Thank you for your recent letter in response to our article. Herein
we hope to clarify some valid points you raised.
It is important to note that FLAIR sequence and T2 –weighted are not
mutually exclusive terms. The images are T2 weighted. As you are aware T2
weighting are generated when there is as long (> 2000ms) interval
between the radiofrequency pulses (long repetition time) and a long (>
80ms) interval between the radiofrequency pulse and the echo (echo time).
FLAIR sequencing produces heavily T2-weighted images with the CSF-nulled
(1). In the article we did not expect the BMJ readership to be able to
distinguish between FLAIR, STIR or other sequences but rather to
understand the fundamental principles which is the basic MRI dichotomy of
T1 and T2 weighting. This is why the weighting of the image was given in
question and the readers were not left to deduce this for themselves.
Admittedly a more challenging version of quiz could be targeted
specifically for radiologists where the images themselves are displayed
and the interrogatee is expected to determine the image weighting,
sequence and findings. We sought however to target the quiz to a wider
audience.
You correctly point out that most patients with intracranial hypotension
develop cystic hygromas rather than subdural haematomas. We never suggest
otherwise in our article. The subdural collections were subdural
haematomas and were labelled as such. They were not subdural hygromas as
these have same signal intensity as CSF on T2 weighted FLAIR sequence
imaging. We do not feel that the term “subdural effusion” is of any
benefit.
Pachymeningeal enhancement is indeed a feature of spontaneous intracranial
hypotension. This is clearly mentioned in the article. This is most
evident following gadolinium enhancement an agent which was not used in
our patient (2).
We are uncertain how Koumellis and co-workers can surmise that most
radiologists would not describe the sagittal sinus as engorged. For our
part the tense circular cross-section is consistent with engorgement. Our
description is also consonant with that of other authors in the literature
(2). Sometimes this is not apparent until comparison is made with post-
treatment images (2).
Downward displacement of more than 5mm of the cerebellar tonsils is
required for the definition of Chiari malformation and not for absolute
descent of the brainstem (3). Where there is descent of less than 5mm
descent is still recognised and termed cerebellar ectopia (3). The bottom
of the foramen magnum is determined by drawing a line from the most
inferior element of the tip of the clivus (basion) to the most inferior
portion of the foramen magnum (opisthion) (McRae’s line)(3, 4). If such a
line is draw in figure 2 it is apparent that there modest but appreciable
protrusion of the cerebellar tonsils.
Well conducted systematic reviews suggest that CT myelography remains the
investigation of choice to diagnose spontaneous CSF leak (2). This is not
to say that MRI imaging can never be effective as a diagnostic tool.
However its ascendancy over CT myelography can not be based on the single
case report cited (5). Part of the problems lays in the fact that there
exists no definitive guidance from an authoratitive body nor clinical
consensus material on the diagnosis and management of spontaneous
intracranial hypotension. This is part of the reason we presented this
informative article. We hope it has increased the knowledge of the BMJ’s
readership in this important but rare condition.
We thank Koumellis and co-workers for their communication.
1. Ryberg JN, Hammond CA, Grimm RC, et al. Initial clinical
experience in MR imaging of the brain with a fast fluid-attenuated
inversion recovery pulse sequence. Radiology. 1994;193:173-180.
3. Symptomatic tonsillar ectopia. Furuya K, Sano K, Segawa H, Ide K,
Yoneyama H. J Neurol Neurosurg Psychiatry. 1998; 64:221-6.
4. Ishikawa M, Kikuchi H, Fujisawa I, et al. Tonsillar herniation on
magnetic resonance imaging. Neurosurgery 1988;22:77–81.
5. Liong WC, Constantinescu CS, Jaspan T Intrathecal gadolinium-
enhanced magnetic resonance myelography in the detection of CSF leak.
Neurology. 2006 Oct 24;67(8):1522
Rapid Response:
Entrusting Image Interpretation to Radiologists?
Dear respondents,
Thank you for your recent letter in response to our article. Herein
we hope to clarify some valid points you raised.
