Effects of cranioplasty in cerebral blood perfusion using quantification with 99m‐Tc HMPAO SPECT‐CT

Syndrome of the trephined or sinking skin flap syndrome is an underdiagnosed condition of craniectomized patients that usually improves after cranioplasty. Among the pathophysiological theories proposed, the changes of cerebral blood perfusion (CBP) caused by cranial defects might have a role in the neurological deficiencies observed. We aim to assess the regional cortex changes in CBP after cranioplasty with Technetium 99m hexamethylpropylene‐amine oxime (99mTc‐HMPAO) SPECT‐CT.


INTRODUCTION
Sinking skin flap syndrome (SSFS) or syndrome of the trephined (SoT) is a difficult-to-diagnose complication of decompressive craniectomy performed for any cause. SSFS has variable clinical presentation with signs and symptoms, such as sunken skin flap in the site of the craniectomy, headache, motor weakness, and language deficits, which can begin in the days or months following craniectomy, in some cases after full or partial recovery. 1 One of the most important characteristics of this syndrome is neurological improvement of many of these patients after the cranial defect is reconstructed with cranioplasty.
Although the previously mentioned signs and symptoms constitute the classical definition of the syndrome, studies published in recent years have shown improvement in clinical and physiological parameters after cranioplasty, even in patients without symptoms of SFSS. 2 Such findings have changed the management of craniectomized patients and the use of cranioplasty has been implemented not only for aesthetic or protective reasons but also to improve patient functionality and prognosis.
The pathophysiology of SSFS is unknown, although different hypotheses have been published, most of them considering the main variable that seems to affect these patients, which is the influence of atmospheric pressure in the brain. Whether it directly harms the brain parenchyma, changes brain metabolism, alters brain blood perfusion, or affects cerebrospinal fluid (CSF) flow (or a combination of the aforementioned factors), the mechanisms underlying SSFS have not been elucidated to date. [2][3] CT or perfusion magnetic resonance has been used to examine cerebral blood perfusion (CBP) changes in these patients and establish different pathophysiological theories. 4 In recent years, positron emission tomography has evolved to become the gold standard method for analyzing brain-blood hemodynamics, although measuring glucose metabolism is not exactly the same as measuring CBP. Moreover, quantification of those changes has typically been limited to brain hemispheres or lobes at most with no evaluation of different cortical regions.
In our study, we aimed to analyze changes in CBP in craniectomized patients before and after cranioplasty using Technetium-99 m hexamethylpropylene-amine oxime (Tc-99 m HMPAO) and SPECT-CT to assess changes in different cortical areas. This radiotracer is a specific tool to measure CBP, not only cerebral blood flow or metabolism, as in previous imaging studies.

METHODS
A total of 28 patients were selected for this study, all of whom had a craniectomy performed to control refractory high intracranial pressure and were due to cranioplasty between October 2016 and November 2019. Patients were scheduled to receive cranioplasty regardless of whether they had classical symptoms of SSFS as a procedure to improve their clinical situation. We recorded the demographic data of each patient, pathology leading to the craniectomy, date of cranioplasty, and any brain hemisphere damage.
Surgeries were scheduled a variable period of time after decompressive craniectomy, depending on the primary cause for decompression, reduction of brain swelling, and patient and surgeon preferences.
As a general rule, procedures were performed after some patient rehabilitation. Autologous bone flaps were the first choice to reconstruct the cranial defects, and when not available, computer-designed polyetheretherketone or methacrylate implants were used.
To evaluate CBP, we used a hybrid imaging technique (SPECT-CT) for morphological imaging, as well as for attenuation correction.
We employed Tc-99 m HMPAO as a radiopharmaceutical because it diffuses into neurons based on blood perfusion of the area. Ten to 20 minutes after injection, the radiopharmaceutical fixes to the neuron, and there is no intracerebral redistribution, resulting in a fixed representation of CBP. Each patient was scheduled for three separate studies performed at different timepoints, including before cranioplasty, 1 week after the surgery, and finally 3 months after surgery.
Imaging studies were performed following the recommendations of the European Society of Nuclear Medicine guidelines. Patients were injected with a 925 MBq dose of Technetium-99 m HMPAO at rest and in decubitus in a quiet room 30-60 minutes before the scan was conducted. Images were acquired in a dual head gamma camera and CT scan (model NM/CT 640, General Electric Healthcare, USA). Images were reconstructed in a 128 × 128 matrix using high-resolution collimators. Processing was performed in Xeleris 4.0 Functional Imaging Workstation (General Electric Healthcare, USA) using filtered back projection reconstruction and Butterworth filtering (critical frequency 0.51, order 10).
Quantification was performed using QBrain software (General Electric Healthcare, U.S.A.), which was added to the Xeleris 4.0 Workstation. The regions of interest are automatically drawn by the software on the CBP image obtained by SPECT and then manually adjusted if needed to correct possible patient positioning issues. A ratio was calculated, comparing the 12 different cortical regions to one reference area, the pons, which was selected due to its location far from the cortical defects observed in these patients, which, in turn, is important to guarantee an adequate quantification of blood perfusion in the different studies. CBP in each cortical region was compared to a database of normal individuals included and validated for clinical use by the manufacturer in the QBrain software. A Z-score was obtained for each brain region that indicated the differences between the patient and normal CBP in each region ( Figure 1).
Statistical analyses were performed using SAS software (version 9.4, SAS Institute Inc., 2012). Descriptive data are presented as the mean ± standard deviation for continuous variables, and categorical variables are expressed as absolute or relative frequencies. Two different methods of statistical analysis were performed for the same sample. First, we used a mixed model effects in which each patient was considered a random effect, while the brain hemisphere (damaged or undamaged) where the region studied was located and the moment in which the SPECT-CT was performed (presurgical, postsurgical 1 week and F I G U R E 1 Segmentation of the cerebral cortex in transverse slices of the brain by postsurgical SPECT-CT. The lines in colors represent the cortical areas studied and quantified afterward using ratio and Z-score. The radiotracer uptake of each area is quantified and compared with a reference area, in our case the pons. P, posterior; A, anterior; R, right; L, left 3 months) were considered fixed effects, and the interaction between the moment and the brain hemisphere was evaluated. This model reliably determines the areas where changes are observed and the value of such variations. Second, we used a T-Student for related samples to assess the variations of CBP in each patient at two different timepoints to clearly determine the magnitude of those changes.
This study was revised and approved by the ethical committee of the Hospital Universitario 12 de Octubre (Reference: Committee for Clinical Research number: 16/361). Informed consent to participate and publication was obtained from all participants in this study.

