Image Analysis

Overview

In the Human Imaging Core, magnetic resonance imaging (MRI) is the most commonly used technique for determining total kidney volume and renal blood flow — and for cardiac analysis — though other methods are employed as appropriate.

The Mayo Clinic Radiology Informatics Laboratory (RIL) assists the Human Imaging Core in coordinating image measurement and analytics.

Analysis of kidney and liver volumes

Renal and liver volumes are obtained with coronal T2-weighted images. Nonenhanced three-dimensional, volume-interpolated, spoiled gradient-echo coronal T1-weighted images are obtained at three millimeters contiguous slice thickness.

Individual kidney volume is measured from the T1-weighted images using a stereologic method — ANALYZE software — and calculated from the set of contiguous images. A number of image measurement tools in the RIL complement these efforts.

As registration and subtraction could likely also help identify regions of change in PKD, the core has a developmental project under way to create a tool suitable for kidney-sized rigid and semi-rigid registration. This would allow for the definition of thresholds and the computation of the volume of change — usually for individual cysts — which could then be summed to create a global measurement.

Renal blood flow measurements

MR renal blood flow measurements can be obtained with thick-section, oblique-axial, two-dimensional, phase-contrast, breath-hold MR angiograms along the course of each renal artery as reference images. MR renal artery flow measurements are then obtained perpendicular to the oblique-axial, two-dimensional reference images of the renal arteries using a cardiac-gated, two-dimensional, fast gradient-echo, phase-contrast pulse sequence.

Flow analysis is performed using FLOW software, while semiautomated techniques are used for definition of the vessel borders in the flow images.

Evaluation of cardiac function

MRI is recognized as the "gold standard" for quantification of ventricular volumes and function. Cardiac function, chamber size and left ventricular mass are determined from short-axis, electrocardiogram-gated, steady-state, free-precession cine images.

After initial localizing scans, two-chamber and four-chamber scout images are acquired and used to prescribe cine short-axis views extending from the base of the left ventricle to the apex. Epicardial and endocardial borders of the left ventricle are traced at end-systole and end-diastole using commercially available software. The end-systolic and end-diastolic volumes, ejection fraction, and left ventricular mass are calculated using Simpson's method. These values are all indexed to body surface area.

Right ventricular volumes, ejection fraction and mass are measured in an identical manner. Cardiac output can be obtained by multiplying the stroke volume (end-diastolic to end-systolic volume) by the pulse rate or by performing a cine phase contrast flow measurement through the proximal ascending aorta.

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