Silver-based surface plasmon waveguide for terahertz quantum cascade lasers.

Terahertz-frequency quantum cascade lasers (THz QCLs) based on ridge waveguides incorporating silver waveguide layers have been investigated theoretically and experimentally, and compared with traditional gold-based devices. The threshold gain associated with silver-, gold- and copper-based devices, and the effects of titanium adhesion layers and top contact layers, in both surface-plasmon and double-metal waveguide geometries, have been analysed. Our simulations show that silver-based waveguides yield lower losses for THz QCLs across all practical operating temperatures and frequencies. Experimentally, QCLs with silver-based surface-plasmon waveguides were found to exhibit higher operating temperatures and higher output powers compared to those with identical but gold-based waveguides. Specifically, for a three-well resonant phonon active region with a scaled oscillator strength of 0.43 and doping density of 6.83 × 1015 cm-3, an increase of 5 K in the maximum operating temperature and 40% increase in the output power were demonstrated. These effects were found to be dependent on the active region design, and greater improvements were observed for QCLs with a larger radiative diagonality. Our results indicate that silver-based waveguide structures could potentially enable THz QCLs to operate at high temperatures.

Gas spectroscopy through multimode self-mixing in a double-metal terahertz quantum cascade laser.

A multimode self-mixing terahertz-frequency gas absorption spectroscopy is demonstrated based on a quantum cascade laser. A double-metal device configuration is used to expand the laser's frequency tuning range, and a precision-micromachined external waveguide module is used to enhance the optical feedback. Methanol spectra are measured using two laser modes at 3.362 and 3.428 THz, simultaneously, with more than eight absorption peaks resolved over a 17 GHz bandwidth, which provide the noise-equivalent absorption sensitivity of 1.20×10-3 cm-1 Hz-1/2 and 2.08×10-3 cm-1 Hz-1/2, respectively. In contrast to all previous self-mixing spectroscopy, our multimode technique expands the sensing bandwidth and duty cycle significantly.

Extraction-controlled terahertz frequency quantum cascade lasers with a diagonal LO-phonon extraction and injection stage.

We report an extraction-controlled terahertz (THz)-frequency quantum cascade laser design in which a diagonal LO-phonon scattering process is used to achieve efficient current injection into the upper laser level of each period and simultaneously extract electrons from the adjacent period. The effects of the diagonality of the radiative transition are investigated, and a design with a scaled oscillator strength of 0.45 is shown experimentally to provide the highest temperature performance. A 3.3 THz device processed into a double-metal waveguide configuration operated up to 123 K in pulsed mode, with a threshold current density of 1.3 kA/cm2 at 10 K. The QCL structures are modeled using an extended density matrix approach, and the large threshold current is attributed to parasitic current paths associated with the upper laser levels. The simplicity of this design makes it an ideal platform to investigate the scattering injection process.

Active phase-nulling of the self-mixing phase in a terahertz frequency quantum cascade laser.

We demonstrate an active phase-nulling scheme for terahertz (THz) frequency quantum cascade lasers (QCLs) under optical feedback, by active electronic feedback control of the emission frequency. Using this scheme, the frequency tuning rate of a THz QCL is characterized, with significantly reduced experimental complexity compared to alternative approaches. Furthermore, we demonstrate real-time displacement sensing of targets, overcoming the resolution limits imposed by quantization in previously implemented fringe-counting methods. Our approach is readily applicable to high-frequency vibrometry and surface profiling of targets, as well as frequency-stabilization schemes for THz QCLs.

Three-dimensional terahertz imaging using swept-frequency feedback interferometry with a quantum cascade laser.

We demonstrate coherent three-dimensional terahertz imaging by frequency modulation of a quantum cascade laser in a compact and experimentally simple self-mixing scheme. Through this approach, we can realize significantly faster acquisition rates compared to previous schemes employing longitudinal mechanical scanning of a sample. We achieve a depth resolution of better than 0.1 µm with a power noise spectral density below -50 dB/Hz, for a sampling time of 10 ms/pixel.

Terahertz inverse synthetic aperture radar imaging using self-mixing interferometry with a quantum cascade laser.