RESULTS
A total of 28 patients were included in this study ( Results are shown for both the ratio and Z-score for each hemisphere as a whole, as well as for the 12 regions in each hemisphere.
We referred to the damaged side as the hemisphere where the cranioplasty was performed, and we referred to the undamaged side as the other hemisphere. Note: All data represent mean ± standard deviation unless otherwise indicated; n = number of participants.

TA B L E 1 Characteristics of the participants
Regarding both hemispheres as a whole, our results with a mixed effects model showed a statistically significant increase in CBP in both hemispheres after cranioplasty, both in ratio (β = .019, p-value = .030 in the SPECT-CT a week after surgery and β = .021, p-value = .015 in the F I G U R E 2 Visualization of the cerebral blood perfusion (CBP) on a presurgical SPECT-CT where the functional images are presented on a template of the normal brain. Defects and reduced perfusion in the frontal, parietal, and parietal lobes of the right hemisphere are evident in the images and consequently provoke reduced cerebral blood perfusion ratios (patient column) and Z-scores. This pattern of cortical damage with preservation of CBP in medial regions of the cortex as well as contralateral hemisphere is found throughout the study subjects. S, superior; I, inferior; L, left; R, right; A, anterior; P, posterior one performed 3 months after, vs. presurgical) and Z-score (β = .220, pvalue = .026 and β = .279, p-value = .005, respectively). The damaged hemisphere exhibited a decreased blood perfusion ratio compared to the undamaged side before and after the reconstruction surgery (β = -.073, p-value < .0001) ( Figure 2). T-Student model statistics only showed a significant increase in the undamaged hemisphere in the 3 months postsurgery SPECT-CT, both in ratio with 2.23% increase (p-value = .048) and Z-score, 30.60% (p-value = .025).
In the analysis for each of the 12 areas, nine of the damaged hemispheres (prefrontal lateral and medial, sensorimotor, occipital lateral, primary visual, precuneus, and both temporal lateral and mesial) presented a significantly lower ratio (Table 2 and Figure 3) and Z-score (Table 3 and Figure 4) for CBP compared to the undamaged side.
Regarding measurement of CBP ratios using a mixed effects model,  The results of the interactions between the moment and the brain hemisphere are only included in the areas of the brain where they were significant. *Significance for p-value <0.05 shown in the undamaged hemisphere where average raise was 88.01% (p-value = .046). with SoT who was evaluated before and after cranioplasty with a CT perfusion scan, which showed an increase in CBP after the procedure. 9 Sarubbo et al. studied changes in CBP before and after cranioplasty (7 days and 3 months, respectively), showing an increase in CBP, although less important at the 3-month study, suggesting that the changes observed begin soon after cranioplasty and may even be temporary. 10 In our center, Paredes  The results of the interactions between the moment and the brain hemisphere are only included in the areas of the brain where they were significant. *Significance for p-value <0.05 F I G U R E 4 Graphics representing average variations of Z-scores (horizontal axis) for each of the 12 different cortical areas studied. Two lines are drawn for each cortical area representing the damaged and undamaged side of the brain. The lines have three points representing the different SPECT-CT studies. The first point represents the presurgical SPECT-CT, the second the first postsurgical study (1 week after the cranioplasty), and the third the last SPECT-CT (3 months after).

