Are you Yoshihisa Oka?

Claim your profile

Publications (26)30.17 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We evaluated neuropsychological function before and after surgery in patients with unruptured cerebral aneurysms. Neuropsychological functions in 43 patients with unruptured cerebral aneurysms were evaluated before and 1 month after surgery. The neuropsychological examination included the Mini-Mental State examination, "Kana-hiroi" test, Kohs Block Design test, and Miyake's Memory test. Then, if scores of even a single test were decreased 1 month after surgery, the tests were performed again 5 months later. In 24 of the 43 subjects, cerebral blood flow (CBF) was measured before and 1 month after surgery by single-photon emission tomography. The outcome in all patients was evaluated as good according to the Glasgow Outcome Score (GOS). In 17 (40%) of the 43 patients, neuropsychological function had deteriorated 1 month after surgery. The most sensitive test applied was the Miyake's Memory test. Of 14 patients with neuropsychological deterioration 1 month after surgery, 6 showed complete recovery, 5 showed partial recovery, and 3 still showed cognitive deterioration 6 months after surgery. Patients over 65 years old, those with anterior communicating artery aneurysms, those operated by interhemispheric approach, or those with systemic diseases showed a greater tendency toward a decline in postoperative neuropsychological function than the other patients. The postoperative CBF and vascular response in the frontal lobe of affected-side was decreased in cases showing cognitive deterioration. These results suggested that the neuropsychological outcomes after surgery for unruptured cerebral aneurysms were not satisfactory.
    Surgical Neurology 05/2003; 59(4):269-75; discussion 275-6. DOI:10.1016/S0090-3019(03)00043-0 · 1.67 Impact Factor
  • Yoshihisa Oka · Kazuhiko Sadamoto ·

    No shinkei geka. Neurological surgery 01/2003; 30(12):1361-3; discussion 1363-5. · 0.13 Impact Factor

  • Surgery for Cerebral Stroke 01/2003; 31(6):436-441. DOI:10.2335/scs.31.436
  • [Show abstract] [Hide abstract]
    ABSTRACT: Dynamic MR imaging provides hemodynamic information about normal and pathologic tissue of the brain. The purpose of our study was to evaluate the usefulness of dynamic MR imaging in the assessment of tumor vascularity and the tumor tissue blood flow of meningiomas. We studied 13 patients with meningiomas using dynamic spin-echo MR imaging. The histological subtypes of meningioma were confirmed by the examination of surgical specimens in all patients, and tumors were meningothelial in 9 cases, fibrous in 2, transitional in 1, and psammomatous in 1. Serial images were obtained every 18-24 sec for 8 minutes and 30 seconds after rapid injection of gadolinium diethylenetriaminepentaacetic acid. Different parameters (time to peak, maximum of signal intensity and the washout ratio) were calculated directly from signal intensity curves. As an indicator of tumor vascularity, microvessel density was counted based on immunohistochemically stained sections and tumor tissue blood flow was measured using an xenon-CT system. The maximum of signal intensity corresponded to the tumor vascularity. With dynamic MR imaging, the time intensity curves (TI curves) were divided into two patterns; type 1 had a steep increase with a peak and type 2 had a slow increase to a peak followed by plateau. The maximum of signal intensity measured from TI curve of dynamic MR imaging correlated significantly with microvessel density (R2 = 0.840, p < 0.0001). Linear regression revealed a significant positive relation between the washout ratio and the tumor tissue blood flow in group showed type 1 on TI curve (R2 = 0.961, p < 0.001). There was also a significant negative correlation between the time to peak and the tumor tissue blood flow (R2 = 0.792, p < 0.01). We suggest that dynamic MR imaging is useful for evaluating hemodynamics of meningiomas.
