Marieke van Rosmalen

Chapter 8 136 T2 hyperintensity and thickening of nerve roots are both important key abnormalities on brachial plexus MRI. However, nerve thickening is easily quantitatively measured, while T2 hyperintensity is not. This is mainly explained by the challenges to develop and perform a method to quantify T2 hyperintensity. The intensity in a T2-weighted image, i.e. how white the signal appears, depends on several factors such as differences between sequence parameters and MRI machines. Already small variations in the MRI scan protocol may lead to large changes in T2 signal intensities. Therefore, measurements of signal intensity on qualitative MRI are difficult to reproduce and validate in other centers. Some studies did measure signal intensity by determining a nerve-to muscle contrast-to- noise ratio. 35 Although interrater reliability was acceptable in this study, reproducibility remains a problem due to technical limitations. Amore reproducible method would be objective quantification of T2 relaxation times by T2 mapping. T2 mapping is an innovative quantitative MRI technique and produces objective measures for the T2 relaxation time in milliseconds of a structure of interest ( chapter 1 ). The diagnostic value of T2 mapping has been widely studied in cardiac tissue but is unknown in the peripheral nerves. 46–48 To obtain quantitative T2 measures of the peripheral nerves some post-processing steps are required which could be a possible limitation for implementation in clinical practice. This limitation will be discussed later in this chapter (see paragraph ‘Implementation of quantitative MRI in clinical practice’ ). MRI and nerve ultrasound Nerve ultrasound is a relatively new tool in diagnosis of chronic inflammatory neuropathies and may show nerve thickening of the nerve roots of the brachial plexus and the peripheral nerves. Nerve ultrasound is a dynamic tool and has a (theoretically) large field-of-view so that the nerves in the arms and, to a lesser extent, legs are easily evaluated in one examination. Furthermore, nerve ultrasound is a relatively cheap diagnostic tool. Because of the characteristics and advantages of nerve ultrasound, recent studies favor a more prominent role for nerve ultrasound in the new diagnostic guidelines for CIDP and MMN. 31,32,49 Nerve conduction studies (NCS) and nerve ultrasound seem to be compatible techniques as NCS has a high specificity (97%) and nerve ultrasound has a high sensitivity (94%). 49 Diagnostic strategies in which both techniques (i.e. NCS and nerve ultrasound) are combined are preferred over replacement of one technique by the other. In these diagnostic strategies nerve ultrasound could be used as a screening tool for CIDP or MMN and treatment could be started after a positive test result, which reduces the number of unnecessary NCS by 56%. 49 If nerve ultrasound does not show nerve (root) thickening, NCS should be performed. However, nerve ultrasound is not widely implemented in clinical practice, particularly not in non-academic centers, although exact numbers are lacking. Moreover, the use of nerve ultrasound requires a specific expertise that is not always available. Nerve ultrasound and MRI differ in many ways. Nerve ultrasound is able to measure nerve root thickness of the brachial plexus and peripheral nerves in one examination, while MRI has a smaller field-of-view and, particularly due to time limitations, only one body structure can be imaged in one examination. MRI, on the other hand, can provide information on T2 signal intensity and enhancement