Q1: What is MRI?
A1: MRI (Magnetic Resonance Imaging) is the abbreviation of magnetic resonance imaging, which is usually referred to as magnetic resonance in daily life, but in fact it is a large medical imaging equipment made by applying the magnetic resonance phenomenon of atomic nucleus. In the early days, it was directly translated as nuclear MR (NMR) in China, but since the people were afraid of “nuclear” and mistakenly thought that it had nuclear radiation, it was later named as magnetic resonance imaging (MRI). Magnetic resonance imaging is widely used in clinical medicine and medical research because of its advantages such as no X-ray, no radiation, high soft tissue resolution, and multi-directional imaging. MR with high field strength also has the advantages of uniform magnetic field, fast scanning speed, and relatively low noise. The patient lies on the examination bed during the examination. This is the same as CT, because there is a magnetic field area formed by the instrument in the MRI room, and thus this magnetic field makes not every patient can do MRI examination, which is the contraindication sign that we often say cannot do MRI examination, and some cases may even be life-threatening, so patients should also pay attention to it themselves before the examination.
Q2: Are there any contraindications to MRI examinations?
A2: Contraindications to MRI examinations are conditions in which the patient is not suitable for MRI examinations or in which the application of MRI examinations will cause adverse consequences. Absolute contraindications include: pacemakers, heart valves, and nerve stimulators are strictly prohibited from MRI examinations (and avoid getting within 5 gauss lines of the MRI scanner); aneurysm clips and metallic foreign bodies in the eye are strictly prohibited from MRI examinations; critically ill patients with various types of resuscitation equipment (except for MRI with demagnetization resuscitation equipment); and patients with high fever. Relative contraindications include: patients with various ferromagnetic metal implants in the body, such as dentures, contraceptive rings, gun shrapnel, metal implants (except titanium alloy), and metal foreign bodies in the scanning area, should be scanned carefully to prevent the movement of metal objects or heat generation that may cause injury to the patient, and metal artifacts may also hinder the diagnosis. Longer scanning time, involuntary movements that cannot be controlled and patients who cannot cooperate, such as coma, convulsions, confusion, metal abnormalities, those prone to epilepsy or cardiac arrest, severe trauma, claustrophobic patients, young children and other patients; pregnant women (within the third month of pregnancy) and infants should be examined with the consent of the physician after the patient’s signature. In addition, items such as watches, credit cards, and disks should not be brought within the 5 gauss line of the scanner to prevent demagnetization or data loss.
Q3: Is preoperative MRI staging examination for rectal cancer different from pelvic MRI examination?
A3: It is very different in terms of examination techniques. Don’t look at the following technical descriptions as relatively simple, but to really do preoperative MRI staging for rectal cancer is to bring many details of progress through these small changes, bringing the advantages of MRI staging to the current extreme, and thus highly respected by radiologists and surgeons. The conventional pelvic MRI examination technique is the basic examination technique for preoperative MRI evaluation of rectal cancer, so I will not go into details here, but let’s mainly look at what are the meticulous tasks to be done for preoperative MRI staging examination of rectal cancer. The preparation for the examination includes reviewing the patient’s previous images, clinical history and previous treatment (such as partial resection or radiotherapy). We have to determine the site and size of the tumor and ask the patient to empty the urine and stool before the examination, and for tumors less than 3 cm, villous adenomas or patients with previous treatment history, 60-120 mL of warm gel for ultrasound should be given as an intrarectal contrast agent depending on the tumor site. Of course, the MRI scanner must also be 1.5T and above in field strength, with scanning coils in 8 channels and above, and the center part of the coil is accurately wrapped around the tumor location with a scan layer thickness of 3 mm. On sagittal T2WI we perform orthogonal axial image scanning of the rectal wall where the tumor occurs. Diffusion-weighted images (DWI) help to identify the primary tumor and small nodules, while 3D FSE T2WI images are a very good way to obtain high-resolution images and multiplanar reconstructed images.
Q4: Why is preoperative MRI staging of rectal cancer performed?
