D15 Morphological image

Topic

Imaging techniques allow for the visualization of bodily structures using different physical principles; consequently, each offers distinct capabilities regarding resolution, contrast, depth, and speed.

Radiography and computed tomography (CT) utilize X-rays, the intensity of which decreases exponentially as they pass through tissues, depending on the tissue's attenuation capacity. CT quantifies this attenuation using the Hounsfield scale: air registers at approximately −1000 units, fat near −100, water at 0, soft tissues roughly between +30 and +60, and cortical bone between +700 and +1000. These differences enable the anatomical differentiation of tissues with varying densities.

Magnetic resonance imaging (MRI) does not employ ionizing radiation. Instead, it relies on the behavior of hydrogen nuclei subjected to strong magnetic fields—typically ranging from 1.5 to 7 teslas. The nuclei spin at a Larmor frequency determined by the field strength; at 1.5 teslas, this frequency is approximately 63 MHz. Following the application of radiofrequency pulses, the nuclei realign with the magnetic field and progressively lose coherence through two processes, each characterized by time constants specific to the tissue type. Different imaging sequences exploit these properties to generate contrast. For instance, specific sequences can detect diffusion changes associated with acute cerebral ischemia as early as 15 minutes after onset; others highlight demyelinating lesions, while gadolinium agents allow for the identification of regions where the blood-brain barrier has been compromised.

Ultrasound imaging utilizes ultrasound pulses—typically in the 1 to 15 MHz range—and reconstructs images by measuring echoes produced at interfaces between tissues with differing acoustic properties. There is a trade-off between resolution and depth: high frequencies allow for the visualization of smaller structures but offer less penetration, whereas low frequencies achieve greater depth at the cost of lower resolution. According to the text, structures on the order of a tenth of a millimeter can be distinguished at around 7.5 MHz, whereas higher frequencies penetrate only a few centimeters. Doppler imaging also utilizes the frequency shift produced by moving red blood cells to measure blood flow velocity.

Positron emission tomography combined with CT (PET-CT) incorporates metabolic information. A modified, radioactive glucose molecule is administered; its uptake allows for the localization of tissues with high glucose consumption. Many tumors exhibit higher uptake than surrounding tissues. Normalizing this uptake based on the administered dose and the patient's weight yields a standardized value; according to the text, values ​​above approximately 2.5 may indicate malignancy, although the threshold depends on the tumor and its location. Regarding lung cancer staging, the text attributes a sensitivity of nearly 90% for disseminated disease and a specificity of around 80% to this technique.

Techniques utilizing X-rays or radioactive substances involve exposure to ionizing radiation. Its effect is expressed in sieverts, taking into account absorbed energy, the type of radiation, and tissue sensitivity. A chest X-ray represents approximately 0.1 mSv, whereas a chest CT scan reaches about 7 mSv, and an abdominal and pelvic CT scan around 10 mSv. Because exposure accumulates over a lifetime, radiological practice aims to use the minimum dose reasonably necessary to answer the clinical question.

Overall, each technique provides different information. Radiography and CT yield images based on X-ray attenuation and are particularly useful when speed or good definition of dense structures is required; MRI offers excellent soft-tissue contrast without ionizing radiation; ultrasound provides real-time dynamic images and allows for the study of blood flow; and PET adds information regarding metabolic activity. The choice therefore depends on the clinical question. By combining anatomy, tissue properties, flow, and metabolism, these techniques allow for the visualization of the body's interior without surgical procedures, enabling the selection—in each situation—of the method that provides the necessary information with the lowest possible risk to the patient.

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