Arterial Spin-Labeling Perfusion MR Imaging Demonstrates Regional CBF Decrease in Idiopathic Normal Pressure Hydrocephalus

Patients and Healthy Controls

Sixty patients were prospectively and consecutively recruited from the waiting list of patients referred to Uppsala University Hospital for evaluation of suspected iNPH. Nine patients were excluded for participating in an ongoing multicenter study of the CSF tap test. Patients with complete inability to walk were excluded because of the intention to study the relationship between CBF and gait and balance function. Other exclusion criteria were secondary or congenital hydrocephalus, contraindications to MR imaging, older than 84 years of age, severe cognitive dysfunction with an inability to give informed consent, refusal to undergo evaluation or shunt surgery, and gait problems that could be explained by other known diseases. The inclusion process is illustrated in the flow chart (Fig 1). Of the 26 patients examined clinically, 17 were classified as having probable iNPH according to the iNPH guidelines.¹⁵ Six patients were classified as having possible iNPH, 1 because of only 1 triad symptom (gait disturbance), 1 due to severe white matter changes, 1 because of a small cyst communicating with 1 of the lateral ventricles, and 3 due to a history of head trauma, probably unrelated to the hydrocephalus. Three patients were considered unlikely to have iNPH, 2 because of symptoms of Parkinson disease and 1 with an aqueductal stenosis. The patients examined in this study have been described in detail in a previous study.¹⁶

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Fig 1.

Flow chart describing the inclusion process of patients.

Controls were randomly recruited from the county of Uppsala using the Swedish population registry and were matched with patients with respect to age (±2 years) and sex. Exclusion criteria were any known neurologic disease, stroke, diabetes mellitus, history of myocardial infarction that required acute treatment or resulted in persistent electrocardiogram changes, dependence on walking aids, or any terminal disease. Antihypertensive medication, aspirin, and common pain medications were allowed. Information was sent to 105 potential elderly controls. Forty-nine did not reply; 31 responded that they did not want to participate or could not participate according to the exclusion criteria. The remaining 25 accepted participation and were examined clinically by a neurologist. MR imaging examination was not possible in 2 of the 25 controls because the MR imaging scanner was upgraded before those 2 controls were examined, and another 2 controls did not match any patient. Thus, a total of 21 matched pairs of patients (15 with probable iNPH and 6 with possible iNPH) and controls were available and included in the statistical analysis. The study was approved by the local ethics committee in Uppsala, Sweden.

Clinical Examination

Neurologists, especially trained physiotherapists, and occupational therapists performed the clinical examinations. Besides a standard neurologic examination, the tests included the modified Rankin Scale, Mini-Mental State Examination (MMSE), incontinence scale,¹⁷ Romberg test, and time and number of steps required to walk 10 m at a maximum pace. Tests of motor function were performed twice, and the mean result was used in the statistical analyses.

Imaging

Details of the imaging method, scanner protocol, and postprocessing have been described previously.¹⁶ The first MR imaging was performed between 8:00 am and 10:00 am. The second MR imaging was performed 60 minutes after the first scan to assess the repeatability of the method. The patients left the scanner after the first investigation. Between the investigations, the patients rested in the supine position in a quiet, isolated room next to the scanner. The patients were not allowed to use caffeine on the day of the MR imaging, and all transportation was performed with the patient in bed or in a wheelchair to minimize variations in CBF. Healthy controls were only examined once with MR imaging.

Scanner and Imaging Protocol

Imaging was performed with a 3T MR imaging scanner (Achieva; Philips Healthcare, Best, the Netherlands) with a 32-channel head coil. A 3D T1-weighted gradient-echo and a T2-weighted FLAIR sequence were included in the protocol. Perfusion MR imaging was performed with a background-suppressed pseudocontinuous ASL sequence, with a single-shot echo planar imaging readout, with the following protocol: FOV = 264 × 264 mm², in-plane resolution = 2.75 × 2.75 mm², section thickness = 5 mm, number of sections = 20, TR = 4100 ms, TE = 14.5 ms, label duration = 1650 ms, postlabeling delay = 1600 ms, and 30 repetitions. To allow quantitative CBF values, we obtained a fast reference scan.

Synthetic MR imaging data were acquired with the multisection, multiecho, and multisaturation delay sequence Quantification of Relaxation Times and Proton density by Multiecho acquisition of a saturation recovery with Turbo spin-echo Readout (QRAPMASTER).¹⁸ Because of technical problems, synthetic MR imaging data were not acquired in 2 patients and in 1 control.

