CT, MRI and ultrasound: acquisition and interpretation limits
Cross-sectional modalities form images through different physical mechanisms. CT reconstructs X-ray attenuation, MRI uses magnetic resonance signals and ultrasound uses acoustic interactions. A technique suitable for one question may not supply the same information as another modality. Selection and protocol design should connect the clinical task, patient factors, safety and the information needed rather than assume that the newest or most complex method is always preferable.
In a conventional CT number definition, water provides the zero reference and air is approximately minus one thousand Hounsfield units. Window level and width determine the displayed range, not the underlying attenuation. A stated window centered at 40 with a width of 400 spans minus 160 to plus 240. Changing the display can reveal a finding but does not change the acquired tissue data. Reconstruction thickness, kernel, field of view, motion and artifacts also affect what can be seen.
Helical pitch relates table travel per rotation to the nominal total beam width. Its relationship with exposure depends on which other parameters are held fixed and how the scanner controls output. It is not safe to claim that increasing pitch always reduces patient dose by a fixed proportion. Bolus tracking responds to a monitored enhancement criterion, while fixed delay protocols rely on timing assumptions that can be affected by circulation and injection factors. No universal liver phase delay or heart-rate prescription is established here.
MRI signal depends on sequence and tissue context. T1 describes longitudinal recovery and T2 describes transverse relaxation; T2-star also reflects field-inhomogeneity effects. Spin-echo refocusing and gradient-echo behavior therefore differ. Fluid is commonly bright on conventional T2-weighted imaging, while suppression methods can change that appearance. Diffusion-weighted brightness should be considered with the apparent diffusion coefficient and other findings so that T2 shine-through is not mistaken automatically for restricted diffusion. Metal can produce artifacts and safety questions that require separate assessments.
Ultrasound transducers use the piezoelectric effect. Higher frequency generally improves certain resolution characteristics while increasing attenuation and limiting penetration. Doppler measurements depend on the angle and the acquisition method; in the ideal stated equation, the cosine term is zero at a right angle. Pulsed-wave sampling can alias when a Doppler shift exceeds the appropriate Nyquist limit. Acoustic enhancement and shadowing convey information about transmission but should be interpreted with the full examination. These principles support protocol reasoning and recognition of limitations, not a guarantee that one image appearance establishes a complete diagnosis.