Power Doppler Imaging Applications 13

With these advantages of power Doppler ultrasound, it has rapidly earned its position as a tool to evaluate and quantify vascularity and perfusion in a wide variety of applications. These applications can be broadly divided into vascular depiction applications that depend on the improved sensitivity of power Doppler and vascular quantification

applications that use these depicted vascular networks to quantify blood flow in the area of interest.

Figure 1-4: Sample Doppler color flow images of human right kidney using (a) color Doppler display mode showing only large vessels and (b) power Doppler mode with higher sensitivity showing the full cortical perfusion network [Downloaded from Power Doppler Sonography, GE Healthcare, Medical Diagnostics: Medcyclopaedia.com according to the term of use attached in Appendix A].

1.5.1 Vascular Depiction Applications

1.5.1.1

Vessel Morphology Depiction Applications

Firstly, power Doppler (PD) has been used in depiction of vessel morphology depending on its improved ability to display continuous vessel segments and better define edges of vascular structures [17]. The ability to display continuous flow has been particularly useful when studying a number of organs and systems, including: evaluating the anatomy of orbital arteries [18], differentiating normal and abnormal fetal anatomical structures [19], studying vascularity of intestinal structures related to Crohn’s disease [20], and in screening thyroid nodules at high risk of malignancy [21]. Another application that has benefited from the improved ability of PD to depict small flow vessels that are possibly running in unfavorable angles to the Doppler beams is transcranial imaging. PD was used to detect and analyze intracranial aneurisms [22-24], imaging small-caliber, low-flow vessels [25, 26] and evaluating morphological and hemodynamic information in patients with severe head injury [27]. In large arteries,

power Doppler’s improved definition of vessel edges enhance the accuracy of luminal and vessel diameters measurements used in diagnosing high-grade stentosis in the renal artery [28] and the carotid artery [29-33].

1.5.1.2

Inflammation Evaluation Applications

The second application is using power Doppler to image and evaluate inflammation specifically in the musculoskeletal tissues [34, 35]. Due to its enhanced sensitivity, PD is valuable in depicting increased flow in vessels that are dilated owing to inflammatory response such as the intra-articular knee vasculature in rheumatoid arthritis patients [36-39]. In addition, PD can be used to distinguish inflammatory and infectious musculoskeletal fluid collections from those that are noninflammatory and may help guide the decision to perform diagnostic biopsy procedure [40].

1.5.1.3

Tumor Vasculature Depiction Applications

The combined effect of power Doppler’s sensitivity to slow flow and improved delineation of tortuous and irregular vessels makes it a promising technique to image intratumoral vessels [17]. Studies assessing vasculature of hepatocarcinoma [41, 42] and analyzing the lymph node involvement and vascular invasion with breast cancer [43] have found power Doppler to be a very effective tool. Moreover, other investigators used power Doppler to differentiate benign and malignant tumors in breast [44, 45], ovarian [46] and adnexal lesions [47].

1.5.2 Vascular Quantification Applications

Three-dimensional power Doppler became available for medical purposes towards the end of the last century [48, 49], giving rise to the possibility of extracting quantifiable objective measures describing full vasculature networks or trees in a volume of interest (VOI). A number of 3-D power Doppler quantification metrics were developed [13, 50, 51] based on the direct correlation of the power Doppler signal and the number or concentration of moving particles and their relation to fractional blood volumes and perfusion in the VOI. In 1999, while studying blood flow in adnexal masses; Pairleitner

et al. presented a standardized set of metrics that could give a mathematical expression of vascularization and flow [52]:

Eqn. 1-2a Eqn. 1-2b Eqn. 1-2c

As defined in [52], VI, also known as color pixel density (CPD), measures the proportion of color voxels in the cube, representing the amount of moving blood in the tissue, FI, the mean power signal of blood flow, represents the intensity of flow at the time of acquisition and VFI is a combination of vascularization and flow indices representing both blood flow and vascularization. The software developed by Pairleitner et al. was later implemented in GE ultrasound scanners under the name VOCALTM: volumetric calculations.

The use of these quantification indices and VOCALTM software, currently known as 3-D power Doppler angiography (3-D PDA), has produced an abundance of research communications in a variety of applications.

Quantifying tumor vascularity is the primary application for 3-D power Doppler angiography, especially as it offers unique ways for assessing women with gynecological cancers [53] such as ovarian [54, 55] and endometrial cancers [56, 57]and for diagnosis of malignant pelvic solid tumors [58]. Moreover, 3-D PDA was shown to serve as a useful tool in distinguishing benign and malignant breast [59, 60] and prostatic tumors [61].

These quantification indices are progressively being applied to studying the feto- placental unit. Attempts to correlate the VI, FI and VFI indices to regional perfusion in fetal brain [62, 63], liver [64] and lungs [65, 66] have been reported. The most promising field in applying 3-D PDA is the analysis of placental vascularity in different stages of normal pregnancy [67-71] as well as adverse pregnancy outcomes [72-75]. These indices

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Vascularization index= color voxels

total voxels in VOI, Flow index=sum of power in colored voxels

color voxels ,

Vascularization flow index =sum of power in colored voxels

have also shown promising results when assessing the endometrium for assisted reproductive techniques and in vitro fertilization treatments [47, 76, 77].