    Nō to shinkei = Brain and nerve 07/2002; 54(7):589-93.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The usefulness for evaluation of cerebral hemodynamics using time-resolved projection MRA was studied in normal volunteers and patients of cerebrovascular diseases. Six normal volunteers and ten patients with cerebrovascular occlusive diseases including 6 of IC occlusion and 4 of post EC/IC bypass surgery underwent time-resolved projection MRA on a 1.5 T clinical MRI system. Projection angiograms are acquired with 2 D-fast SPGR sequence with a time resolution of approximately one image per second, 40 images being acquired consecutively before and after bolus injection Gd-DTPA. And all images were calculated by complex subtraction from the background mask in a work station. In normal volunteers, the quality of images of time-resolved projection MRA was satisfactory. The arteries from internal carotid artery through M 2 segment of middle cerebral artery and all major venous systems were well portrayed. In 4 cases of IC occlusion who were assessed the collateral flow through the anterior communicating artery and posterior communicating artery, there were delayed to demonstrate the ipsilateral MCA. However, in 2 cases of IC occlusion that were assessed the collateral flow through leptomeningeal anastomosis, ipsilateral MCA and collateral circulation were not demonstrated. In all patients of post EC/IC bypass surgery, the patency of EC/IC bypass could be evaluated as properly with time-resolved projection MRA as 3 D-TOF MRA. Although the temporal and spatial resolutions are insufficient, time-resolved projection MRA was powerful non-invasive method to evaluate the cerebral hemodynamics via the basal communicating arteries in IC occlusion and identify the patency of EC/IC bypass.
    Nō to shinkei = Brain and nerve 06/2001; 53(5):463-7.

  • CARS 2001. Computer Assisted Radiology and Surgery. Proceedings of the 15th International Congress and Exhibition, Berlin, Germany, June 27-30, 2001; 06/2001
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report an adult onset patient with moyamoya disease showing acute progress after contralateral vascular reconstructive surgery. A 47-year-old female developed cerebral infarction in the left corona radiata. A magnetic resonance (MR) angiography and a cerebral angiogram revealed severe stenosis extending from the terminal portion of left internal carotid artery (ICA) to the M1 portion. The right ICA showed slight stenosis. We performed direct bypass surgery (STA-MCA anastomosis) on the affected left side. MR angiography 1 month after surgery revealed the progressive stenosis of the C1 portion of the right ICA. While measurement of cerebral blood flow (CBF) showed a slight impairment of vascular reactivity to acetazolamide loading in the region of the right MCA, we continued without vascular reconstructive surgery for the right side because there was no ischemic attack. The patient had a transient sensory disturbance of the left upper extremity 16 months after surgery. MR angiography and a cerebral angiogram revealed more progressive stenosis extending from the right ICA to the M1 portion. CBF study showed a low CBF at rest and a negative response to acetazolamide loading in the region of the right MCA. Direct bypass surgery was performed on the right hemisphere. Follow-up study revealed an increment of rest CBF and improvement of vascular reactivity. We underlined the necessity for careful postoperation observation of progressive contralateral arterial stenosis using MR angiography and CBF study in adult onset patients with moyamoya disease.
    No shinkei geka. Neurological surgery 05/2000; 28(4):373-8. · 0.13 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In comparison with 99mTc-ECD SPECT, the usefulness for evaluation of cerebral blood flow by the perfusion images using MRI with FAIR sequence was studied in ischemic stroke patients. Among 27 cases, 15 patients showed lacunar infarctions and 12 patients showed cortical infarctions determined by T2 weighted MR images. FAIR images were obtained as single images at the slice position running through the basal ganglia or corona radiata. The inversion times(TI) were varied, ranging from 800 to 1,400 msec. In 15 patients without definite low perfusions in the SPECT study, FAIR images showed sequentially proxymal arterial branches in early phase and distal arterial branches or capillary beds in the cortical tissues in a late phase as the TI was elongated. Nine of the 12 patients with low cerebral perfusions in the SPECT study showed perfusion defects in FAIR imaging. Five of the 12 patients with a small low cerebral perfusion area in the SPECT study showed a delay of the depiction of cortical arteries along with TI elongation. In 3 patients, ischemic lesions demonstrated by the SPECT study was not shown in the MRI study because of motion artifacts. In conclusion, FAIR imagings are considered to be useful in the evaluation of cerebral flow dynamics in the ischemic stroke patients.
    Nō to shinkei = Brain and nerve 03/2000; 52(2):151-6.