A4: The reason why preoperative MRI staging is getting more and more attention is that the mainstream treatment for rectal cancer is: preoperative neoadjuvant therapy followed by surgical resection for properly staged rectal cancer. In the past decade or so, Total Mesorectal Excision (TME) has been widely used, which has led to a significant decrease in the local recurrence rate of rectal cancer, from 38% to less than 10%. known as the rectal annuloplasty margin). However, even with the TME approach, the presence of tumors or malignant lymph nodes 1 mm within the rectal annuloplasty margin indicates a high probability of local recurrence. Therefore, a reliable preoperative imaging evaluation is important for surgical planning.
In recent years, MRI has rapidly become a reliable and reproducible clinical examination technique because of its high resolution image quality, its ability to predict with high specificity the absence of tumor invasion at the rectal annuloplasty margin, the relationship between the tumor and the annuloplasty margin, and the depth of tumor invasion beyond the lamina propria. It is also used to help surgeons choose the surgical procedure.
Q5: How is MRI used for preoperative T-staging of rectal cancer?
A5: Here I think it is necessary to give you a brief review of the anatomy of the rectum. The anatomy of the rectum is divided into three segments based on the distance from the anal verge, from the dentate line to the sigmoid-rectal junction, which is about 15 cm.
Upper 1/3 segment: more than 10 cm from the anal verge, the anterior wall of which is covered by the peritoneum in a reflexive manner. The height of this attachment point is variable, especially in women, and this is an important site to be carefully evaluated for tumor penetration of the rectal wall for implantation and metastasis.
middle 1/3 segment: 5-10 cm from the anal verge, here the rectum is completely wrapped by rectal mesenteric fat, most patients with middle rectal cancer will undergo TME and preserve the anal sphincter, and patients have a high quality of life
Lower 1/3 segment: within 5 cm from the anal verge, including the rectum and rectal mesentery located between the anal raphe and the rectal muscle attachment, the anal raphe forms the apex of the puborectal fossa, and the rectal mesentery here is significantly narrower and very challenging to evaluate.
Those who have read “[One question, one answer, easy to understand] 10 questions about colorectal cancer” may remember the histopathological features of tumor T-staging, but the images obtained from MRI examination are more similar to what is seen in general, so the criteria for T-staging by MRI of rectal cancer are based on histopathological features, but differ.
Tx: primary tumor cannot be assessed.
T0: No evidence of primary tumor.
T1: tumor invades the submucosa: tumor is seen as low signal within the submucosa, and abnormal signal replaces the submucosa but does not extend to the circumferential muscle layer.
T2: tumor invaded but did not penetrate the lamina propria: tumor within the lamina propria showed moderate signal (higher than the signal of the lamina propria and lower than the signal of the submucosa); the outer longitudinal muscle layer was replaced by moderate signal of tumor but did not break through the outer rectal muscle layer (longitudinal muscle) to invade the rectal mesenteric fat.
T3: tumor breaks through the intrinsic muscle layer and invades the subplasma layer (upper 1/3 of rectum) or rectal mesenteric fat (lower 2/3 of rectum): tumor penetrates the outer rectal muscle layer (longitudinal muscle) with moderate signal, showing a broad base or nodular prominence (without fine burr).
T3a: tumor penetrates the intrinsic muscle layer <1 mm.
T3b: The tumor breaks through the intrinsic muscle layer by 1~5 mm.
T3c: tumor breaks through the intrinsic muscle layer >5~15 mm.
T3d: tumor broke through the intrinsic muscle layer >15 mm.
T4: Tumor invaded other organs: tumor showed abnormal signal invading adjacent organs, tumor signal broke through the peritoneal reflex line.
Note: The version of TNM staging will be updated periodically, and different scholars will make some modifications to it. For example, some scholars have modified the T3 stage of rectal cancer MRI into the following three subtypes according to the radiological report template of the Radiological Society of North America (RSNA), which is essentially a combination of the above T3a and T3b into a new T3a stage for the convenience of MRI evaluation, with some minor adjustments: T3a*: tumor breaks through the lamina propria <5 mm; T3b*: tumor breaks through the lamina propria 5~10 mm; T3c*: tumor breaks through the intrinsic muscular layer >10 mm. the author also supports and uses this modified staging method, which is more practical in the clinic].
Q6: How can MRI do the best job of discriminating T2 stage from T3 stage?
