SPI Sample Questions & Answers

The scenario describes color flash or clutter, which is most often caused by the color gain being set too high. The system becomes overly sensitive and amplifies electronic noise or subtle tissue motion, displaying it as color. The first and most effective step is to reduce the color gain until the random noise disappears, leaving only the signal from true blood flow.

Tissue harmonic imaging relies on harmonic frequencies generated within the tissue, not transmitted by the probe. These harmonic signals are strongest in the main beam and weaker in the weaker parts like side lobes and grating lobes. Furthermore, since harmonics are generated deeper in the tissue, superficial reverberation artifacts are significantly reduced. This results in a cleaner image with improved contrast resolution and reduced clutter.

The decibel scale is logarithmic. A change of -10 dB corresponds to a tenfold decrease in intensity. Conversely, +10 dB is a tenfold increase. A change of -3 dB corresponds to halving the intensity, and +3 dB corresponds to doubling it. Since the intensity is reduced to 1/10th, the change is -10 dB.

This statement is true. The purpose of the matching layer is to reduce the large acoustic impedance mismatch between the high impedance of the PZT crystal and the low impedance of soft tissue (skin). By having an intermediate impedance, the matching layer facilitates more efficient energy transfer from the transducer into the body, improving sensitivity and image quality.

The dead zone is the region close to the transducer face where imaging is not possible. It is caused by the time it takes for the system to switch from transmit to receive mode and by reverberation within the transducer itself. Measuring the dead zone on a phantom assesses the system's ability to resolve structures in the extreme near field. A larger dead zone can be caused by a cracked crystal or a detached backing material.

When the PRF is already maxed out, other methods must be used to combat aliasing. The Doppler shift is directly proportional to the transducer frequency. Switching to a lower frequency transducer will decrease the measured Doppler shift for the same velocity, bringing it below the Nyquist limit and resolving the aliasing. Adjusting the baseline only shifts the waveform up or down but does not increase the Nyquist limit. Increasing the wall filter would remove low-velocity signals, not help with high-velocity aliasing. Using CW Doppler is another solution, but switching the transducer frequency is a direct way to solve the issue with PW Doppler.

Spatial compounding is a technique where the ultrasound beam is electronically steered to acquire multiple frames of the same anatomical region from different angles. These frames are then averaged together in real-time. This process reduces speckle artifact, minimizes shadowing, and improves the definition of tissue boundaries and specular reflectors. The main trade-off is a reduction in temporal resolution (frame rate).

Snell's Law describes the bending of a wave as it passes from one medium to another. Refraction requires two conditions: 1) an oblique angle of incidence (not 90 degrees), and 2) a difference in the propagation speeds of the two media. If the beam strikes the boundary at normal incidence (90 degrees), it will travel straight through without bending, regardless of the propagation speeds. If the propagation speeds are identical, refraction will also not occur, but normal incidence is the more fundamental condition that always prevents it.

Power Doppler (or color power angio) displays the amplitude, or power, of the Doppler signal rather than the frequency shift. This makes it more sensitive to the presence of flow, especially slow flow in small vessels. It is also less dependent on the insonation angle. Its main disadvantages are that it provides no information on velocity or direction and is more susceptible to flash artifact from tissue motion.

Comet-tail artifact is a type of reverberation artifact. It occurs when the sound beam encounters a very strong, small reflector (like a surgical clip, cholesterol crystal, or calcification). The sound reverberates within the object itself, creating a series of closely spaced echoes that appear as a bright, tapering trail deep to the structure.