We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit.

The impact of athrosclerosis on cognition and disability in multiple sclerosis patients: the ATHUS score.

AIM: Atherosclerosis of the large arteries of the neck could be related to the cognitive and motor deficit. We investigated if the atherosclerosis of common carotid and femoral arteries in patients with multiple sclerosis (MS) is directly linked with a disability and has an inverse relationship with cognitive performance. METHODS: We enrolled, in this prospective study, a random sample of 105 patients with MS and 22 healthy controls. All participants received a comprehensive neuropsychological assessment. The physical disability was quantified with the Expanded Disability Status Scale (EDSS). We utilized ultrasound of the carotid and femoral arteries to evaluate the degree of stenosis and intima-media thickness (IMT). We created a novice ultrasound index of atherosclerosis (ATHUS score) based on the arterial stenosis and the IMT of the carotid and femoral arteries. We then compared the results of the psychometric assessment and EDSS with the ATHUS score. RESULTS: The analysis demonstrated that higher cognitive function is correlated with lower values of ATHUS score (p =0.01). Also, there was a direct correlation between the ATHUS score and EDSS (p =0.001). CONCLUSION: Our results suggest that the degree of atherosclerosis, as calculated by the ATHUS score, is directly related to low cognitive score and higher sensory and motor disability. HIPPOKRATIA 2019, 23(2): 81-86.

Cerebral cysticercosis: treatment with praziquantel. Report of two cases.

Two patients with cerebral cysticercosis are described: one with a long-standing infection with multiple intracerebral calcifications, and one who had become infected only recently. Treatment with praziquantel 50 mg/kg body weight daily for 14 days resulted in clinical and radiological improvement in the first patient, although she obviously still harbored a number of viable non-calcified parasites. The second patient was completely cured by the same praziquantel regimen. Both patients needed temporary corticosteroid treatment to suppress development of intracranial hypertension. Computerized tomographic scanning and regular measurement of serum antibody titers against cysticercal and echinococcal antigen seem appropriate methods for follow-up of the effect of chemotherapy.

The missing temporal crescent.

PURPOSE: We studied clinically the representation of the monocular temporal crescent in the human visual cortex and noted the importance of using the perimetric techniques best suited to detect this visual field defect and to study patients in whom the temporal crescent is missing. METHODS: Goldmann perimetry and high-resolution magnetic resonance imaging were performed in two patients with vascular lesions located in the anterior striate cortex. RESULTS: A monocular visual field defect, the missing temporal crescent, was found on the side contralateral to the lesion. CONCLUSIONS: The perimetric-magnetic resonance imaging correlation is in exquisite agreement with recent information about the representation of the visual field in the human primary visual cortex. Reports of this specific perimetric finding are rare, in part because of underdetection with currently used perimetric techniques that concentrate on the central 30 degrees of the visual field.

Preoperative embolization of the head and neck: indications, patient selection, goals, and precautions.

Preoperative embolization was performed in 39 patients with 44 paragangliomas of the head and neck. Because of their complex vascular supply and their relation to vital structures such as the internal carotid artery and the lower cranial nerves, paragangliomas of the temporal bone represent challenging lesions to both the neuroradiologist and the otoneurosurgeon. Detailed classification by high-resolution CT and recognition of the multi- or monocompartmental vascular composition and of dangerous situations by selective angiography are essential prerequisites for safe and effective devascularization of paragangliomas of the temporal bone. Major complications that may occur if embolic material reaches intraaxial vessels through anastomoses between external carotid artery branches and the internal carotid and/or the vertebral artery can be avoided with the use of specific precautionary techniques. Palsies of the facial and lower cranial nerves can also be avoided if reabsorbable material is used for embolization of vessels supplying cranial nerves in asymptomatic patients. In selected cases with significant supply from the internal carotid artery, special interventional techniques, including embolization of the pericarotid tumor portion through the caroticotympanic artery and pre- or peroperative balloon occlusion of the petrous internal carotid artery, allow radical removal of extensive paragangliomas of the temporal bone. Techniques and selection of materials for embolization of carotid body, vagal body, and other paragangliomas of the head and neck mainly depend on the vascular composition of the tumor and on the specific vascular territory in which the tumor is located. In this series, preoperative embolization significantly improved surgical conditions of paragangliomas of any location in the head and neck and proved to represent an essential prerequisite for successful surgery of extensive paragangliomas of the temporal bone.

