Microvascular Flow Imaging: A State-of-the-Art Review of Clinical Use and Promise

Abstract

Vascular imaging with color and power Doppler is a useful tool in the assessment of various disease processes. Assessment of blood flow, from infarction and ischemia to hyperemia, in organs, neoplasms, and vessels, is used in nearly every US investigation. Recent developments in this area are sensitive to small-vessel low velocity flow without use of intravenous contrast agents, known as microvascular flow imaging (MVFI). MVFI is more sensitive in detection of small vessels than color, power, and spectral Doppler, reducing the need for follow-up contrast-enhanced US (CEUS), CT, and MRI, except when arterial and venous wash-in and washout characteristics would be helpful in diagnosis. Varying clinical applications of MVFI are reviewed in adult and pediatric populations, including its technical underpinnings. MVFI shows promise in assessment of several conditions including benign and malignant lesions in the liver and kidney, acute pathologic abnormalities in the gallbladder and testes, and superficial lymph nodes. Future potential of MVFI in different conditions (eg, endovascular repair) is discussed. Finally, clinical cases in which MVFI correlated and potentially obviated additional CEUS, CT, or MRI are shown.

Graphical abstract

Figure 1:

Images of hepatocellular carcinoma in a 16-year-old girl with cryptogenic cirrhosis who was found to have a focal liver lesion increasing in size on serial US images. (A, B) Longitudinal gray-scale image through the right liver lobe. (A) On baseline US image, the lesion (arrows) was slightly hyperechoic with a hypoechoic halo. (B) Microvascular flow imaging showed hypertrophic peripheral vessels (thick arrows) with abnormal central vessels in a haphazard distribution (thin arrows). (C) Contrast-enhanced US of the focal liver lesion (arrows) showed early arterial enhancement at 26 seconds after administration of contrast agent, with more avid arterial enhancement at 31 seconds and subsequent late washout at 3 minutes 42 seconds.

Figure 2:

Longitudinal images through the left liver lobe of hemangioma in a 39-year-old man referred for investigation of microscopic hematuria, noted to have an incidental solitary liver lesion at US. (A) B-mode scan showed an isoechoic lesion (long arrows) with an eccentric hypoechoic component (short arrow). (B) At microvascular flow imaging, there were peripheral globular vessels observed in the periphery (arrows) of the focal liver lesion. (C) A contrast-enhanced US examination depicted globular enhancement at 18 seconds (short arrows), centripetal filling of the lesion (long arrows), with incomplete filling attributed to either a thrombus or scar and hyalinization (short arrow) at 39 seconds of contrast agent administration.

Figure 3:

Longitudinal image through the right liver lobe in a 14-year-old girl with a background fatty liver and an incidental focal liver lesion. (A) B-mode US showed a focal liver lesion (arrows) of low reflectivity compared with the background liver parenchyma, measuring 58 × 47 × 36 mm. (B) Microvascular flow image of the lesion showed a so-called spoke wheel distribution of vessels (arrows). (C) Contrast-enhanced US performed at the arterial phase confirmed this pattern of spoke wheel vascularization (horizontal arrow), with central vessels at 13 seconds after contrast agent injection and hyperenhancement of the lesion (vertical arrow) at 38 seconds after contrast agent administration. The lesion had complete fill-in and no washout, with no observed central scar. A subsequent MRI scan verified the findings at US of a focal nodular hyperplasia (not shown).

Figure 4:

Hemorrhagic hepatic cyst in a 55-year-old man with hepatic mass. (A) Gray-scale image with color Doppler showed a solid mass with isoechoic echotexture in the liver without color flow at Doppler. (B) Corresponding microvascular flow imaging (MVFI) did not show any internal vascularity; however, some of the posterior aspects were considered too deep for definitive characterization. (C) The lesion was hyperintense at contrast-enhanced T1-weighted MRI. (D) No enhancement was identified on the subtraction image, confirming the assessment at MVFI.

