Modern neurosurgery has clear requirements for neuroimaging techniques. The diagnosis of glioma takes into account the size and extent of the tumor, and the proximity of the tumor to important surrounding structures (including important arteries, cortical veins, and functional areas of the cortex). Neuroimaging plays the most important role in the planning of glioma surgery. MRI is superior to CT and can show the extent of invasion of the lesion.
When we suspect glioma we should consider in the following order.
1. Is it a glioma, its location, size, extent, and relationship to surrounding vital structures?
2. What is the nature of the tumor (astrocytoma, oligodendroglioma and glioblastoma multiforme or other diseases, etc.)?
3. Can the surgery be performed safely? What is the location of the dermal flap, bone flap, cortical dissection, the adjacent relationship between the functional area of the cortex and the tumor, and the location of the cortical drainage vein?
In general, CT is superior in diagnosing the characteristic calcification of oligodendroglioma or the acute stage of intra-tumor hemorrhage, and this examination should be done. Low-grade gliomas appear as hypointense on CT images and as high signal occupancy on MRI T2-weighted images (T2WI), neither of which is enhanced on strengthening scans. However, up to 25-30% of low-grade glioma cases may have contrast enhancement on CT and MRI T2-weighted images. According to traditional diagnostic criteria, contrast-enhanced images are only present in patients with high-grade gliomas because high-grade gliomas disrupt the blood-brain barrier and the contrast agent leaks out. However, up to 25% of patients with high-grade gliomas can have no contrast-enhanced images. Table 2-1 shows the MRI imaging features of common gliomas.
Table 2-1-1 MRI imaging features of adult gliomas
Histological type
T1WI
T2WI
Edema
Enhanced image
Features
Astrocytoma
Low signal
High signal
Mild
No enhancement
Cortical white matter junction
Oligodendroglioma
Low signal
High signal
Mild
sometimes enhanced
Cortical white matter junction
Mesenchymal astrocytoma
Low signal
More extensive
High signal
Severe
inhomogeneous
Enhanced
Glioblastoma multiforme
Low signal
more extensive
High signal
Severe
heterogeneous, ring-like, multifocal
Occurs in the deep white matter
MRI coronal images are useful to show whether the tumor is invasive to the right or left hemisphere. For example, MRI coronal images are most likely to show that glioblastoma multiforme in the frontal lobe grows invasively to the contralateral side via anterior crossed fibers. MRI not only helps to diagnose the tumor qualitatively but also to distinguish whether the tumor is infiltrative or expansive, and MRI sagittal-coronal-axial scan can help to plan the surgery.
Non-tumor diseases that must be differentiated on imaging include intracerebral hematoma (especially in the subacute to chronic phase), hemorrhagic infarction, venous infarction, multiple sclerosis, some white matter lesions, encephalitis, and brain abscess.
1. The value of neuroimaging to determine the aggressiveness of glioma
CT started to be used in the field of neurosurgery after the mid-1970s. Enhanced CT scans (contrast-enhanced CT scans) of gliomas show increased signal in the lesion area and high or equal density in the surrounding area. Autopsy and serial stereotactic biopsy studies have confirmed that the high signal areas of supratentorial gliomas are the solid part of the tumor. Surrounding areas of high-density enhancement are areas of peritumoral edema mixed with tumor cells. The degree of malignancy of glioma cannot be determined based on the signal level in the area of signal enhancement, but at least it can indicate that the stronger the signal, the higher the vascular density in the tumor area. Comparison scans by autopsy and enhanced CT revealed that the tumor area determined from CT images was 2 cm smaller than the actual area of glioma, and that CT was not a good judge of residual glioma.
