11.2 Color & Spectral Analysis: Measurements and Waveforms

Color Doppler: Direction and Presence of Flow

Color Doppler overlays a color map on the grayscale image, assigning a hue to each pixel based on the mean Doppler shift — and therefore on the direction of flow relative to the transducer. The default convention is summarized by the mnemonic BART: Blue Away, Red Toward. Red means flow moving toward the transducer; blue means flow moving away. The single most-tested point is what color does not mean: red does not mean artery and blue does not mean vein. One artery curving through the field can appear red, then blue, then red again as its direction relative to the probe changes. The color map is also operator-invertible, so the only safe habit is to read the color bar at the edge of the image, which shows which hue is assigned toward and which away.

Aliasing in color Doppler — apparent color reversal from red to blue within a vessel at high velocity — is a sampling (Nyquist) phenomenon, not a change in true flow direction, and is managed by increasing the pulse repetition frequency (scale) or lowering the baseline.

Presence Versus Absence of Flow

Just as important as direction is simply documenting whether flow exists. High-yield applications:

• Cystic versus vascular — an anechoic tubular structure with no color flow is a fluid structure (cyst, dilated duct); flow confirms a vessel. This is how the common bile duct is distinguished from the hepatic artery and portal vein in the porta hepatis.
• Testicular torsion — absent or markedly decreased flow in the symptomatic testis compared with the normal side supports torsion.
• Ovarian torsion — diminished or absent flow raises suspicion, though preserved flow does not exclude it.
• Thrombus — no flow within a vein (for example, a non-compressible femoral vein or the portal vein lumen) indicates thrombosis.
• Tissue viability — absent flow supports infarction or avascular necrosis in the appropriate setting.

Spectral Doppler: Quantifying the Waveform

Spectral (pulsed-wave) Doppler displays the full range of velocities in the sample volume over time. From the traced waveform, the system measures:

The resistive index (RI), also called the Pourcelot index, reflects downstream vascular resistance: the more diastolic flow is preserved, the lower the RI. Because the ratio divides by PSV, it is independent of the Doppler angle — a major practical advantage when vessels cannot be insonated at a proper angle.

Worked example: A renal interlobar artery shows PSV = 100 cm/s and EDV = 25 cm/s. RI = (100 − 25) ÷ 100 = 0.75. A native renal RI above approximately 0.7 is generally considered abnormal, seen with medical renal disease, obstruction, or transplant dysfunction, so 0.75 would be reported as elevated. For PI, if the same vessel had a mean velocity of 45 cm/s, PI = (100 − 25) ÷ 45 ≈ 1.67.

High-Resistance Versus Low-Resistance Waveforms

Waveform shape reflects the vascular bed being supplied:

• Low-resistance (continuous forward flow) — organs that need constant perfusion show systolic peaks with continuous forward diastolic flow. Examples: internal carotid artery, renal artery, hepatic artery, the celiac artery in any state (it feeds the liver and spleen), the superior mesenteric artery after a meal, and the uterine arteries in pregnancy. A normal internal carotid RI is roughly 0.5–0.7.
• High-resistance (pulsatile, little or reversed diastolic flow) — beds at rest that do not need continuous perfusion. These beds become low-resistance on demand — a limb artery after exercise, the SMA 30 to 60 minutes after eating. The classic example is the triphasic waveform of a fasting peripheral artery (for example, the common femoral artery): sharp systolic upstroke, brief flow reversal in early diastole, then a small forward component. The external carotid artery is also high-resistance, which helps distinguish it from the internal carotid (along with the temporal artery tap causing visible oscillations in the external carotid tracing). The superior mesenteric artery converts from high-resistance (fasting) to low-resistance after a meal.

Changing an expected pattern is the abnormality: a peripheral artery that loses its reversal component and shows continuous forward flow suggests proximal disease or vasodilation; a renal or hepatic artery that becomes high-resistance suggests downstream parenchymal or venous problems.

Tardus Parvus: The Signature of Proximal Stenosis

When a significant stenosis lies proximal (upstream) to the sample site, the distal waveform loses its brisk character and becomes tardus parvus — from Latin, tardus = slow/late, parvus = small. Its two features:

• Tardus — a delayed systolic upstroke (prolonged acceleration time, typically > 70 ms in the renal literature), reflecting the slow pressure rise beyond the obstruction
• Parvus — low amplitude with a rounded, blunted peak, reflecting reduced velocity distal to the lesion

Clinically, finding tardus parvus waveforms in the renal parenchymal arteries prompts a search for renal artery stenosis proximally; the same pattern in hepatic arteries after liver transplant raises concern for hepatic artery stenosis. The companion direct criterion at the stenosis itself is focal velocity elevation — in renal artery stenosis, a renal-aortic ratio (RAR) of PSV above about 3.5 with renal PSV above roughly 180–200 cm/s. Recognize the logic for the exam: at the stenosis, velocities are high; downstream, the waveform is small and slow.