Pediatric applications of abdominal vascular Doppler imaging: Part I

Effects of breath-hold and cardiac cycle on the MRI appearance of the aorta and inferior vena cava in t2 abdominal imaging

OBJECTIVE

The purpose of this article is to review the MR principles of blood signal and normal flow-related phenomena seen in the aorta and inferior vena cava (IVC) on T2-weighted images of the abdomen and to discuss the physiologic and physical basis of signal alterations with breath-hold imaging. We define time-of-flight (TOF) loss and list the factors that affect it; explain the physiologic effects of breath-hold imaging on both aortic and IVC waveforms and velocity; state which abdominal T2 imaging techniques are most susceptible to variable TOF effects and explain why based on physiologic effects and MR principles; and describe three trouble-shooting techniques to confirm that unexpected signal does not reflect pathology.

CONCLUSION

T2 images customarily result in TOF loss and dark blood signal. Abdominal MRI relies extensively on breath-hold imaging techniques that may alter markedly the flow velocity in normal vessels. This marked flow change results in signal heterogeneity and variable TOF loss especially with faster sequences filling k-space in shorter time frames. Breath-hold imaging decreases k-space fill time, and of the breath-hold T2 techniques, single-shot fast spin-echo decreases k-space fill time the most. Atypical blood signal during breath-hold imaging may mimic pathology, but abnormal findings can be verified by review of other sequences and planes of imaging.

Normal Doppler spectral waveforms of major pediatric vessels: specific patterns

Every major vessel in the human body has a characteristic flow pattern that is visible in spectral waveforms obtained in that vessel with Doppler ultrasonography (US). Spectral waveforms reflect the physiologic status of the organ supplied by the vessel, as well as the anatomic location of the vessel in relation to the heart. In addition, the waveforms may be affected by age- and development-related hemodynamic differences. For example, adults tend to have higher flow velocities, whereas neonates, particularly those born prematurely, have higher resistance to flow, especially in the cerebral and renal vascular beds. As Doppler US is performed with increasing frequency for vascular evaluation in children, the recognition of normal flow patterns has become imperative. Familiarity with the waveforms characteristic of specific veins and arteries in children is important. In addition, an understanding of the hemodynamic factors involved provides a useful basis for interpreting waveform abnormalities.