing contrast, each modalityalso has different abilities to resolve fine detail in the patient. Spatial resolutionrefers to the ability to see small detail, and an imaging system has higher spatial resolutionif it can demonstrate the presence of smaller objects in the image. The limitingspatial resolution is the size of the smallest object that an imaging system canresolve.Table 1-1 lists the limiting spatial resolution of each imaging modality used inmedical imaging. The wavelength of the energy used to probe the object is a fundamental limitation of the spatial resolution of an imaging modality. For example,optical microscopes cannot resolve objects smaller than the wavelengths of visiblelight, about 500 nm. The wavelength of x-rays depends on the x-ray energy, buteven the longest x-ray wavelengths are tiny-about one ten-billionth of a meter.This is far from the actual resolution in x-ray imaging, but it does represent the theoreticlimit on the spatial resolution using x-rays. In ultrasound imaging, the wavelengthof sound is the fundamental limit of spatial resolution. At 3.5 MHz, thewavelength of sound in soft tissue is about 0.50 mm. At 10 MHz, the wavelengthis 0.15 mm.