The contrast created in the image is determined in particular by the difference in T1 relaxation times between fat and water. Fat has a high signal intensity (white) and water a low signal intensity (black).
Question: Why does fat have a high signal intensity on a T1 weighted image?
When a second radiofrequent pulse of 90 degrees is given, water will not have fully recovered in the longitudinal plane. After the second pulse, fat will make a larger deflection than water and create more transversal magnetization (fig. 10).. As a reminder, signals can be received and processed only in the transversal plane. The higher the transversal magnetization of the tissue, the more signal is received. When radiofrequent pulses are repeated, fat will contribute more to the final MRI image and therefore be represented as a high signal (white).
Figure 10. By repeating radiofrequent (RF) pulses, fat will have more transversal magnetization than water.
In practice, T1 weighted images are used mostly to evaluate normal anatomy. Remember that only a few structures have a high signal intensity (= white) on a T1 weighted image: fat, blood, gadolinium (= contrast), melanin, protein (e.g. high-protein cysts). A high signal can also be seen in specific MRI artifacts and accumulation diseases (not discussed further in this module). Water and collagenous tissue (ligaments, tendons, scars) have a lower signal intensity on a T1 weighted image (fig. 11).
Figure 11 Signal intensities in T1 weighted image. Depending on protein content, the tissue may have an intermediate or high signal intensity (SI).
The spearhead of T1 weighted imaging is visualizing normal anatomy, particularly the musculoskeletal system (fig. 12). When the signal intensity of the fat-containing bone marrow (high on T1!) is replaced by a low signal intensity, beware of bone marrow edema or bone marrow infiltration (fig. 13/14).
Figure 12. T1 weighted image in transversal direction of the upper legs. Normal anatomy.
Figure 13. Wrist X-ray of the left hand: no abnormalities. T1 weighted image in coronal direction: fracture line midpolar in the scaphoid bone (red line) with reactive bone edema.
Figure 14. X-ray of left upper leg/left knee: lytic lesion in the femur with multilayered periosteal reaction and soft tissue mass (PA diagnosis: osteosarcoma). The T1 weighted image clearly visualizes the permeative cortex destruction and the breakthrough into soft tissues. Note also the abnormal low signal intensity of the bone marrow (fat has been replaced by tumor).
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