Postoperative lumbar CT: technique, results, and indications.

Seventy-seven patients with recurrent radicular symptoms after operation for lumbar disk herniation were examined by plain computed tomography (CT) and by intravenously enhanced CT. With the latter technique, scar tissue and recurrent disk herniation can be distinguished: scar tissue shows definite contrast enhancement whereas recurrent disk herniation remains unenhanced. Nerve roots surrounded by scar tissue are often visualized on the postcontrast scan as rounded lucencies. Symmetrical undisplaced nerve roots were identified in 88% of cases of hypertrophic scar formation; this finding excludes recurrent disk herniation. Dural calcifications were found in five patients with hypertrophic scar formation. The authors conclude that CT with contrast enhancement should be the method of choice for evaluating patients with recurrent radicular symptoms after operation for disk herniation.

High-resolution CT investigation of nonchromaffin paragangliomas of the temporal bone.

Twenty-six cases of surgically verified nonchromaffin paragangliomas (NCPs) of the temporal bone were investigated by contrast-enhanced, thin-section, multiplanar high-resolution computed tomography (CT). Based on the high-resolution CT findings the tumors were classified according to a recently introduced surgical classification of NCP into four main types (A, B, C1-C3, and D1-D3). With high-resolution CT tumors were correctly classified in all cases, as was shown with intraoperative correlation. On the basis of these findings, high-resolution CT should replace conventional tomography and should be regarded as complementary to selective angiography in the preoperative evaluation of NCP.

High-resolution CT of the normal and abnormal fallopian canal.

A two-part anatomic and clinical high-resolution computed tomographic (HRCT) study of the fallopian canal was conducted. From the correlation of HRCT images of eight specimen temporal bones with their corresponding anatomic sectional images, it was evident that the full length of the fallopian canal can be accurately visualized. An axial section demonstrates the labyrinthine segment, geniculate ganglion fossa, and proximal part of the tympanic segment, whereas a Stenver projection is used for the tympanic segment, second knee, and mastoid segment. In clinical studies axial sections simultaneously visualized the proximal parts of the fallopian canal in 82% of 28 cases, whereas Stenver projections simultaneously visualized the distal parts in 75% of 16 cases of acute facial nerve palsy. Twenty-one patients with intratemporal facial nerve palsy and six patients with congenital atresia of the external auditory canal were also examined. HRCT was highly accurate in detecting and defining neoplastic, inflammatory, and congenital lesions of the fallopian canal. A lower rate of detection was recorded for traumatic lesions.

Idiopathic, herpetic, and HIV-associated facial nerve palsies: abnormal MR enhancement patterns.

PURPOSE: To determine specific criteria that can be used to define normal versus abnormal MR contrast enhancement of the facial nerve. METHODS: Twenty-three patients with acute unilateral inflammatory peripheral facial nerve palsy were examined on a 1.5-T MR using multiplanar T1-weighted spin-echo sequences before and after injection of gadopentetate dimeglumine. These MR patterns were compared with those of healthy control subjects. RESULTS: The normal facial nerve usually showed a mild to moderate enhancement of the geniculate ganglion and the tympanic-mastoid segment. The intracanalicular-labyrinthine segment did not enhance. All patients showed abnormal enhancement of the distal intracanalicular and the labyrinthine segment. An intense enhancement could be observed in the geniculate ganglion and the proximal tympanic segment, especially in herpetic palsy. Associated enhancement of the vestibulocochlear nerve was seen in herpetic and idiopathic palsy. Enhancement of the inner ear structures was detected only in herpetic palsy. CONCLUSIONS: Abnormal contrast enhancement of the distal intracanalicular and the labyrinthine facial nerve segment is observed in all patients and is the only diagnostically reliable MR feature proving an inflammatory facial nerve lesion. The intense enhancement of the geniculate ganglion and the proximal tympanic segment is possibly correlated with the reactivation of the latent infection in the sensory ganglion. The abnormal enhancement results from breakdown of the blood-peripheral nerve barrier and/or from venous congestion in the venous plexuses of the epi- and perineurium.

