Dear Editor,

We read with great interest and enthusiasm the article “Comparison of posterior cranial fossa morphometric measurements in Chiari type I patients with and without syrinx cavity on magnetic resonance imaging” [1]. The aim of that study was to compare the posterior fossa measurements in magnetic resonance imaging (MRI) of Chiari malformation type 1 (CHM1) patients with and without a syrinx within each group and with a (non-CHM) control group. In the study it was concluded that all the posterior cranial fossa lengths were shorter in patients with a syrinx, but the anterior posterior (AP) length of the posterior fossa was found to be the only measurement that was significantly different in the CHM1 patients with a syrinx. The posterior cranial fossa measurements were significantly lower in the patients with a syrinx measurement that exceeded more than half that of the spinal cord. Therefore, the maximum anteroposterior measurement of the syrinx (S)/anteroposterior measurement of the spinal cord at the same level (C) ratio can be a useful tool to determine the underlying aetiology. As mentioned, the key factor of this study is measurement of posterior fossa volumes. The researchers set specific landmarks to determine the boundaries of the posterior fossa.

In this article there was insufficient explanation of how the McRae line (ML) is formed and how it should be measured.

As Baysal et al. [2] mentioned, the lines of the craniocervical junction (CJ) are used primarily for the diagnosis of CJ pathologies, such as Chamberlain and McRae lines first determined in direct radiography. Chamberlain in 1939 [3] defined the line of the hard palate to opisthion, then in 1953 McRae [4] identified on direct radiography the line from the basion to the opisthion.

In mid-sagittal MRI or computed tomography images, the basion is the end tip of the clivus and the opisthion is the tip of the occipital bone, and they collectively form the margins of the foramen magnum.

The McRae line is traced between the basion and opisthion, the most anterior and posterior midsagittal points of the foramen magnum margin, respectively [1].

The given figures for the posterior fossa measurements, especially Figure 6, fail to emphasise the correct demonstration of ML, as it is clearly seen that the tip of the line which the author has drawn does not reach the opisthion.

Korbecki et al. [6] in Figure 9 of their article “Imaging of cerebrospinal fluid flow: fundamentals, techniques, and clinical applications of phase-contrast magnetic resonance imaging” clearly demonstrate the ML successfully.

Inaccurate measurement of ML not only puts the credibility of the findings of this study in jeopardy but also can mislead inexperienced radiologists about correctly evaluating craniocervical junction pathologies in daily practice.