A novel, fast entropy-minimization algorithm for bias field correction in MR images

Disrupted development and integrity of frontal white matter in patients treated for pediatric medulloblastoma

Background

Treatment of pediatric medulloblastoma is associated with known neurocognitive deficits that we hypothesize are caused by microstructural damage to frontal white matter (WM).

Methods

Longitudinal MRI examinations were collected from baseline (after surgery but before therapy) to 36 months in 146 patients and at 3 time points in 72 controls. Regional analyses of frontal WM volume and diffusion tensor imaging metrics were performed and verified with tract-based spatial statistics. Age-adjusted, linear mixed-effects models were used to compare patient and control images and to associate imaging changes with Woodcock-Johnson Tests of Cognitive Abilities.

Results

At baseline, WM volumes in patients were similar to those in controls; fractional anisotropy (FA) was lower bilaterally (P < 0.001) and was associated with decreased Processing Speed (P = 0.014) and Broad Attention (P = 0.025) performance at 36 months. During follow-up, WM volumes increased in controls but decreased in patients (P < 0.001) bilaterally. Smaller WM volumes in patients at 36 months were associated with concurrent decreased Working Memory (P = 0.026) performance.

Conclusions

Lower FA in patients with pediatric medulloblastoma compared with age-similar controls indicated that patients suffer substantial acute microstructural damage to supratentorial frontal WM following surgery but before radiation therapy or chemotherapy. Additionally, this damage to the frontal WM was associated with decreased cognitive performance in executive function 36 months later. This early damage also likely contributed to posttherapeutic failure of age-appropriate WM development and to the known association between decreased WM volumes and decreased cognitive performance.

Measurable Supratentorial White Matter Volume Changes in Patients with Diffuse Intrinsic Pontine Glioma Treated with an Anti-Vascular Endothelial Growth Factor Agent, Steroids, and Radiation

BACKGROUND AND PURPOSE

Assessing the response to treatment in infiltrative brain tumors by using lesion volume-based response criteria is challenging. We hypothesized that in such tumors, volume measurements alone may not accurately capture changes in actual tumor burden during treatment. We longitudinally evaluated volume changes in both normal-appearing supratentorial white matter and the brain stem lesions in patients treated for diffuse intrinsic pontine glioma to determine to what extent adjuvant systemic therapies may skew the accuracy of tumor response assessments based on volumetric analysis.

MATERIALS AND METHODS

The anatomic MR imaging and diffusion tensor imaging data of 26 patients with diffuse intrinsic pontine glioma were retrospectively analyzed. Treatment included conformal radiation therapy in conjunction with vandetanib and dexamethasone. Volumetric and diffusion data were analyzed with time, and differences between time points were evaluated statistically.

RESULTS

Normalized brain stem lesion volume decreased during combined treatment (slope = -0.222, P < .001) and increased shortly after completion of radiation therapy (slope = 0.422, P < .001). Supratentorial white matter volume steadily and significantly decreased with time (slope = -0.057, P < .001).

CONCLUSIONS

Longitudinal changes in brain stem lesion volume are robust; less pronounced but measurable changes occur in the supratentorial white matter. Volume changes in nonirradiated supratentorial white matter during the disease course reflect the effects of systemic medication on the water homeostasis of normal parenchyma. Our data suggest that adjuvant nontumor-targeted therapies may have a more substantial effect on lesion volume changes than previously thought; hence, an apparent volume decrease in infiltrative tumors receiving combined therapies may lead to overestimation of the actual response and tumor control.

Prognostic factors that increase the risk for reduced white matter volumes and deficits in attention and learning for survivors of childhood cancers

OBJECTIVE

In children, CNS-directed cancer therapy is thought to result in decreased cerebral white matter volumes (WMV) and subsequent neurocognitive deficits. This study was designed as a prospective validation of the purported reduction in WMV, associated influential factors, and its relationship to neurocognitive deficits in a very large cohort of both acute lymphoblastic leukemia (ALL) and malignant brain tumors (BT) survivors in comparison to an age similar cohort of healthy sibling controls.

PROCEDURES

The effects of host characteristics and CNS treatment intensity on WMV were investigated in 383 childhood cancer survivors (199 ALL, 184 BT) at least 12 months post-completion of therapy and 67 healthy siblings that served as a control group. t-Tests and multiple variable linear models were used to assess cross-sectional WMV and its relation with neurocognitive function.

RESULTS

BT survivors had lower WMV than ALL survivors, who had less than the control group. Increased CNS treatment intensity, younger age at treatment, and greater time since treatment were significantly associated with lower WMV. Additionally, cancer survivors did not perform as well as the control group on neurocognitive measures of intelligence, attention, and academic achievement. Reduced WMV had a larger impact on estimated IQ among females and children treated at a younger age.