  • [Show abstract] [Hide abstract]
    ABSTRACT: A 58-year-old female presented with right conjunctival chemosis and right abducens nerve paresis. Cerebral angiography demonstrated a right carotid-cavernous sinus fistula associated with persistent primitive trigeminal artery. The fistula was treated by introducing detachable coils through the transvenous approach, as the detachable balloon was not available. Follow-up angiography performed 14 days after the embolization revealed complete disappearance of the carotid-cavernous sinus fistula due to thrombosis, which was presumably accelerated by the coils. Transvenous coil embolization should be considered as an alternative treatment for high-flow carotid-cavernous sinus fistula, but only if transarterial balloon embolization is not successful or unavailable.
    Neurologia medico-chirurgica 02/2000; 40(1):61-4. DOI:10.2176/nmc.40.61 · 0.72 Impact Factor
  • K Kusunoki · Y Oka · M Saito · K Sadamoto · S Sakaki · H Miki · K Nagasawa ·
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigated age-related changes in the visibility of intracranial arteries on magnetic resonance angiography (MRA) and the influence of risk factors for stroke. We studied 230 adult patients without specific neurological deficits. MRA was performed using the three-dimensional time-of-flight technique with a spoiled gradient-recalled acquisition sequence. We classified internal carotid artery (IC) and the horizontal (M1) and distal (beyond M2) middle cerebral segments into 4 grades. Linear regression revealed a significant negative relation between age and the quality of demonstration on MRA. For IC and M1, the score was significantly lower in subjects with risk factors than in those without. The distal MCA was poorly seen in patients without a history of hypertension or lacunar infarcts. A marked correlation was observed between visibility and age patients with no history of hypertension, diabetes mellitus and hyperlipidaemia. We suggest that atherosclerotic change and decline in flow velocity with normal ageing are factors leading to decreased visibility on MRA.
    Neuroradiology 12/1999; 41(11):813-9. DOI:10.1007/s002340050847 · 2.49 Impact Factor
  • Y Kumon · K Zenke · K Kusunoki · Y Oka · K Sadamoto · S Ohue · S Ohta · S Sakaki ·
    [Show abstract] [Hide abstract]
    ABSTRACT: We examined the diagnostic use of isotropic diffusion-weighted (DW) MRI in 140 consecutive patients suspected of or diagnosed as having an ischaemic stroke. Isotropic DW imaging could demonstrate the lesion responsible for the clinical deficit in patients with multiple cerebral infarcts at an early stage, even small lesions without a perifocal oedema or mass effect. Accurate diagnosis by DW images may, however, be difficult about 2 weeks after the onset of stroke.
    Neuroradiology 11/1999; 41(10):777-84. DOI:10.1007/s002340050841 · 2.49 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Endothelial nitric oxide synthase (eNOS) may play an important role in the regulation of tumor blood flow and vascular permeability. However, there have been no reports describing alterations of eNOS expression in relation to malignant progression in human astrocytic tumors. We immunohistochemically studied the relationship between eNOS expression in tumor vasculature and malignancy in supratentorial astrocytic tumors. Tissue samples were obtained from 12 patients with low-grade astrocytomas, 10 with anaplastic astrocytomas, and 17 with glioblastomas. Normal brain tissue samples were obtained from four patients with other brain diseases. Immunohistochemical staining was performed using the avidin-biotin complex method, with polyclonal anti-eNOS antibody, and the levels of eNOS expression in endothelial cells were evaluated as slight, moderate, or intense on the basis of eNOS immunoreactivity. The proliferative potential was assessed as the MIB-1 staining index for tumor cells. The expression of eNOS was slight in all specimens of normal brain tissue, slight in 7 and moderate in 5 specimens of low-grade astrocytoma, slight in 2, moderate in 6, and intense in 2 specimens of anaplastic astrocytoma, and moderate in 5 and intense in 12 specimens of glioblastoma. The MIB-1 staining index (mean+/-standard deviation) was 0.2+/-0.2% for normal specimens, 1.8+/-0.6% for low-grade astrocytomas, 9.6+/-6.9% for anaplastic astrocytomas, and 18.5+/-7.7% for glioblastomas. The MIB-1 staining indices for slight, moderate, and intense eNOS expression were 2.0+/-2.3%, 10.8+/-9.8%, and 16.9+/-7.7%, respectively. Expression of eNOS in tumor vessels was significantly correlated with histological grade and proliferative potential. These findings suggest that astrocytic tumor vessels possess higher activity for nitric oxide production than do normal vessels.