It is important to note that FLAIR sequence and T2 –weighted are not
mutually exclusive terms. The images are T2 weighted. As you are aware T2
weighting are generated when there is as long (> 2000ms) interval
between the radiofrequency pulses (long repetition time) and a long (>
80ms) interval between the radiofrequency pulse and the echo (echo time).
FLAIR sequencing produces heavily T2-weighted images with the CSF-nulled
(1). In the article we did not expect the BMJ readership to be able to
distinguish between FLAIR, STIR or other sequences but rather to
understand the fundamental principles which is the basic MRI dichotomy of
T1 and T2 weighting. This is why the weighting of the image was given in
question and the readers were not left to deduce this for themselves.
Admittedly a more challenging version of quiz could be targeted
specifically for radiologists where the images themselves are displayed
and the interrogatee is expected to determine the image weighting,
sequence and findings. We sought however to target the quiz to a wider
audience.
You correctly point out that most patients with intracranial hypotension
develop cystic hygromas rather than subdural haematomas. We never suggest
otherwise in our article. The subdural collections were subdural
haematomas and were labelled as such. They were not subdural hygromas as
these have same signal intensity as CSF on T2 weighted FLAIR sequence
imaging. We do not feel that the term “subdural effusion” is of any
benefit.
Pachymeningeal enhancement is indeed a feature of spontaneous intracranial
hypotension. This is clearly mentioned in the article. This is most
evident following gadolinium enhancement an agent which was not used in
our patient (2).
We are uncertain how Koumellis and co-workers can surmise that most
radiologists would not describe the sagittal sinus as engorged. For our
part the tense circular cross-section is consistent with engorgement. Our
description is also consonant with that of other authors in the literature
(2). Sometimes this is not apparent until comparison is made with post-
treatment images (2).
Downward displacement of more than 5mm of the cerebellar tonsils is
required for the definition of Chiari malformation and not for absolute
descent of the brainstem (3). Where there is descent of less than 5mm
descent is still recognised and termed cerebellar ectopia (3). The bottom
of the foramen magnum is determined by drawing a line from the most
inferior element of the tip of the clivus (basion) to the most inferior
portion of the foramen magnum (opisthion) (McRae’s line)(3, 4). If such a
line is draw in figure 2 it is apparent that there modest but appreciable
protrusion of the cerebellar tonsils.
Well conducted systematic reviews suggest that CT myelography remains the
investigation of choice to diagnose spontaneous CSF leak (2). This is not
to say that MRI imaging can never be effective as a diagnostic tool.
However its ascendancy over CT myelography can not be based on the single
case report cited (5). Part of the problems lays in the fact that there
exists no definitive guidance from an authoratitive body nor clinical
consensus material on the diagnosis and management of spontaneous
intracranial hypotension. This is part of the reason we presented this
informative article. We hope it has increased the knowledge of the BMJ’s
readership in this important but rare condition.
We thank Koumellis and co-workers for their communication.
1. Ryberg JN, Hammond CA, Grimm RC, et al. Initial clinical
experience in MR imaging of the brain with a fast fluid-attenuated
inversion recovery pulse sequence. Radiology. 1994;193:173-180.
2. Schievink WI. Spontaneous spinal cerebrospinal fluid leaks and
intracranial hypotension. JAMA 2006;295:2284-96
3. Symptomatic tonsillar ectopia. Furuya K, Sano K, Segawa H, Ide K,
Yoneyama H. J Neurol Neurosurg Psychiatry. 1998; 64:221-6.
4. Ishikawa M, Kikuchi H, Fujisawa I, et al. Tonsillar herniation on
magnetic resonance imaging. Neurosurgery 1988;22:77–81.
5. Liong WC, Constantinescu CS, Jaspan T Intrathecal gadolinium-
enhanced magnetic resonance myelography in the detection of CSF leak.
Neurology. 2006 Oct 24;67(8):1522
Competing interests:
None declared
Competing interests: No competing interests