CONCLUSIONS

Survivors of childhood cancer that have undergone higher intensity therapy at a younger age have significantly less WMV than their peers and this difference increases with time since therapy. Decreased WMV is associated with significantly lower scores in intelligence, attention, and academic performance in survivors.

Comparing segmented ASL perfusion of vascular territories using manual versus semiautomated techniques in children with sickle cell anemia

PURPOSE

Elevated cerebral blood flow (CBF) in sickle cell anemia (SCA) is an adaptive pathophysiologic response associated with decreased vascular reserve and increased risk for ischemia. We compared manual (M) and semiautomated (SA) vascular territory delineation to facilitate standardized evaluation of CBF in children with SCA.

MATERIALS AND METHODS

ASL perfusion values from 21 children were compared for gray matter and white matter (WM) in vascular territories defined by M and SA delineation. SA delineated CBF was compared with clinical and hematologic variables acquired within 4 weeks of the MRI.

RESULTS

CBF measurements from M (MCA 82 left, 79 right) and SA (MCA 81 left, 81 right) delineated territories were highly correlated (R = 0.99, P < 0.0001). Bland-Altman plots had close-fitting limits of agreement of -1.8 to -3.5 lower limit and 0 to 1.8 upper limit. SA vascular territory delineation was comparable to the expert delineation with a kappa index of 0.62-0.85 and was considerably faster. Median territorial CBF values did not differ by gender or age. WM perfusion in the posterior cerebral artery territories was positively correlated with degree of hemolysis (R = 0.58, P = 0.01 left, 0.73, P < 0.001 right) and negatively correlated with hemoglobin (R = -0.48; P = 0.03 left; -0.47; P = 0.04 right) and hemoglobin F (R = -0.42; P = .09 left; -0.47; P = 0.049 right).

CONCLUSION

We established the validity of the SA method, which in our experience was much faster than the M method for delineation of vascular territories. Associations between CBF and hematologic variables may demonstrate pathophysiologic changes that contribute to clinical variation in CBF.

Evaluation of memory impairment in aging adult survivors of childhood acute lymphoblastic leukemia treated with cranial radiotherapy

BACKGROUND

Cranial radiotherapy (CRT) is a known risk factor for neurocognitive impairment in survivors of childhood cancer and may increase risk for mild cognitive impairment and dementia in adulthood.

METHODS

We performed a cross-sectional evaluation of survivors of childhood acute lymphoblastic leukemia (ALL) treated with 18 Gy (n = 127) or 24 Gy (n = 138) CRT. Impairment (age-adjusted score >1 standard deviation below expected mean, two-sided exact binomial test) on the Wechsler Memory Scale IV (WMS-IV) was measured. A subset of survivors (n = 85) completed structural and functional neuroimaging.

RESULTS

Survivors who received 24 Gy, but not 18 Gy, CRT had impairment in immediate (impairment rate = 33.8%, 95% confidence interval [CI] = 25.9% to 42.4%; P < .001) and delayed memory (impairment rate = 30.2%, 95% CI = 22.6% to 38.6%; P < .001). The mean score for long-term narrative memory among survivors who received 24 Gy CRT was equivalent to that for individuals older than 69 years. Impaired immediate memory was associated with smaller right (P = .02) and left (P = .008) temporal lobe volumes, and impaired delayed memory was associated with thinner parietal and frontal cortices. Lower hippocampal volumes and increased functional magnetic resonance imaging activation were observed with memory impairment. Reduced cognitive status (Brief Cognitive Status Exam from the WMS-IV) was identified after 24 Gy (18.5%, 95% CI = 12.4% to 26.1%; P < .001), but not 18 Gy (8.7%, 95% CI = 4.4% to 15.0%; P = .11), CRT, suggesting a dose-response effect. Employment rates were equivalent (63.8% for 24 Gy CRT and 63.0% for 18 Gy CRT).

CONCLUSIONS

Adult survivors who received 24 Gy CRT had reduced cognitive status and memory, with reduced integrity in neuroanatomical regions essential in memory formation, consistent with early onset mild cognitive impairment.

Retrospective correction of nonuniform illumination on bi-level images

We propose a novel method for correcting the effect of nonuniform illumination on a bi-level image. The proposed method is based on a penalized nonlinear least squares objective function that measures the binariness of an image and the roughness of illumination. Compared with conventional methods, it has the advantages of 1) not suffering from a trivial minimizer, 2) not requiring tuning of design parameters, and 3) effective optimization. In addition, it yields a unique solution since the minimization of the objective function is well-posed. In simulations and experiments, the method showed better accuracy and speed than the conventional entropy-based method.

