In order to compare the multimodal MRI characteristics of different brain tumour types, the imaging features of the three tumour types acquired from conventional and diffusion MRI sequences are described in Table 2.2. The table also illustrates the typical variation of grey-level intensities of the three tumour types in relation to grey-level intensities of normal tissues: white matter, grey matter and CSF. The estimated range of grey-level intensities of the three tumour types obtained from conventional and diffusion MRI widely overlap. In the literature, the grey-level intensities of the three tumour types PA, EP and MB decrease in that order for ADC images. For other image types, such a pattern has not been reported. However on T2 images MB tends to have lower mean grey-level intensity than PA and EP. This is because MB consists of mostly solid mass and therefore has higher homogeneity. Furthermore, the grey-level intensities of PA and EP based on conventional MRI are noted to have a wider range than MB.
Table 2.2: MRI features for medulloblastoma, pilocytic astrocytoma and ependymoma. Estimated grey-level intensity range is drawn to compare the three tumour types in relation to signal intensity of grey matter, white matter and CSF of each image type. The boxes represent estimated grey-level intensity range.
Image type Medulloblastoma Pilocytic Astrocytoma Ependymoma T2 - MB is heterogeneous
with mostly hyperintense solid mass relative to grey matter and white matter. - PA is heterogeneous with hyperintense solid mass. - Hyperintense cystic components relative to normal brain, and isointense relative to CSF.
- EP is heterogeneous with hyperintense solid mass relative to both grey and white matter. - Cystic components
could have similar signal intensity as fluid. Pre- contrast T1 - MB is homogeneous with isointense to hypointense solid mass relative to white matter and mostly hypointense relative to grey matter.
- PA is heterogeneous with hypointense solid mass and hypointense cystic component relative to grey and white matter.
- EP is hypointense relative to both grey and white matter.
Post- contrast T1 - MB is heterogeneous or patchy enhancement. - PA is heterogeneous lesion with non- enhancing cystic components. - EP is heterogeneous enhanced soft tissues of tumours and non-enhanced components. Intensity Solid mass of MB Cystic region
White matter Grey matter CSF
Solid mass of EP/PA
Intensity Solid mass of MB
Cystic region
Grey matter White matter CSF
FLAIR - MB is
heterogeneous with isointense solid mass relative to normal brain tissue.
- PA is heterogeneous with hyperintense solid mass - Liquid can be differentiated from lesions vividly. - EP is isointense to hyperintense relative to both grey and white matter - Fluid can be hyperintense if contain proteinaceous fluid. ADC - MB is heterogeneous and has lower intensity than PA.
- PA is heterogeneous and higher intensity than MB.
- EP is heterogeneous and intermediate intensity between MB and PA.
Pilocytic astrocytomas, medulloblastomas and ependymomas have different levels of aggressiveness and distinct molecular genetic types, which result in diverse imaging features, e.g. homogeneous and heterogeneous. However, medulloblastomas have a higher propensity to be more homogeneous and reduced free water compared to pilocytic astrocytomas and ependymomas (77, 78, 87). Some cases such as pilocytic astrocytomas without prominent cystic component or ependymomas with large cystic
Intensity
CSF Grey matter White matter
Cystic region Solid tumours
Intensity CSF
White and grey matter Solid mass of MB
Solid mass of EP Solid mass of PA
Cystic region
Intensity
CSF White matter Grey matter
Cystic region Solid mass of MB
Solid mass of EP/PA Proteinaceous fluid
component can be seen and can be visually misdiagnosed. Pilocytic astrocytomas and ependymomas can be more likely found in supratentorial region than medulloblastomas. Ependymomas can also be found in spinal cord. However, generally, all of these tumour types arise mostly in the posterior fossa rather than the supratentorial region and spinal cord (91, 92).
