D16 Functional image and structural parameters

Topic

Functional imaging and structural parameters go beyond merely displaying an organ's anatomy; they quantify specific tissue properties, such as stiffness, density, or functional capacity.

Pulse wave velocity directly measures arterial stiffness. Each heartbeat generates a pressure wave that propagates through the arteries; the speed of this wave increases as the arterial wall becomes stiffer. Measuring this velocity—typically between the neck and the groin—provides an indication of aortic stiffness. According to the text, it is one of the most important independent cardiovascular predictors after age and blood pressure, with every 1 m/s increase associated with a 14% rise in the risk of a major cardiovascular event.

The Moens-Korteweg equation establishes a physical relationship between arterial wall elasticity and wave propagation velocity. A young, elastic aorta exhibits velocities of approximately 4–6 m/s, whereas an aged, calcified aorta can exceed 10–12 m/s. Hypertension progressively promotes this stiffening; conversely, a stiff aorta causes pressure waves to reflect back toward the heart sooner, thereby increasing pulse pressure. This raises the cardiac workload and can damage organs such as the brain, kidneys, and heart, creating a feedback loop involving hypertension, arterial stiffness, and vascular injury.

Bone densitometry uses two different X-ray energy levels to distinguish bone-induced attenuation from that caused by soft tissue, thereby allowing for the calculation of bone mineral density. This value is compared against the peak bone density of young adults of the same sex using the T-score. According to the text, a value lower than −2.5 standard deviations defines osteoporosis, while values ​​between −1 and −2.5 indicate osteopenia. Density can also be compared with that of individuals of the same age. By combining this data with factors such as age, sex, body mass index, previous fractures, smoking, corticosteroid use, and rheumatoid arthritis, the risk of sustaining a fracture over the next ten years can also be estimated.

Hepatic elastography applies a principle similar to that used to assess arterial stiffness. A vibrator generates a low-frequency mechanical wave—around 50 Hz—the propagation speed of which increases as liver tissue becomes stiffer. A healthy liver exhibits stiffness of less than approximately 6 kPa; advanced fibrosis reaches values ​​of 9–12 kPa, and cirrhosis exceeds approximately 12 kPa. This technique allows for the non-invasive monitoring of fibrosis in conditions such as chronic viral hepatitis and non-alcoholic fatty liver disease; according to the text, it correctly identifies or rules out cirrhosis in over 80% of patients, considerably reducing the need for repeated biopsies.

Doppler echocardiography allows for the assessment of another functional property: the left ventricle's ability to relax and fill between heartbeats. To do this, it combines the velocity at which blood flows through the mitral valve during filling with the movement velocity of the mitral annulus itself during cardiac muscle relaxation. The ratio between the two provides a non-invasive estimate of ventricular filling pressure. According to the text, a ratio exceeding 14 suggests a pressure greater than 15 mmHg, comparable to that obtained via invasive catheter measurement. This information is particularly useful for identifying heart failure in patients whose blood ejection fraction remains normal, but whose ventricle struggles to relax and fill adequately.

Taken together, these methods convert the physical properties of tissues into quantifiable parameters. Pulse wave velocity measures aortic stiffness and vascular risk; densitometry quantifies bone mineral loss and helps estimate fracture risk; Elastography measures the progression of liver fibrosis without the need for repeated biopsies, while cardiac Doppler estimates filling pressure and diastolic function without the insertion of a catheter.

Thus, functional imaging complements traditional anatomical imaging: it goes beyond merely showing the appearance of an organ, allowing for the measurement of tissue stiffness, density, or function, and enabling the quantitative monitoring of the progression of cardiovascular, bone, and liver diseases through predominantly non-invasive procedures.

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