A6: Although some studies have shown high accuracy of MRI in T staging, these results have failed to be reconfirmed by large scale studies and clinical patients. The reasons for this are manifold. First, the accuracy of MRI in T-staging of rectal cancer is very directly related to the experience of the radiologist, and thus there is significant variation from physician to physician and between two repeat readings by physicians when assessing T-staging of rectal cancer. The second is that current MRI techniques have difficulty in differentiating early stage T3 tumors (T3a+b) from stage T2 tumors. Again, although MRI can accurately assess advanced T3 stage tumors, analyzing minor imaging presentations to distinguish early T3 stage tumors from T2 stage tumors requires an evaluating physician with considerable experience and images of very good quality. In conclusion, experienced physicians and high quality examination images are the most important elements, both of which will continue to improve over time, making MRI only better, not extreme, in distinguishing stage T2 from stage T3 rectal cancer.
[Note: Clinical medicine is a combination of science and clinical experience, and clinical experience is also quite important, which is an indispensable value for the existence of physicians. Many people have become fascinated with big data in recent years, and I would like to advise sensible patients: If big data is really that useful in medicine, then robots can be allowed to rule humans, in which case why do we need doctors? I think this rhetorical question should make you understand what I mean. Humans definitely rule the robots, not the other way around. Therefore, big data is a very good thing, but it is only a good helper to help doctors to do better clinical treatment, not to replace doctors, which is the inevitable result of technological development.
Q7: How does MRI evaluate the rectal mesentery and rectal loop margin?
A7：The reason why this question is important is that most rectal cancers are detected at stage T3, and the prognosis of patients with these tumors correlates with the depth of extramural invasion of rectal cancer.MRI is a useful tool to assess the depth of extramural invasion of rectal cancer, and applying the T3 stage subtype modified from the RSNA radiology report template mentioned in A5 to assess the depth of rectal cancer invasion of rectal mesentery and whether the rectal fascia is involvement, can better assess the prognosis. In fact, this is the clinical implementation of MRI for the detailed assessment of T3 stage.
Here we will clarify once again what is the rectal mesentery and rectal loop margin. The rectal mesentery is the fatty tissue structure that surrounds the rectum and contains lymph nodes, blood vessels, multiple fibrous septa, and other structures. The rectal annuloplasty margin is a ring of fine fiber structure formed by the rectal mesenteric fascia, which is actually similar to a cylindrical structure with an inverted equilateral trapezoid in longitudinal section and the rectum in the middle. The rectal loop is an important anatomical structure for surgical resection of TME. The outer posterior aspect of the rectal mesenteric fascia is clearly shown, and the anterior wall of the rectal mesenteric fascia is closely adjacent to Denonvillier’s fascia in male patients and is indistinguishable.
The concept of MRI criteria for maximum extra-mural invasion depth of rectal cancer and the concept of MRI showing the shortest distance between the rectal cancer tumor and the rectal mesenteric fascia are also very important to answer this question. The former shows that the prognosis of patients with rectal cancer with a maximum extra-mural invasion depth >5 mm is significantly worse than that of patients with stage T3 tumors ≤5 mm, while there is no significant difference in prognosis between stage T2 and T3 patients when the extra-mural invasion depth of stage T3 tumors is <2 mm. The latter is a quantitative analysis to assess whether the rectal circumferential margin is involved, and helps to compare data from follow-up examinations. When the shortest distance between the rectal cancer tumor and the rectal mesenteric fascia is found >1 mm on histopathology, the local recurrence rate of the tumor will be significantly reduced.
The accuracy of MRI in assessing the anterior invasion of low rectal cancer is as high as 83%, while the accuracy of MRI for posterior invasion is only 22%, which means that although there is no other method to replace MRI for the assessment of low rectal cancer, it is not omnipotent. We are looking forward to the continued advancement of imaging technology and earlier breakthroughs in this topic.
Q8: Can MRI evaluate whether rectal cancer has invaded the anus?
A8: Yes, it can. MRI is the best imaging method to show the anal sphincter of the lower rectum, which consists of the internal anal sphincter (smooth muscle) and the external anal sphincter complex (consisting of the anal levator, puborectalis and external anal sphincter). The internal anal sphincter (smooth muscle) and the external anal sphincter complex (composed of three skeletal muscles: anal levator, puborectalis, and external anal sphincter) are composed of the annulus recti, which is connected to the internal anal sphincter, and the longitudinal longitudinal muscle of the outer rectum, which continues with the intermuscular layer of the internal and external anal sphincter. Coronal MRI images can best assess whether there is tumor invasion on the superior border of the puborectalis muscle, and if the superior border of the puborectalis muscle is not invaded by tumor, anal preservation surgery is generally feasible. When the tumor involves the anal sphincter, it may be necessary to remove part of the sphincter and do coloanal reconstruction, but when the anal sphincter is widely invaded by the tumor, there is no way to preserve the anus.