DISCUSSION
respectively. 2 CBP improvement was observed in all craniectomized patients, regardless of whether they presented with SoT symptoms.
In the same study, no statistically significant clinical relationship was observed between those results and clinical improvement, although a greater increase in CBP was identified in patients with a better clinical outcome.
Shahid and colleagues studied CBP in brain lobes and basal ganglia 1 week before and 3 months after cranioplasty. CBP assessment was obtained with SPECT-CT using technetium-99 m ethyl cysteinate dimer (99 mTc-ECD), 11 whose pharmacokinetics differ slightly from 99 mTc-HMPAO. They found that CBP increased after cranioplasty in the frontal and occipital lobes and decreased in the other lobes, although statistically significant changes were only found in occipital and basal ganglia regions. The decrease in blood flow in the parietal, temporal, and basal ganglia was attributed to redistribution of the CBP after cranioplasty. Matsumura presented a case in 1996 of a young child with a cranial defect in the right parietofrontal region. 12 The young patient was studied before and after reconstructive surgery using 99 mTc-HMPAO, which showed a decrease in CBP in the damaged area that subsequently normalized.
To the best of our knowledge, we present the first series of patients whose CBP was evaluated using 99 m-Tc HMPAO SPECT-CT. Moreover, we did not find other studies analyzing blood perfusion changes at the regional level or comparing those variations to a normal population, as we have done using the normal brain database in our quantification software.
First, we must consider that we are not measuring absolute variations in blood flow but rather changes in the ratios between the different brain regions using the pons as a reference. Quantification using ratios has one major advantage: general blood perfusion alterations in the whole brain also alter the reference area used to compare the cortex; as such, we consider those values to be more accurate than absolute values. Another aspect of the study worth considering is that the damaged side of the brain is prone to possible quantification errors by the software due to the anatomical damage that these patients suffer, which is especially significant in parietal and frontal lobes, where parenchymal damage is more frequent.
We identified an increase in CBP on both sides of the brain after cranioplasty, and the ratios increased bilaterally in eight of 12 regions examined in the second study and in three regions in the third SPECT-CT, which was more significant in the evaluation performed 3 months after surgery. This difference was observed both in the ratios of CBP and Z-score, which is consistent with published evidence using other imaging techniques and adds more evidence indicating that CBP increases in both hemispheres after cranioplasty, [2][3][4][9][10][11][12][13] and that CBP changes seem to play a key role in the pathophysiology of SoT.
Regarding specific regions, the general pattern evidenced an increase in CBP after surgery, especially in the second study, while the tendencies in the third study were more variable. For example, temporal regions exhibited a decrease in CBP, even lower than in the presurgical examination, which, according to Shahid, might indicate redistribution of CBP from the temporal lobes to other areas of the brain that are typically damaged. In our case, we did not observe the same results in the parietal lobes. 11 The cingulate cortex demonstrated a particular pattern on the damaged side, since both anterior and posterior regions of this cortical area are the only regions where the blood perfusion ratio in the week after cranioplasty reaches higher ratios than the undamaged side of the brain both in ratios and Z-scores. The posterior cingulate of the damaged side is the only area of the brain that exhibited a statistically significant increase in blood perfusion ratio in the second study, both in Note: Statistically significant results are highlighted (*). a Ratio percentage difference of ratio between the presurgical study (1) and the one performed a week after cranioplasty (2). b Ratio percentage difference of ratio between the presurgical study (1) and the one performed 3 months after cranioplasty (3).
ratios and Z-score assessment. The percentage of variations in CBP is below 5% in case of ratios (Table 4) and mostly perceptible with quantification analysis that is nowadays a common tool. To our knowledge, this is the first time that significant changes in CBP have been identified in a specific region of the brain in this patient population.
In a review article by Leech  Note: Statistically significant results are highlighted (*). a Z-score percentage difference of ratio between the presurgical study (1) and the one performed a week after cranioplasty (2). b Z-score percentage difference of ratio between the presurgical study (1) and the one performed 3 months after cranioplasty (3).
autism, and depression. Traumatic brain injury induces a reduction in metabolism and blood perfusion in the damaged area, which is related to attention deficits with difficulties sustaining attention, producing attentional lapses and decreased cognitive performance. Although we did not perform any clinical evaluation in this study, dysfunction of this area was related to similar symptoms in craniectomized patients and could serve as evidence for further studies of the function of this brain region.
We are aware of several limitations in our study, the most relevant likely being the possible miscalculation of regional CBP due to anatomical anomalies. This limitation is hardly solvable due to the heterogeneity of anatomical alterations among patients and timepoints.
However, all the imaging studies have been processed with the same software and parameters, and the relevant findings of regional CBP alterations are mostly found in areas far from the harmed regions.
Other relevant limitations are related to the heterogeneous patient population in our study, with different causes leading to decompres-sive craniectomy, a wide range of time intervals between surgeries, and different materials used to reconstruct the cranial defect. Another limitation of this study is that we only evaluated patients in decubitus and not in a standing or sitting position, which induces changes in cerebral blood and cerebrospinal fluid flows. As multiple comparisons have been made with T-Student analysis, we have a risk of a Type 1 error, although we find this as assumable for two reasons: first the study uses also a mixed effects model to assess the CBP changes and second because we try to avoid a Type 2 error as this work is mainly exploratory. The possibility of evaluating CBP changes in different positions is a clear advantage of this technique, which might be a good follow-up study.
Finally, we are aware that the most interesting way forward in this line of study is to correlate these findings with changes in the clinical evolution of these patients.