Figure 5:

(A) Gray-scale and color Doppler US images in an adult patient with elevated liver function tests without clinically significant color flow within the left portal vein (LPV). (B) Spectral Doppler image showed slow velocity flow in the left portal vein. (C) Microvascular flow imaging depicted internal vascularity, which confirmed patency and obviated further investigations.

Figure 6:

(A) Gray-scale US image shows irregular gallbladder wall in a 65-year-old female patient with upper abdominal pain. There is a fluid collection in the liver adjacent to the gallbladder, as seen. (B) Power Doppler mode shows only limited vascularity in the most superficial portion of the gallbladder wall. (C) Microvascular flow image and gray-scale image depicted diffuse vascularity in the gallbladder wall, without definite blood flow in the irregular portion of the gallbladder wall of concern. This increased confidence in diagnosis of gallbladder wall perforation, with communication with the adjacent intrahepatic collection. (D) Spot image from cholecystostomy tube placement confirmed focal perforation of the gallbladder with contrast leak into the adjacent collection.

Figure 7:

Images in a 67-year-old man who was positive for COVID-19 and who presented with fever and abdominal pain. (A) Gray-scale US image showed a large isoechoic mass in the gallbladder lumen, and gallbladder tumor was in the differential. (B) Limited color Doppler was observed in the gallbladder wall; therefore, confidence was low that absence of flow in the gallbladder mass indicated sludge versus tumor. (C) Increased depiction of vascularity of the gallbladder wall and adjacent liver on microvascular flow image with no internal vascularity in the gallbladder mass improved confidence in a diagnosis of tumefactive sludge. (D) Follow-up CT confirmed lack of contrast-enhanced mass in the gallbladder.

Figure 8:

Incidentally detected renal cystic lesion in a 53-year-old male patient (Bosniak type IV). (A) A mixed cystic and solid lesion with little internal vascularity on color Doppler images. (B) At microvascular flow imaging, greater discrete internal vascularity (arrows) was observed within the solid component of the mass and septations, increasing diagnostic confidence in diagnosis of Bosniak type IV renal cell carcinoma. (C) Subsequent contrast-enhanced US imaging depicted vascular septations and mural nodules, consistent with cystic renal cell carcinoma. (D) Subsequent CT confirmed presence of cystic renal cell carcinoma.

Figure 9:

Images in a 28-year-old man with history of trauma. (A) US Doppler image depicted a heterogeneous area in the right kidney with relative paucity of flow in the mid and lower pole. Hypoechoic area (white arrow) within infarcted zone represented laceration with hemorrhage and fluid, which was observed at CT. (B) Noncontrast microvascular flow image clearly demarcated the area without vascularity that was confirmed on the (C) subsequent contrast-enhanced CT image.

Figure 10:

Images in a 50-year-old man with no predisposing factors and sudden onset of pain in the left testis. (A) Color Doppler image shows hypoechoic area in the left testis with no definite internal vascularity. (B) Microvascular flow image better depicted surrounding vascularity extending up to the margins of the hypoechoic area (arrow) compared with the color Doppler image, which increased its diagnostic value. (C) Contrast-enhanced US (CEUS) showed no internal vascularity consistent with segmental infarction. The CEUS image gave the best definition of the vascularized borders. Contraction and resolution were identified over 6 months of serial US.

Figure 11:

Longitudinal US image in the spleen in a 14-year-old boy who fell on his elbow while playing soccer, sustaining a grade 4 splenic laceration. (A) Day 5 postinjury US image showed a small anechoic lesion within the splenic fracture plane (long arrow) adjacent to splenic vessels (short arrow). (B) Microvascular flow image showed a pseudoaneurysm of the splenic artery (arrow), confirmed on a (C) contrast-enhanced US (CEUS) image (arrow) at 55 seconds after contrast agent administration. A follow-up examination 3 days later, with both microvascular flow imaging (D) and CEUS (E), showed that the pseudoaneurysm (arrows in D and E) had spontaneously thrombosed without any intervention.