Table 1.
Pearls and Pitfalls
| PITFALL | PEARL |
|---|---|
| DWI restriction in an area not immediately adjacent to or within the scar. | Always match the bed position of the T2 scar to DWI to avoid calling DWI restriction in an area where there was no tumor. |
| DWI sequences are T2WI by nature (with fat saturation): Anything with a long T2 relaxation time like fluid will be bright, most frequently the lumen or radiation-induced submucosal edema. | Always refer to the matching ADC map to ensure that bright DWI signal is not a T2 shine-through. In cases of true restriction, the bright area on DWI should be dark on ADC. |
| Ulcers may be edematous and show DWI bright signal from T2 effects. They can also trap a bubble of air and cause an artifact and lead to false-positive DWI restriction. | Stricture is a common response to therapy as is ulcer. The presence of a stricture may limit the endoscopic visualization of the tumor. But also, it may cause diffusion restriction by limiting the mechanics of the normal wall muscle and thereby restrict proton motion as well. |
| Rectal filling may result in high signal which limits one’s appreciation of DWI restriction if present. | Residual or regrown tumor has brighter restriction than that of the normal wall on DWI. When the normal wall has very little or no restriction, interpretation is easy; when the normal wall restricts, it can be helpful to narrow the window/level to appreciate the “extra signal.” Occasionally, a very good, reliable response is indicated by a lower-than-usual signal compared with the normally restricting wall. |
| A point of view of frame-shift interpretation can occur when looking at the treated tumor bed: There is often atrophy (and even ballooning out of) the wall where the tumor was and edema of the opposite wall can be misinterpreted as tumor. Always compare with the baseline MRI for tumor location and attachment points. | The collapsed normal mucosa typically has a bright T2 signal and can appear tri-radiate or with more extensions (“Mercedes Benz Sign”) and is truly easily recognized as such. The collapsed mucosa with bright T2 signal should also demonstrate T2 shine-through on DWI. |
| Air most often causes DWI artifact, and when present on several slices, one should avoid attempting interpretation and recommend micro-enema for follow-up in all cases. | Mismatched interpretations between DWI and endoscopy at the same time usually favor endoscopy. For example, if DWI seems to show restriction and endoscopy shows no suspicious findings, most likely MRI is incorrect for any number of reasons (80%), but one small series indicated that MRI may be detecting submucosal tumor before it reaches the mucosa and can be seen at endoscopy, so not all false positives are actually MRI false findings. |
| Adenomas often co-exist with cancer, but they lack the genetic mutations of cancer and may not respond to therapy and instead be left over as residual tissue. This may or may not show DW restriction. Also, these may or may not contain tumor and so are often prophylactically locally excised. | Artifacts are common and may appear as one or more of the following:
|
| Be aware of interventions other than simple endoscopic visualization (e.g., biopsy, TAE, EMR/ESD/hemorrhoidal banding); they may lead to granulation tissue with potential DWI restriction and false positivity. | Do not forget to look outside the primary tumor site in the mesorectum. Residual lymph nodes that are > 0.5 cm are considered suspicious and one cannot call cCR, even if the primary tumor bed is normal. The same goes for residual tumor deposits (size unknown) and EMVI. |
| Over-reliance on the ADC map can be problematic. Recall that its purpose is solely to distinguish between diffusion restriction (ADC dark) and T2 shine-through (ADC bright). If an ADC region of interest is dark, the DWI region of interest must always be bright. There are situations in which DWI/ADC ROI are dark and this is not tumor. This is called “T2 dark-through.” |