Diffusion Weighted Imaging (DWI) is a technique that utilizes the Brownian motion of water molecules to reflect the internal condition of lesions. It combines magnetic resonance imaging (MRI) scans or enhancement sequences to achieve clear diagnoses. The advantages include non-invasive operation, ease of use, and a high detection rate for small lesions. In the acute phase of cerebral infarction, lesions can be detected within 30 minutes, providing valuable time for thrombolytic therapy. It also has significant advantages in differentiating benign and malignant tumors and evaluating malignant tumors post-treatment. The Apparent Diffusion Coefficient (ADC) is another reference metric derived from the DWI sequence, which can indirectly reflect the density of lesions. Extensive clinical research has shown that ADC values are valuable for determining the benign or malignant nature of tumors and the specific staging of cerebral infarction. By combining the advantages of DWI and ADC characteristics, one can obtain as much morphological and metabolic information about lesions as possible, aiding in the qualitative and staging of lesions, significantly improving diagnostic accuracy, and providing the most effective information for selecting the optimal treatment plan.
The specific clinical applications are as follows:
① Applications in the nervous system: Most cerebral infarctions can be detected on DWI as diffusion restriction within 30 minutes after the infarction, with ADC values decreasing to their lowest between 8 to 32 hours, lasting for 3 to 5 days. DWI shows high signals in the acute phase, while the ADC map shows low signals.
② Applications in brain tumor diagnosis: The DWI presentations of gliomas, metastatic tumors, and meningiomas vary. Some lesions present high signals on DWI, while ADC may show low or high signals. The size of the ADC value depends on the tumor density; high-grade gliomas have significantly lower ADC values than low-grade gliomas, primarily because higher malignancy levels in gliomas lead to more cells, smaller intercellular spaces, and increased cellular atypia, which restricts water molecules further. Conversely, a decrease in ADC values tends to indicate malignancy or atypical meningiomas, making DWI significant for distinguishing between benign and malignant meningiomas. Furthermore, lymphomas, due to their high tumor density, larger nuclei, and lower extracellular water content, show significantly high signals on DWI and low signals on ADC. Combining DWI with other MRI sequences can reliably differentiate lymphoma from other types of tumors.
③ Applications in abdominal tumor diagnosis: a. Malignant tumors on DWI typically present restricted high signals, with low ADC values. This application is widely used for diagnosing and evaluating treatment effects of malignancies in various organs such as the liver, pancreas, uterus, adnexa, digestive tract, bones, and breasts. For patients post-chemotherapy, observing a decrease in restricted high signals on DWI and an increase in ADC values can indicate effective treatment; otherwise, it is ineffective. b. Inflammatory lesions, tumor-like lesions, and benign tumors may show slightly restricted high signals or unrestricted low signals, with ADC maps presenting equal or high signals and higher values. For example, focal hyperplastic nodules, hepatocellular adenomas, breast adenomas, and fibromas. A few cases, such as abscesses, may show restricted high signals on DWI, with ADC values being equal or low, and lower numbers, which can be distinguished by enhancement methods. Other inflammatory or tumor-like lesions may show slightly restricted or unrestricted signals, with slightly high or equal low signals, and higher ADC values.
④ Applications in prostate tumor diagnosis: DWI aids in the diagnosis, staging, differential diagnosis, and efficacy evaluation of prostate cancer. Most prostate cancer lesions present high signals on DWI, while prostatitis usually shows equal signals. The average ADC value of prostate cancer is higher than that of prostatitis. Additionally, metastatic lymph nodes and bone metastases from prostate cancer can also show significant high signals on DWI.
⑤ Whole-body DWI technology: Whole-body DWI requires full-body or full-length cross-sectional scanning and three-dimensional reconstruction of images, ultimately forming PET-like images. Whole-body DWI helps in detecting systemic metastatic lesions in advanced malignant tumors; it can also be used for efficacy evaluation of hematological malignancies and systemic metastases.
As mentioned above, the advantages of DWI technology and ADC values in MRI diagnosis and treatment are increasingly prominent, and they are being applied more frequently in the diagnosis and treatment of cerebrovascular diseases and tumor lesions. (MRI Room Liu Yuan)