Cerebellocerebral diaschisis is the likely mechanism of postsurgical posterior fossa syndrome in pediatric patients with midline cerebellar tumors

BACKGROUND AND PURPOSE

PFS occurs in approximately 25% of pediatric patients receiving surgery for midline posterior fossa tumors. Increasing evidence suggests that PFS represents a complex supratentorial cortical dysfunction related to surgery-induced disruption of critical cerebellocerebral connections. The purpose of this study was to determine whether a consistent surgical damage pattern may be identified in patients with PFS by early postoperative anatomic imaging analysis of the pECP and to test whether DSC can detect corresponding changes in cerebral cortical perfusion to indicate a secondary, remote functional disturbance, which could suggest a diaschisis-like pathomechanism.

MATERIALS AND METHODS

Eleven patients with postoperative PFS were evaluated retrospectively and were paired with age- and sex-matched control subjects in whom PFS did not develop. MR imaging work-up included DSC within 3 to 4 weeks after surgery as well as early postoperative anatomic imaging to evaluate components of the pECP.

RESULTS

DSC showed significant decreases in CBF within frontal regions (P < .05) and a trend to global cerebral cortical hypoperfusion in patients with PFS. Logistic regression analysis suggested a strong (potentially predictive) relationship between bilateral damage to pECP and the development of PFS (P = .04).

CONCLUSIONS

Our data suggest that the primary cause of PFS is the bilateral surgical damage to the pECP. This leads to a trans-synaptic cerebral cortical dysfunction (a form of bilateral crossed cerebellocerebral diaschisis), which manifests with DSC-detectable global, but dominantly frontal, cortical hypoperfusion in patients with patients with PFS compared with age- and sex-matched control subjects.

Scaling errors in measures of brain activity cause erroneous estimates of effective connectivity

Effective connectivity (EC) is the collective term for various measures of the interaction between the nodes in a network of neurons or neural populations during a certain experimental condition. Here, I investigated three types of EC that differ with respect to signal normalization, and therefore measure different aspects of neural interactions. Unnormalized EC measures pure connection strength. Amplitude-scaled EC measures the combined influence of signal amplitude and connection strength on neural activity. Finally, normalized EC measures the influence of one node on the activity of another relative to all influences on that node. With a theoretical analysis, I investigated the sensitivity of EC to signal scaling (the ratio of the amplitude of the measured signal and the underlying neural activity) and found that scaling affects the conclusions of the analysis of unnormalized EC severely, whereas normalized EC is not affected by the scaling problem. In an analysis of previously published hemodynamic response functions (Handwerker, D. A., Ollinger, J. M., D'Esposito, M., 2004. Variation of BOLD hemodynamic responses across subjects and brain regions and their effects on statistical analyses. Neuroimage 21, 1639-1651), I tested the predictions of the theoretical analysis. The empirical analysis indicated that signal scaling contributes to a large extent to measurement errors of unnormalized EC, although hemodynamic response function shape variability also contributed. Normalized EC, on the other hand, was only affected by shape differences and not by scaling. In addition to being more accurate, normalized EC is also an appropriate type of measure of neural interactivity if one is interested in the relative influence of one node on another, rather than absolute connection strengths per se.

Quantitative morphologic evaluation of magnetic resonance imaging during and after treatment of childhood leukemia

INTRODUCTION

Medical advances over the last several decades, including CNS prophylaxis, have greatly increased survival in children with leukemia. As survival rates have increased, clinicians and scientists have been afforded the opportunity to further develop treatments to improve the quality of life of survivors by minimizing the long-term adverse effects. When evaluating the effect of antileukemia therapy on the developing brain, magnetic resonance (MR) imaging has been the preferred modality because it quantifies morphologic changes objectively and noninvasively.

METHOD AND RESULTS

Computer-aided detection of changes on neuroimages enables us to objectively differentiate leukoencephalopathy from normal maturation of the developing brain. Quantitative tissue segmentation algorithms and relaxometry measures have been used to determine the prevalence, extent, and intensity of white matter changes that occur during therapy. More recently, diffusion tensor imaging has been used to quantify microstructural changes in the integrity of the white matter fiber tracts. MR perfusion imaging can be used to noninvasively monitor vascular changes during therapy. Changes in quantitative MR measures have been associated, to some degree, with changes in neurocognitive function during and after treatment.

CONCLUSION

In this review, we present recent advances in quantitative evaluation of MR imaging and discuss how these methods hold the promise to further elucidate the pathophysiologic effects of treatment for childhood leukemia.