MRI is superior to CT in determining the boundaries of glioma and postoperative recurrence. MRI can determine the postoperative residual glioma well, with an accuracy rate of more than 77%. Since postoperative enhancement within 72 hours (preferably within 24 hours) is not affected by postoperative artifacts, this technique can be used to determine the amount of postoperative residual glioma when conditions permit, and can be used as a baseline slice for future patient review to determine whether the tumor is a postoperative recurrence or radionecrosis. If the postoperative review MRI shows an area of enhancement, it may be a high risk area for future recurrence. Grade III and IV astrocytomas have aggressive growth in the cerebral hemispheres, and the main body of the tumor is a heterogeneous area of enhancement, but even enhanced MRI does not provide a good indication of the invasive area of the glioma. We know that the MRI image of glioma actually contains two parts: (1) the parenchymal part of the tumor, i.e., the main part of the tumor; and (2) the invasive margin part of the tumor. The parenchymal part of the tumor is the part of the blood-brain barrier disruption, which is shown as enhanced image on MRI. This part of the tissue is tumor tissue without normal neural tissue, which is generally non-functional and can be removed. The tumor invasion margin part is the part of the tumor invading into the surrounding normal neural tissue. This part of the tissue can be shown as abnormal on T2 image, and it is not necessary to blindly expand the scope of surgical resection in the surgical management, and the parenchymal part of the tumor should be resected mainly, and lobectomy can be done in special cases.
Earnest studied a group of untreated gliomas, including grade III or IV gliomas, by comparing stereotactic biopsy with enhanced CT and MRI, and found that MRI was superior to CT, but biopsy of the part of brain tissue that showed normal performance on MRI T2-weighted images could still reveal tumor cells. In contrast, Johnson concluded that the invasive border of glioma in the white matter region of the brain could be better determined by MRI T2-weighted images. This raises the question of what image is reflected in the T2-weighted image. Current studies suggest that T2-weighted images reflect an edematous zone, demyelinated nerve fibers, and other degraded tissues, rather than true cellular tissue or atypical proliferating tumor cells. However, there is no other means to better determine the boundary of tumor tissue, so the T2-weighted image is still used as a more reliable indicator to reflect the boundary of tumor tissue in clinical practice.
2.The value of neuroimaging to determine postoperative radiation necrosis of glioma
Currently, radiation therapy including stereotactic radiosurgery is a conventional treatment for glioma, but radiation necrosis (Radionecrosis) caused by radiation therapy to normal brain tissue is also common in clinical practice. Masciopinto performed biopsies on 10 cases of gliomas with high suspicion of recurrence after radiation therapy and found that 70% had postoperative tumor recurrence and 30% had radiation brain necrosis. In conclusion, the existing conventional imaging examinations cannot distinguish well between radiation brain necrosis and tumor recurrence. CT and MRI images of necrotic areas are very similar to those of postoperative glioma recurrence and are difficult to distinguish. The area of necrosis may also show occupying signs, destruction of local structures of brain tissue, and slow clinical progression (can be unchanged for more than 10 months); the enhancing areas are mostly located in the white matter, away from the primary lesion, and may show ring-like enhancing foci. As with recurrent gliomas, radionecrosis foci present as enhancing images on CT and as a markedly enhancing signal on MRI T2-weighted images. If the diagnosis of radionecrosis is established clinically, doctors should not take it lightly and should review it regularly to closely observe the imaging changes of the radionecrosis lesions.
3.The value of neuroimaging to determine postoperative residual glioma
The postoperative residual of glioma judged by neuroimaging can evaluate the effect of surgical treatment on the one hand, and can be used as a prognostic indicator after surgical treatment on the other hand. The amount of postoperative residual tumor can directly affect the survival period. At present, we do not have a good method to determine the amount of postoperative residual tumor. In clinical practice, we usually review the enhanced CT and MRI within 72 hours after surgery to determine the amount of postoperative residual tumor. However, the value of neuroimaging to determine postoperative residual glioma is still controversial, because the enhanced portion of postoperative tumor may not necessarily represent the residual tumor, and the ectopic scar from intraoperative vascular injury may also be shown as enhanced image. The postoperative enhancement of vascular injury peaks at 7 days postoperatively and disappears after 4 weeks. It is believed that early postoperative review of enhanced CT and MRI can determine the amount of tumor remaining after surgery with relative accuracy, and the review of CT should be done within 4 days after surgery, and the review of MRI should be done within 2 days after surgery, preferably within 24 hours after surgery.