Table 1

Description of the 15 retinal vascular parameters measured for each retinal photograph and relevant retinal zone

ParameterDescriptionRetinal zone
CRAECentral retinal arteriolar equivalent calibreSummary measures of vascular equivalent calibre representing the equivalent single-vessel parent width for the six largest arterioles and venules. Based on the Knudston-Parr-Hubbard formula.24B
CRVECentral retinal venular equivalent calibreB
AVRArteriole–venular ratioDefined as the ratio of CRAE to CRVE. CRAE/CRVE=AVRB
FDaFractal dimension of arteriolar networkGlobal summary measure of branching complexity of the retinal vascular tree reflecting how thoroughly the branching pattern fills two-dimensional spaces. Larger values represent a more complex pattern. Calculated from the skeletonised line tracing using the box-counting method.C
FDvFractal dimension of venular networkC
TORTaCurvature tortuosity arterioleIntegral of the curvature squared along the vessel path, normalised by the total path length. Measurements are summarised to represent the average tortuosity of arterioles and venules separately, with smaller values reflecting straighter vessels.C
TORTvCurvature tortuosity venuleC
Num1stBaNumber of first branching arteriolesThe number of arterioles and venules with a first bifurcation, branch or daughter vessel, in zone C.C
Num1stBvNumber of first branching venulesC
BCaBranching coefficient arterioleCalculated from average Num1st measurements. Branching coefficient reflects the relationship between parent vessels and branches. Defined as the summed square of the mean vessel widths of each branch or daughter vessel divided by the square of the mean width of the parent vessel.C
BCvBranching coefficient venuleC
JEaJunctional exponent deviation for arteriolesCalculated from average Num1st measurements. Junctional exponent deviation reflects the deviation from the optimum ratio of vessel widths at a bifurcationC
JEvJunctional exponent deviation for venulesC
LDRaLength-to-diameter ratio arterioleLength-to-diameter ratio is the length of the vessel from the midpoint of one bifurcation to the midpoint of the next bifurcation. It is expressed as a ratio to the diameter of the parent vessel at the first bifurcationC
LDRvLength-to- diameter ratio venuleC
• Retinal vessels within the retinal zones of interest (0.5–1.0 (zone B) and 1.0–2.0 (zone C) disc diameters away from the optic disc margin) were traced automatically by the software.