Perfusion MR Imaging Postprocessing

For perfusion quantification, the compartment model proposed by Alsop and Detre was used.¹⁹ The magnetization difference (perfusion-weighted signal) is given by where M_0a is the arterial blood magnetization, α is the labeling efficiency, f is the perfusion, T_1a is the T1 of arterial blood, τ is the labeling duration, and w is the postlabeling delay. Labeling efficiency was set to 70%, T_1a was set to 1664 ms, and M_0a was estimated with the signal intensity of CSF (see Alsop and Detre¹⁹ for more detail).

As an anatomic reference, the FLAIR images were coregistered to the perfusion maps by using a 12-degree affine transform (elastix; http://elastix.isi.uu.nl/). To minimize the effect of nonideal image coregistration and to reduce the intrasubject variation caused by varying delineation of regions, we spatially smoothed the CBF maps (Gaussian kernel with 4-mm full width at half maximum, Fig 2).

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Fig 2.

Example of ASL perfusion maps from a healthy control without (A) and with (B) smoothing, as well as from a patient with iNPH without (C) and with (D) smoothing.

ROIs and Morphologic Evaluations

Twelve different ROIs were drawn manually on transverse T2 FLAIR images using the in-house software Eval Gui (developed by Markus Nilsson, Lund University, Lund, Sweden). We covered the following regions: cerebellum, pons, high-convexity cortex, medial frontal cortex, lentiform nucleus, medial temporal lobe, supplementary motor area, thalamus, frontal WM, lateral WM, superior WM, and periventricular WM. The number of sections and voxels for all ROIs is listed in Table 1. Four different ROIs are illustrated in Fig 3, and all ROIs in both patients and controls are illustrated in the On-line Appendix. All sections of all scans were visually inspected, and care was taken not to include artifacts or large vessels in the ROIs. The ROIs were drawn by the first author (J.V., 4 years of experience) and reviewed by the last author (E.-M.L., >30 years of experience). When ROIs were drawn, the investigator was blinded to the patients' clinical data. CBF values were calculated for every voxel, and a mean CBF was provided in each ROI.

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Fig 3.

FLAIR images that were coregistered to the ASL images and used to draw ROIs. A–D, Healthy controls. E–H, Patients with iNPH. A and E, High-convexity cortex. B and F, Lateral periventricular WM. C and G, Frontal WM. D and H, Superior WM. All ROIs above were drawn in 2 adjacent sections, 1 of which is shown here.

For descriptive purposes, we assessed the following morphologic MR imaging features: the Evans index,²⁰ the callosal angle,²¹ and deep white matter hyperintensities (DWMH) according to the visual grading scale of Fazekas et al.²² Disproportionately enlarged subarachnoid space hydrocephalus was defined as a combination of enlarged ventricles (Evans index of >0.3), narrow sulci at the high convexity or parafalcine and dilated Sylvian fissures (Table 2).³ The measurements of morphologic features were performed by the last author (E.-M.L.). She was blinded to CBF data.

Volumetric Measurement

Measurements of CSF volumes were performed by using the QRAPMASTER sequence,¹⁸ which provides quantification of longitudinal relaxation time (T1), transverse relaxation time (T2), and proton density. The software SyMRI Brain Studio (SyntheticMR, Linköping, Sweden) uses combinations of T1, T2, and proton density to estimate voxelwise partial volumes of GM, WM, and CSF, which allow volumetric estimation of these tissues. The volume of both lateral ventricles was calculated semiautomatically in both patients and controls after manual delineation on each section that included the lateral ventricles.

Statistical Analysis

The mean CBF in each ROI from the 2 scans of the patients was used in the statistical analyses. Each patient was paired with an age- and sex-matched control. A paired-samples t test was used for the difference in CBF between patients and controls. Differences in descriptive data and comorbidity were analyzed with the Wilcoxon signed rank test or the McNemar test. Difference in age was tested with the Mann-Whitney U test. Correlations were assessed with the Spearman correlation. An intraclass correlation coefficient (2-way mixed, single measures) was used to calculate the repeatability of CBF between the 2 investigations in patients. For the analysis of CBF in patients and controls in the 12 ROIs, the Holm-Bonferroni correction for multiple comparisons was applied to reduce type I errors. Holm-Bonferroni correction works by sorting the N P values from the multiple tests and testing the smallest P value against α/N, the second smallest against α/(N-1), and so on. As soon as a null hypotheses is not rejected, all subsequent null hypotheses are also deemed not rejected, resulting in an effective significance threshold. With α = .05, the effective significance threshold for ROI analysis in patients and controls was P < .006. Correlations were tested among all 12 ROIs, and the 4 clinical tests as well as the 4 morphologic imaging features. Only correlations with r > 0.60 were considered strong and included in the results section. All analyses were performed with SPSS Statistics for Macintosh, Version 22.0 (IBM, Armonk, New York).