Q9: Can MRI evaluate the invasion of rectal cancer to other pelvic organs?
A9: MRI is good for preoperative assessment of primary rectal cancer that most often invades the prostate and seminal vesicles in men and the uterus and vagina in women. Also, preoperative MRI assessment of the presacral fascia for tumor invasion and sacral nerve root involvement is important for surgery. It is generally believed that surgical resection of the tumor is still possible in case of tumor invasion of the proximal sacrum or nerve roots above the level of sacral 2 vertebrae.
MRI assessment of the relationship between the pelvic wall and the tumor is best performed on high-resolution coronal or sagittal images, as conventional large FOV images can easily lead to underestimation of the relationship between the tumor and adjacent important pelvic wall structures such as blood vessels or nerves.
The pelvic wall structures adjacent to the rectum are the common iliac artery, internal iliac artery, external iliac artery, ureter, pear-shaped muscle, internal foramina and sacral nerve roots in the region of the greater sciatic foramen, which are covered by the intrapelvic fascia (also known as the mural fascia of the pelvis). In some areas, especially at the level of the superior rectum, the internal pelvic fascia may be adjacent to or even fused with the mesenteric fascia of the rectum (also known as the visceral fascia of the pelvis). In the anterior sacral region, there is an anterior sacral fascia that continues with the mural fascia of the pelvis and is covered with sacral nerve roots and sacral veins. The rectal mesenteric fascia runs along the anterior aspect of the anterior sacral fascia and is separated from the anterior sacral fascia by a potential posterior rectal space, which provides a natural level of resection for TME surgery. At the level of the mid-rectum, the visceral and mural layers of the intrapelvic fascia are generally distinguishable on MRI. However, in the lower and upper rectum, these structures are often difficult to distinguish, and when rectal cancer invades the rectal mesenteric fascia in these areas it is also likely to mean pelvic wall involvement.
Q10: Are there any current advances in MRI for local lymph node N staging?
A10: Meta-analyses in previous years have shown that the current threshold for determining lymph node involvement by lymph node size or number has not been found to be better, but in clinical work, lymph node size is still the most reliable predictor of lymph node metastasis. There is no uniform consensus on the criteria for lymph node size, ranging from any detectable lymph node being considered lymph node involvement to lymph nodes larger than a certain standard (ranging from 3 mm to 10 mm) being considered as lymph node involvement. However, given the increasingly thin layer thickness (<1 mm) and faster scanning speed (<0.3 s/r) of MDCT, we believe that 4 mm is a more desirable threshold, which is confirmed by pathology studies. In addition, uneven lymph node enhancement and unclear/irregular/burred lymph node margins are also important signs for determining lymph node involvement, and the widespread use of DWI provides new methods for us to rapidly detect and localize lymph nodes.
For lymph nodes within the rectal mesenteric fascia, all lymph nodes will be removed by TME, but their benignity and definite malignant lymph nodes in relation to the rectal mesenteric fascia should be carefully evaluated preoperatively. If a malignant lymph node or tumor invasion is within 1 mm of the rectal mesenteric fascia, this MRI evaluation finding is very important for the surgeon because more care should be taken at the circumferential margin during surgery to ensure that the surgical margin is clean and free of tumor invasion. Preoperative evaluation of the pelvic wall lymph nodes is also important because untreated malignant lymph nodes can lead to tumor recurrence. If malignant lymph nodes outside the rectal mesentery are identified in time, preoperative radiotherapy can be used to treat them with wide coverage of the irradiated field and to extend the surgical resection. In addition, removal of these lymph nodes is also required for N-staging, and the number of lymph nodes currently required for rectal cancer surgery is 12 or more. Overall, the work of MRI in N staging is far more challenging than T staging, and it is hoped that with the advancement of technology and the continuous research efforts of radiologists, a better solution will be found.