= Solution
In <pulse-echo ultrasound>, a transducer emits a pulse and records returning echoes. For a straight round-trip path in a medium of assumed <sound speed> $c$, an echo delay $t$ gives depth $z=ct/2$. <Reflections> depend on acoustic-impedance changes, while attenuation, scattering and propagation geometry affect the received amplitude.
An <A-mode ultrasound scan> displays echo amplitude against inferred depth along one beam. A <B-mode ultrasound scan> maps received amplitude to brightness and assembles many beam directions or translated beam lines into a spatial image. An <M-mode ultrasound scan> repeats one beam line and displays its depth profile versus time, making motion along that line visible. Thus the display axes and acquisition pattern differ even when the pulse-echo principle is shared.
For a pulse lasting $\tau_p$, a simple axial-separation estimate is $c\tau_p/2$. Lateral resolution depends on aperture, wavelength, focusing and depth. Shorter pulses and broader bandwidth can improve axial resolution; an assumed <sound speed> or a multiple-reflection path can instead misplace an echo. Brightness is not a direct universal map of composition.
<Doppler ultrasound> uses <frequency> or pulse-to-pulse <phase> changes from moving scatterers. In a simple backscatter geometry, $\Delta f\simeq2f_0v\cos\theta/c$. Inferring <speed> requires the beam-to-motion angle; pulsed sampling can alias frequencies, and an unknown angle leaves a velocity-projection ambiguity. This describes generic acoustic acquisition rather than a clinical diagnosis.
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