    Neurosurgery 08/1999; 45(1):24-8; discussion 29. DOI:10.1097/00006123-199907000-00006 · 3.62 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: MR angiograms from 200 neurologically normal subjects aged 30 to 79 years were analyzed to assess the influence of aging and hypertension on the degree of the--visualization of the cerebral arteries. The degree of MR visualization of the cerebral arteries, including the IC, M 1, and distal MCA, was evaluated on a 4-degree scale. IC and M 1 differ in appearance. The MR gradings of each cerebral arteries showed a significant (p < 0.001) negative correlation with age. Hypertension, which is a risk factors for arteriosclerosis, significantly reduced MR grade. The degree of visualization of M 1 and the distal MCA decreased significantly with age in the normotensive group. Therefore, the decrease visualization of the cerebral arteries appears to involve not only cerebral arteriosclerosis but other factors associated with aging.
    Nō to shinkei = Brain and nerve 04/1999; 51(3):221-6.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Some adult patients with moyamoya disease have been treated successfully by indirect revascularization alone, although surgical indications and hemodynamic changes for these patients have not been fully explored. To examine surgical indications for this procedure, we studied the regional cerebral blood flow (rCBF) and angiographic findings in adult patients with moyamoya disease preoperatively and postoperatively. On 17 hemispheric sides of 12 adult patients with moyamoya disease treated surgically with a combination of various indirect procedures, mainly by encephaloduroarteriosynangiosis, we retrospectively evaluated changes in rCBF using xenon-133 single photon emission computed tomography, angiographic collateral formation, and clinical results. Preoperatively, the rCBF values in the cortices at the bypass site at rest and after acetazolamide loading were lower than normal. The rCBF values were significantly increased after revascularization, approaching normal, except for incomplete recovery of vascular reactivity. The extent of postoperative neovascularization from implanted tissues fed by the external carotid artery system was more developed, in parallel with the preoperative decrease in resting and loading rCBF values. One-third of the operated sides exhibiting both a low rCBF at rest and impaired vascular reactivity in the noninfarcted cortices achieved good revascularization over two-thirds of the middle cerebral artery territory, accompanied by rCBF improvement and moyamoya vessel regression. Enough potential for neovascularization in the noninfarcted cortices was indicated that the resting rCBF was lower than 50 ml/100 g per minute (below the normal value by 2 standard deviations) and did not increase more than that value after loading, even in a 40-year-old patient who presented with a hemorrhage. Clinically, 11 patients (92%) had good results at the 4-year follow-up, whereas 1 patient (8%) with unsatisfactory neovascularization and a lesser extent of moyamoya vessel reduction experienced rebleeding. We conclude that for the surgical treatment of adult patients with moyamoya disease, indirect procedures, mainly encephaloduroarteriosynangiosis, are recommended for patients with lower rCBF and no or negative vascular reactivity in the noninfarcted cortices, as well as for those who have no indication for the direct procedure. It is possible to determine these indications by a xenon-133 inhalation single photon emission computed tomographic study including an acetazolamide challenge test.
    Neurosurgery 05/1998; 42(4):752-7; discussion 757-8. DOI:10.1097/00006123-199804000-00043 · 3.62 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We conducted neuropsychological testing of 86 patients who had undergone operation for subarachnoid hemorrhage (SAH) due to ruptured aneurysm 6 months to 1.5 years after onset. All patients were assessed as Good Recovery according to the Glasgow Outcome Scale. The battery of neuropsychological tests consisted of the Mini Mental State Examination (MMS), Kohs's block design test, Paired associate learning test, and “Kanahiroi” test.All patients examined by MMS 1.5 years after SAH were normal. Visuoconstructional cognition examined by Kohs's block-design test was impaired in about 40% and long-term memory was impaired in 20-60%. Further, 30% of the patients had frontal dysfunction investigated by “Kanahiroi” test. Multivariate analysis proved significant harmful effects of the neurological grade on onset (Hunt & Kosnik grade) on “Kanahiroi” test and paired associate learning test.In a series of neuropsychological tests, the scores slightly improved by 1 year after SAH, whereas almost none of the patients exhibited improvement in test scores. These findings suggest that neurophycological deficits after SAH was permanent.
