Core Conclusion: At b=1000, liver lesions appear as high signals on both DWI and ADC images, which may seem contradictory theoretically, but can actually occur. This usually indicates that the signal attenuation behavior of the lesion at this specific value of b=1000 does not conform to a simple mono-exponential model, and its “high signal” nature may be different.
Let’s break down the basic principles of DWI and ADC:
· DWI Sequence: Displays the original signal intensity after applying a diffusion gradient (b value). The higher the signal, the more severely the diffusion of water molecules is restricted (but it is also affected by the T2 effect).
· ADC Image: A parameter map calculated from images at at least two different b values (for example, b=0 and b=1000), quantifying the diffusion coefficient of water molecules, attempting to remove the influence of the T2 effect. The lower the ADC value, the more restricted the diffusion.
Under normal circumstances:
· Lesions with restricted diffusion (such as malignant tumors, abscesses): Appear as high signals on DWI (because water molecules cannot escape, leading to reduced signal attenuation), and as low signals on ADC images (ADC value is low).
· Lesions without restricted diffusion (such as cysts, hemangiomas): Appear as low signals on DWI (water molecules diffuse freely, leading to significant signal attenuation), and as high signals on ADC images (ADC value is high).
Why do both show high signals?
This situation is referred to as the “T2 Shine-Through Effect” or the extreme or special manifestation of the “T2 Washout Effect.” Specifically for liver lesions, the possible reasons are as follows:
1. T2 Shine-Through Effect – Most common reason
This is the first reason that needs to be considered.
· Mechanism: The signal intensity of the DWI image depends not only on the diffusion capability of water molecules but also strongly on the T2 relaxation time of the tissue. If a lesion has a very long T2 value (i.e., it appears as a very bright high signal on T2-weighted images), then even if the ADC value is not low (indicating no restricted diffusion), it can still maintain a high signal intensity on the high b value DWI image. This is because its initial signal (the signal at b=0) is too strong, and even after diffusion-weighted attenuation, the remaining signal is still high.
· Calculation explanation: DWI signal ≈ (T2 weighted) × exp(-b × ADC) If the T2 signal of the lesion is extremely high, even if the exp(-b × ADC) term is not large (ADC value is not low), the product of the two can still be large, leading to a high signal on DWI. The ADC image is calculated, and if the lesion has high signals at both b=0 and b=1000, but the attenuation ratio conforms to the characteristics of free diffusion, the calculated ADC value will not be low, and may even be high.
· Corresponding liver lesions:
· Liver cysts: A typical representative. Contains pure water, T2 time is extremely long, appears bright on T2WI, and often appears bright on DWI (b=1000), but has a very high ADC value (also bright).
· Hemangiomas: Especially some fluid-rich hemangiomas, T2 time is long, and will also show high signals on both DWI and ADC.
· Other fluid-rich lesions: Such as choledochal cysts, necrotic cystic areas after treatment, etc.
2. The selected b value is not high enough
· Mechanism: The b value is the strength of diffusion weighting. The higher the b value, the stronger the sensitivity to diffusion, but at the same time, the signal-to-noise ratio of the image will decrease. For the liver, b=1000 s/mm² is a commonly used value, but sometimes it may not be sufficient to completely “suppress” the signals of lesions with moderately high ADC values.
· Situation: A certain lesion has a moderate ADC value (not completely free diffusion, but not severely restricted), at b=1000, its signal may still display as moderate or slightly high on DWI. The calculated ADC value is also at a moderate level, which may show as equal or slightly high signal on the ADC image (depending on the machine’s grayscale settings and contrast). If a higher b value (such as b=1500) is used, the signal of this lesion on DWI may be effectively suppressed, becoming a low signal.
3. Vascular lesions or perfusion effects
· Mechanism: At lower b values (such as b=50, b=100), the DWI signal is significantly affected by blood flow (perfusion) in capillaries. Although the perfusion effect is greatly reduced at b=1000, it does not completely disappear. For some lesions with extremely rich blood supply (such as hypervascular metastases, hemangiomas), the residual perfusion effect may contribute to their high signal on DWI. When calculating the ADC value, if the low b values include the perfusion effect, the calculated ADC value may be biased high.
4. Image interpretation errors
· The ADC image is a parameter map that needs to be interpreted correctly: The high signal on the ADC image represents a high ADC value, indicating fast diffusion. When doctors view the images, they may sometimes subconsciously correlate the DWI and ADC images as if they were “inverted,” mistakenly interpreting the high signal areas on the ADC image as restricted diffusion. It must be remembered: on the ADC image, the lower the signal, the more severely the diffusion is restricted.
Clinical Response Strategies:
When encountering lesions that show high signals on both DWI and ADC, radiologists will take the following steps for differentiation:
1. Compare with T2-weighted images (T2WI): This is the most critical step. If the lesion also shows a significant high signal (bright) on T2WI, then the possibility of the T2 Shine-Through Effect is very high. Cysts and hemangiomas usually show a significantly high signal on T2WI as a “bright light sign.”
2. Observe the absolute value of ADC: Measure the ADC value of the lesion. If the ADC value is very high (for example, > 1.8 × 10⁻³ mm²/s), it strongly supports a benign cyst or hemangioma. Malignant tumors usually have lower ADC values (< 1.2-1.4 × 10⁻³ mm²/s).
3. Combine with other sequences and clinical information: Comprehensive observation of enhancement patterns in enhanced scans (arterial phase, portal phase, delayed phase), patient history, alpha-fetoprotein (AFP) levels, and other tumor markers to make a final diagnosis.
· Cysts: No enhancement.
· Hemangiomas: Marginal nodular enhancement, centripetal filling.
· Metastatic tumors / liver cancer: Usually have characteristic enhancement patterns (such as “fast in and fast out”), and on DWI they are usually high signals accompanied by low ADC signals.
Summary: Liver lesions showing high signals on both DWI and ADC images at b=1000 are primarily due to the T2 Shine-Through Effect, commonly seen in benign lesions such as liver cysts and hemangiomas. Diagnoses should never be made solely based on DWI/ADC, and must comprehensively consider T2WI performance, specific ADC values, and enhancement scan characteristics to make accurate judgments.