detection of cerebrospinal fluid ( csf ) flow within syrinx and within aqueduct [ normal pressure hydrocephalus ( nph ) ] has prognostic significance : syringes with positive flow void sign respond to syrinx subarachnoid shunting ; nph with positive flow void sign responds to ventriculoperitoneal shunting . t2-weighted images showing flow voids is the usual imaging evidence of csf flow . we tried to see whether switching off the flow compensation ( flow comp off ) makes the csf flow voids within syrinx ( syringomyelia ) and within aqueduct ( nph ) more obvious ( to confirm csf flow ) . in these illustrative cases , the following observations were made : 
 faint flow voids were seen within cervical and thoracic syrinx on sagittal t2-weighted [ figure 1a and b ] and axial t2-weighted [ figure 1c and e ] images . switching off the flow compensation [ figure 1d and f ] made the flow voids more obviousfaint flow voids were seen within aqueduct on sagittal t2-weighted [ figure 2a ] image . switching off the flow compensation faint flow voids were seen within cervical and thoracic syrinx on sagittal t2-weighted [ figure 1a and b ] and axial t2-weighted [ figure 1c and e ] images . switching off the flow compensation [ figure 1d and f ] made the flow voids more obvious faint flow voids were seen within aqueduct on sagittal t2-weighted [ figure 2a ] image . switching off the flow compensation [ figure 2b ] made the flow voids more obvious . sagittal t2-weighted images showing syrinx in cervical ( a ) and thoracic ( b ) cord ; faint flow voids ( arrows ) are seen within the syrinx . ( c , d ) : axial t2-weighted images of cervical cord with ( c ) and without ( d ) flow compensation ; flow voids ( arrows ) are more obvious in d ( without flow compensation ) . ( e , f ) : axial t2-weighted images of thoracic cord with ( e ) and without ( f ) flow compensation ; flow voids ( arrows ) are more obvious in f ( without flow compensation ) sagittal t2-weighted image shows faint flow voids ( arrow ) . ( b ) flow voids ( arrow ) are more obvious ( after switching off the flow compensation ) flow compensation ( fc in ge / philips)/gradient motion rephasing ( gmr in siemens)/motion artifact suppression technique ( mast in piker ) or gradient moment nulling ( gmn ) is a gradient pulse used for artifact reduction , often used to suppress flow artifacts in magnetic resonance imaging ( mri ) of head , abdomen , chest and spine . extra gradient pulses ( called gradient lobes ) are added to produce the even - echo rephasing effect on the first echo ( thus eliminating first - echo dephasing ) . rephasing is attained without having to use a double - echo sequence . for gradient echo acquisitions , this type of fc only corrects for first - order ( i.e. , constant velocity ) flow . correction of higher - order motion ( like acceleration or jerk ) requires additional gradient lobes , which lengthen the cycle , thus lengthening tr and minimum te ( thus reducing the number of slices ) . it is possible to apply fc along each and all of the three ( x , y and z ) co - ordinates . fc is used with t2-weighted spin echo ( se ) [ not compatible with fast spin - echo ( fse ) sequences ] and gradient echo acquisitions ; but not with t1-weighted spinal imaging as it increases the signal intensity of csf and may decrease contrast between csf and spinal cord / cauda equina . in contrast to presaturation , gmn can not suppress artifacts due to pulsatile blood flow in spin - echo images . syringomyelia is a fluid - filled cavity or syrinx within the spinal cord , formed by csf dissecting into surrounding white matter ; hence , it is not lined by ependyma . it is very difficult to separate the two by imaging or histopathology ; hence , the term syringohydromyelia is frequently used . syringomyelia may be idiopathic or occur as a complication of trauma , meningitis , hemorrhage , tumor or arachnoiditis . chiari malformation ( a congenital abnormality where lower part of cerebellum protrudes into cervical canal ) , or other craniovertebral junction abnormalities . post - traumatic syringomyelia is a relatively infrequent , but potentially devastating complication that can occur anytime ( 1 month-45 years ) following traumatic spinal cord injury . rostral or caudal cyst extension may be caused by the turbulence of csf flow or a syringomyelia usually presents in third or fourth decades of life ( mean age of 30 years ) and usually involves the cervical cord . clinical presentations include sensory , motor and autonomic symptoms , painless ulcers of hands , neurogenic arthropathies ( charcot 's joints of shoulder , elbow or wrist ) . there is a marked lack of correlation between cavity size and severity of clinical symptoms . surgical treatments include suboccipital and cervical decompression , laminectomy and syringotomy ( dorsolateral myelotomy ) , fourth / terminal ventriculostomy , and neuroendoscopic surgery . myelography ( performed in special situations where mri can not be performed ) may show widening of cord and complete subarachnoid block . mri is the imaging modality of choice ; real - time ultrasonography may be feasible in young children and thin patients . flow voids noted on t2-weighted images of syrinx indicate csf flow ; these are better seen on t2 images without flow compensation . phase - contrast flow studies which measured