T2-weighted images are sensitive to tissue pathology and show well-defined tumour delineation (94). The cystic components on T2-weighted images have higher intensity than solid mass. Medulloblastomas on T2-weighted images generally have less cystic components and lower signal intensity compared to pilocytic astrocytomas and ependymomas. T2-weighted images and pre-contrast T1-weighted images show complementary information but pre-contrast T1-weighted images exhibit faint tumour boundary. Post-contrast T1-weighted images can enhance some pathological tissues and increase image contrast on the enhanced region. The cystic and solid portions are hypointense and hyperintense to normal tissues on both pre- and post-contrast T1- weighted images. On post-contrast T1-weighted images, medulloblastomas can have higher intensity than pilocytic astrocytomas and ependymomas. The review of the MRI features of brain tumours on FLAIR images are limited compared to on T1- and T2- weighted images. However, they are claimed to be useful for distinguishing fluid contents. Two main types of fluid show different signal intensity on FLAIR. Fluid like CSF is supressed, while fluid filled with proteinaceous content is hyperintense to normal tissues.
The restricted diffusion, due to the high intrinsic cellularity of tumours, is usually found in high grade lesions. This restricted diffusion results in lower signal intensity on ADC and
MD maps of medulloblastomas, compared to pilocytic astrocytomas and ependymomas. The review of brain tumours on FA maps is limited. However, it has been observed that FA maps exhibit hypointense cystic components compared to heterogeneous solid mass of brain tumour.
The degree of heterogeneity and intensity of tumours, relative to those of normal tissues, cannot completely differentiate the three tumour types through visual inspection. For example, the medulloblastoma, pilocytic astrocytoma and ependymoma in Figure 2.26 have similar morphological shape, with a fluid area, and are found at the same anatomical location. The medulloblastoma images obtain from conventional and diffusion MRI in Figure 2.26 have a large portion of proteinaceous fluid resulting in higher signal intensity on most image types. The pilocytic astrocytoma and ependymoma images obtained from conventional and diffusion MRI in Figure 2.26 have similar morphological shape and may not be visually differentiated. In general, the separation among medulloblastomas with more cystic component; pilocytic astrocytomas with inclusive solid mass; and ependymomas with excessive cystic components are still questionable.
Medulloblastoma case MB718 Pilocytic Astrocytoma case PA808 Ependymoma case EP639 T2 FLAIR Pre- contrast T1 Post- contrast T1 ADC FA
Figure 2.26: Difficulty in differentiating the three tumour types. Imaging features of medulloblastoma, pilocytic astrocytoma and ependymoma are shown based on conventional and diffusion MRI.
Summary
2.4
This chapter reviewed the concepts of MR-based image acquisition and MRI characteristics of the three most common paediatric brain tumour types. The MR-based image acquisition for MR sequences includes T2, T1, FLAIR, DWI and DTI, as well as the reconstruction of diffusion MRI producing ADC, MD and FA maps. Various imaging parameters and their effects on image contrast were discussed. This review provides understanding for MRI features of healthy and pathological tissues, particularly brain tumours, which are the material used throughout this thesis.
The typical MRI characteristics of the three most common paediatric brain tumours: pilocytic astrocytoma, medulloblastoma and ependymoma, as observed on conventional and diffusion MRI were reviewed. In this thesis, subtypes of embryonal, astrocytic and ependymal tumours are examined. Embryonal tumours include classic medulloblastoma (grade IV) and desmoplastic/nodular medulloblastoma (grade IV). Astrocytic tumours include pilocytic astrocytoma (grade I). Ependymal tumours include ependymoma (grade II) and anaplastic ependymoma (grade III). Majority of cases are frequently found in the posterior fossa region of the brain and fewer cases are found in supratentorial region.
The distinctive genetic types of brain tumours contribute to different imaging characteristics, which can appear homogeneous and heterogeneous. The imaging features also depend on imaging sequences, which can enhance abnormal tissues, such as solid mass, cyst, necrosis, calcification and haemorrhage, with different contrast.
The visual diagnosis by using the relative intensity to normal brain tissue and level of heterogeneity is relatively difficult to differentiate these tumour types, with total accuracy. As part of this thesis, texture analysis methods are investigated to better understand such image-based tumour characteristics and potentially provide appropriate imaging markers for computer-aided diagnosis.