Magnetic Resonance Imaging (MRI) uses powerful magnetic fields and radio waves to generate detailed pictures of internal organs and tissues. Unlike X-rays or CT scans, MRI excels at visualizing soft tissues, such as the brain, spinal cord, and ligaments. Patients often encounter technical language like “signal intensity,” which measures the brightness or darkness of an area on the image. This article explains what a finding of “increased signal” means in a clinical context.
The Basics of MRI Signal Generation
MRI works by measuring the energy released by the hydrogen protons found primarily in water molecules throughout the body. When a person is placed inside the scanner, the strong magnetic field causes these protons to align. Radiofrequency pulses are then used to briefly knock the protons out of alignment, and the “signal” is the energy they emit as they return to their resting state.
The brightness or darkness of a tissue, known as signal intensity, reflects the density and behavior of these water protons. Radiologists use different pulse sequences, or filters, to highlight various tissue properties. The two most common are T1-weighted and T2-weighted images.
T1-weighted images highlight anatomical details, where substances like fat appear bright. T2-weighted images are highly sensitive to water content and are used to highlight potential pathology. On a T2 image, tissues with higher water concentration generate a stronger signal. Therefore, an “increased signal” most often refers to a bright area seen on a T2-weighted image, which indicates abnormal water accumulation.
Interpreting Hyperintensity: What Increased Signal Looks Like
The technical term for an increased signal on an MRI is “hyperintensity,” meaning the area is brighter than the surrounding tissue. This brightness is a relative measure, comparing the area of interest to the expected appearance of healthy tissue nearby. On a standard T2-weighted image, a hyperintense area appears as a distinct, bright white spot.
The appearance of brightness directly translates to a high concentration of free-moving water or fluid within that tissue. Normally fluid-filled structures, such as cerebrospinal fluid (CSF) or cysts, appear very bright on T2 images. When a normally solid tissue begins to accumulate excess fluid, it becomes hyperintense.
The hyperintensity indicates that the tissue’s normal structure has been disrupted, allowing for the abnormal pooling of water. This accumulation is the body’s common response to numerous events, ranging from a simple bruise to a serious neurological condition.
Clinical Significance of Increased Signal
The finding of an increased signal, or hyperintensity, is a non-specific sign that requires careful correlation with the patient’s symptoms and medical history. The underlying cause of the excess water is often inflammation or edema, which is the swelling that occurs in response to injury, irritation, or infection.
For example, a bone bruise resulting from trauma shows an increased signal in the bone marrow due to microscopic fluid leaks. Conditions like tendinitis or arthritis involve inflammation that increases the water content in the affected tendons or joints, making them appear bright on the T2 image.
In the brain and spinal cord, increased signal can indicate more serious pathology, such as demyelination. Diseases like Multiple Sclerosis (MS) cause the immune system to attack the protective myelin sheath around nerve fibers, leading to inflammation and fluid accumulation. These demyelinating lesions appear as distinct, bright white spots.
Vascular events, such as an acute ischemic stroke, also cause a localized increase in signal intensity. When blood flow is blocked, cells die and swell, leading to a rapid influx of water into the affected brain tissue. Tumors and other abnormal masses often display hyperintensity because they frequently have a high water content or are surrounded by edema.
Some areas of mild hyperintensity can be related to normal variants or age-related changes, especially in older individuals. Because the finding of an increased signal can represent a wide spectrum of causes, a specialist must interpret the image in the context of all clinical information to arrive at a definitive diagnosis.