velocities in syrinx ( cyst ) and pericystic subarachnoid spaces ( pcss ) , showed higher systolic and diastolic cyst velocities in large cysts and in patients with poor clinical status . postoperatively , decrease of systolic and diastolic cyst velocities and parallel increase of systolic pcss velocities were noted . diastolic cyst velocities correlated with preoperative clinical status of patients and postoperative satisfactory foraminal enlargement ( based on visibility of cisterna magna ) . state ( ciss ) , used in syrinx imaging to detect subarachnoid webs in syrinx , shows less flow voids than t2 images ; so it is not useful in detecting flow within syrinx . switching off the flow compensation ( flow comp off ) makes the flow voids more obvious and helps confirm csf flow within syrinx . normal pressure hydrocephalus ( nph ) is a disease that presents clinically with gait apraxia , dementia , and incontinence and radiologically as chronic communicating hydrocephalus . a potentially treatable cause of dementia , it may account for as many as 10% of cases of dementia . as the name suggests , mean csf opening pressure in patients with nph is within the normal range ( < 18 cm h2o or 13 mm hg with the patient in lateral decubitus position ) . since first described in 1965 , nph and its treatment by surgical ( csf ) shunting have been the focus of much investigation . diagnostic imaging in nph shows ventricular enlargement without sulcal widening ( venticulosulcal disproportion ) , generally more in frontal and temporal horns of lateral ventricle . the corpus callosum may be bowed upward and the cerebral gyri flattened against the inner table of the skull . csf production ( ~500 ml / day ) is mainly in choroid plexus of lateral ventricles . from there , csf flows through third ventricle , cerebral aqueduct , fourth ventricle , and foraminae of lushka and magendi . traditional teaching that csf is absorbed by arachnoid villi and superior sagittal sinus has been challenged by studies suggesting a role for extracranial lymphatics . velocity of csf flow is expected to be maximum in the narrowest part of ventricular system ( the aqueduct ) . this aqueductal flow is pulsatile : antegrade ( caudal- into 4 ventricle ) during cardiac systole and retrograde ( rostral- into 3 ventricle ) during cardiac diastole . increased aqueductal csf flow void has been noted in mr images of patients with communicating hydrocephalus . this signal loss occurs more often in nph than in healthy individuals , but not in acute communicating hydrocephalus or atrophy . clinically diagnosed as nph , extent of flow voids on proton density - weighted , non  flow - compensated conventional spin - echo images highly correlated with favorable response to csf diversion . later studies disapproved of the csf flow void as they used flow - compensated conventional spin - echo or fast spin echo ( which is intrinsically flow compensated due to the multiple pairs of 180 pulses ) techniques . increased peak velocity through aqueduct can be quantified with cine phase - contrast mri , and has been of proved value in selecting patients who are likely to respond to ventriculoperitoneal shunt surgery . patients with aqueductal csf stroke volume ( acsv ) > 42 l were more likely to respond to surgery . the positive predictive value of response to shunt surgery for acsv > 42 l was 100% , while for acsv < 42 l , it was 50% . the value 42 l is machine dependent , and may vary with machine manufacturers and with the different softwares for calculation . so , it is recommended that normal baseline values be calculated in each center performing csf flow studies by studying elderly patients without symptoms of nph . hyperdynamic csf flow ( which supports a diagnosis of shunt - responsive nph ) is to be declared when acsv is twice the normal value . other factors that may indicate shunt responsiveness include duration of presurgical symptoms ( < 6 months appears favorable ) , onset of gait disturbance before dementia , temporary pre - shunt symptom relief from a csf tap test ( removal of ~40 ml csf through lumbar puncture ) and absence of significant cerebral vascular disease . some institutions followed the following protocol : in patients with symptoms of nph , routine mr scan using conventional ( not fast ) spin echo is performed . if prominent aqueductal csf flow void is detected , patient is considered for shunt surgery ; if not , quantitative phase - contrast flow study is done . mri without flow compensation is thus a good technique of confirming flow within aqueduct and predicting response to shunt surgery . the simple process of switching off the flow compensation ( flow comp off ) makes the flow voids more obvious and helps confirm csf flow within syrinx ( syringomyelia ) and within aqueduct ( nph ) . it does not require much time or expertise ; can be done in most mr scanners and can predict response to surgery .
flow compensation , a gradient pulse used for artifact reduction , often used to suppress cerebrospinal fluid ( csf ) flow artifacts in spinal magnetic resonance imaging ( mri ) , can be switched off to make the csf flow voids within syrinx ( syringomyelia ) and within aqueduct [ normal pressure hydrocephalus ( nph ) ] more obvious ( thus confirming csf flow ) . 
 it is a simple method which does not require much time or expertise .