    Surgery for Cerebral Stroke 01/1998; 26(2):96-100. DOI:10.2335/scs1987.26.2_96
  • [Show abstract] [Hide abstract]
    ABSTRACT: Some types of carotid-ophthalmic artery aneurysms are still difficult to clip successfully because their exposure requires opening the cavernous sinus and/or retracting the optic nerve. It is useful to know the complications and to determine the type of aneurysm preoperatively for the management of carotid-ophthalmic artery aneurysms. The operative results in 15 patients with asymptomatic unruptured carotid-ophthalmic artery aneurysms were surveyed. The aneurysms were small in all the patients, and they underwent direct operation. Four patients presented with other ruptured aneurysms, four with other diseases (infarction, trauma, or pituitary adenoma), and seven were evaluated with magnetic resonance angiography for symptoms such as vertigo or headache. Among them, five had carotid cave aneurysms and one had paraclinoid aneurysm. Neck clipping was performed in 13 patients. Postoperatively, ipsilateral visual loss was encountered in one patient, and ipsilateral visual field defect was encountered in three patients. The visual field defect was lower nasal quadrant hemianopsia in two patients and lower hemianopsia in one patient. The cause of this complication was suspected to be retraction and/or the heat of the drill near the optic nerve. It seemed to be possible to distinguish the carotid cave or the paraclinoid aneurysm from the other carotid-ophthalmic aneurysms using carotid angiography preoperatively. When direct operation is performed for a carotid-ophthalmic artery aneurysm, care must be taken to avoid optic nerve injury caused by the retraction and/or the heat of the drill.
    Surgical Neurology 12/1997; 48(5):465-72. DOI:10.1016/S0090-3019(97)00175-4 · 1.67 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Expansive laminoplasty of several types has been proposed for patients with cervical multisegmental stenotic myelopathy to reduce postlaminectomy complications. Its effectiveness has not been fully explored by evaluating long-term results and magnetic resonance imaging (MRI) findings before and after surgery. We conducted a 5-year follow-up study of 22 patients with cervical spondylotic myelopathy and/or ossification of the posterior longitudinal ligament surgically treated with expansive laminoplasty. The operative results were examined using the Japanese Orthopedic Association (JOA) disability scale, with reference to the findings of MRI, computed tomography, and radiography. Postoperative improvement was observed in 18 (81.8%) of the 22 patients. In 11 patients the percentage recovery of the JOA score was higher than 50% (average: 83.1%), while in the remaining 11 patients it was lower than 50% (average: 20.1%). Factors contributing to incomplete recovery appear to be related mainly to cord degeneration with atrophy (depicted as a T2-high intensity area) and to specific factors such as long symptom duration, age higher than 70 years, deterioration due to trauma, severe cord compression, radiculopathy, and kyphotic cervical curvature. In cervical myelopathy, patients with multisegmental stenosis, expansive laminoplasty can be expected to provide a favorable outcome by providing sufficient cord decompression and stabilization of the cervical spine, when the stenotic cervical canal is enlarged to the normal range (over 12 mm residual anteroposterior diameter and 200 mm2 residual canal area). The efficacy can be restricted by various factors, especially irreparable cord degeneration.
    Surgical Neurology 09/1997; 48(3). DOI:10.1016/S0090-3019(97)00166-3 · 1.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We examined time-dependent changes in protein synthesis and in the immunoreactivities of representative contraction-related structural proteins in smooth muscle cells of canine basilar arteries after experimental subarachnoid hemorrhage (SAH). Protein synthesis was assessed by the percentage of polyribosome-forming ribosomes to total ribosomes (aggregation rate), a morphological index of the activity of protein synthesis. The aggregation rates in prostaglandin F2 alpha-(PGF 2 alpha) and 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced contracted basilar arteries were 70.0 +/- 7.0% and 71.4 +/- 8.7%, respectively, quite similar to the value in normal basilar artery (73.0 +/- 8.0%). In the single-SAH group with little delayed histological changes in the basilar arteries, the aggregation rate was significantly decreased to 30.5 +/- 6.4% by 24 h after the SAH, and recovered to 52.3 +/- 9.0% and 70.2 +/- 7.6% at 7 and 14 days postSAH, respectively, when the vasospasm was moderately and completely ameliorated. In contrast, in the double-SAH group in which the basilar arteries developed delayed smooth muscle cell death and long-lasting arterial contraction, a significant decrease in the aggregation rate (25.0 +/- 5.0% on day 4) persisted for 14 days. The in vitro incorporation of [3H]-leucine in the basilar arterial cells was also significantly suppressed 4 and 7 days after the initial SAH (1.2 +/- 0.4 and 1.4 +/- 0.3 x 10(3) dpm/mg protein) in the double-SAH group, as opposed to no significant decrease in the basilar artery at 7 days postSAH in the single-SAH group (1.9 +/- 0.6 x 10(3) dpm/mg protein). The immunoreactivity of alpha-smooth muscle actin, a contractile protein, demonstrated by immunohistochemistry and immunoblots, was not altered for up to 14 days even in the double-SAH group, but that of calponin and of h-caldesmon, contraction-inhibiting proteins, was markedly reduced 4-14 days after the initial SAH. Persistent impairment of protein synthesis and relative reduction of immunoreactivities of the contraction-inhibiting proteins were observed in arteries with severe vasospasm and loss of smooth muscle cells, as noted in the double-SAH subjects. These abnormalities may cooperate to cause cerebral arterial narrowing accompanied by degeneration of smooth muscle cells after SAH.
    Journal of Cerebral Blood Flow & Metabolism 12/1996; 16(6):1335-44. DOI:10.1097/00004647-199611000-00031 · 5.41 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A 10-month-old girl presented with mental retardation and enlarged head, and a 3-year-old girl presented with epilepsy, both due to suprasellar arachnoid cysts. Magnetic resonance imaging demonstrated the suprasellar arachnoid cyst and the relationship between the cyst and surrounding structures. The 10-month-old patient with accompanying hydrocephalus was treated by fenestration of the cyst wall into the basal cistern. Postoperatively, bilateral subdural effusion appeared and subdural-peritoneal shunting was required. Thereafter, she regained the ability to walk. Two years postoperatively, the cyst reenlarged but communication between the cyst and surrounding cisterns and her development was normal, so she has been followed conservatively. The 3-year-old patient presented without hydrocephalus and was treated by cyst-peritoneal shunting using a Codman-Medos programmable valve through craniotomy. Postoperatively, the cyst was reduced and there has been no recurrence for 2 years and 5 months. We consider that cyst-peritoneal shunting is the most effective treatment for suprasellar arachnoid cyst.
    Neurologia medico-chirurgica 11/1996; 36(10):721-4. DOI:10.2176/nmc.36.721 · 0.72 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A 70-year-old female presented with gait disturbance and dementia. Cerebrospinal fluid examination revealed mild aseptic meningitis. Magnetic resonance images showed multiple cystic lesions and mural nodules in some of the cysts in the cerebral parenchyma. Metrizamide computed tomography clearly demonstrated multiple intracisternal cysts. Some cysts were surgically excised and a ventriculoperitoneal shunt was implaced. Postoperatively, her gait disturbance and dementia disappeared. Histological examination revealed cysticercosis with viable larva. Ten months after the initial surgery, she presented with right hemiparesis due to enlargement of the remaining cysts in the left frontal lobe. These cysts were removed. Postoperative neurological examination showed slight right hemiparesis. We recommend ventriculoperitoneal shunting for hydrocephalus due to cisternal cysticercosis and praziquantel administration for parenchymal lesions.
    Neurologia medico-chirurgica 10/1996; 36(9):654-8. DOI:10.2176/nmc.36.654 · 0.72 Impact Factor