Spatial relationship between vestibular schwannoma and facial nerve on three-dimensional T2-weighted fast spin-echo MR images.

BACKGROUND AND PURPOSE: During surgical removal of a vestibular schwannoma, correct identification of the facial nerve is necessary for its preservation and continuing function. We prospectively analyzed the spatial relationship between vestibular schwannomas and the facial nerve using 3D T2-weighted and postcontrast T1-weighted spin-echo (SE) MR imaging. METHODS: Twenty-two patients with a unilateral vestibular schwannoma were examined with MR imaging. The position and spatial relationship of the facial nerve to adjacent tumor within the internal auditory canal (IAC) and cerebellopontine angle cistern (CPA) were assessed on multiplanar reformatted 3D T2-weighted fast spin-echo (FSE) images and on postcontrast transverse and coronal T1-weighted SE images. The entrance of the nerve into the bony canal at the meatal foramen and the nerve root exit zone along the brain stem were used as landmarks to follow the nerve course proximally and distally on all images. RESULTS: The spatial relationship between vestibular schwannoma and facial nerve could not be detected on postcontrast T1-weighted SE images. In 86% of the patients, the position of the nerve in relation to the tumor was discernible on multiplanar reformatted 3D T2-weighted FSE images. In tumors with a maximal diameter up to 10 mm, the entire nerve course was visible; in tumors with a diameter of 11 to 24 mm, only segments of the facial nerve were visible; and in tumors larger than 25 mm, the facial nerve could not be seen, owing to focal nerve thinning and obliteration of landmarks within the IAC and CPA. CONCLUSION: Identification of the facial nerve and its position relative to an adjacent vestibular schwannoma is possible on multiplanar reformatted 3D T2-weighted FSE images but not on postcontrast T1-weighted SE images. Detection of this spatial relationship depends on the tumor's size and location.

Transcranial color-coded Doppler sonography of intracranial aneurysms before and after endovascular occlusion with Guglielmi detachable coils.

BACKGROUND AND PURPOSE: Our purpose was to evaluate the ability of transcranial color-coded Doppler sonography (TCCD) to 1) identify Guglielmi detachable coils (GDCs) within intracranial aneurysms, 2) show endovascular aneurysmal occlusion and patency of parent and branch arteries, 3) determine the flow velocities within parent arteries and major branches before and after treatment, and 4) assess persistence of aneurysmal occlusion. METHODS: The sonographic appearance of GDCs was established experimentally by TCCD (2 to 2.5 MHz), which was then performed in 40 patients with 43 aneurysms occluded by GDCs. The patency of parent arteries and major branches was assessed qualitatively and compared with the immediate posttherapeutic angiographic appearance in every patient. Flow velocities were selectively measured and compared before and after treatment in 21 parent arteries and 24 major branches. Follow-up TCCD studies performed in 26 patients were compared with angiographic (16 cases) and MR angiographic (10 cases) findings for signs of recanalization of the treated aneurysms. RESULTS: The GDCs were identified experimentally and in the patients as hyperechoic structures of the size and shape, and in the location of, the treated aneurysm in 41 of 43 cases. TCCD in accordance with angiography showed a lack of flow in 42 aneurysms and the presence of flow signal in one large aneurysm. Patency of the parent artery was shown in 40 aneurysms and in all branches. Follow-up TCCD showed the coils unchanged in 23 of 26 cases. In three large aneurysms, TCCD indicated recanalization and reappearance of a flow signal separate from the parent artery. CONCLUSION: TCCD is a reliable, noninvasive means to assess parent artery and major branch patency and to reveal a lack of hemodynamic compromise in the vicinity of aneurysms after endovascular therapy. On follow-up examinations, TCCD was able to detect signs of aneurysmal recanalization.

Tributary venosinus occlusion and septic cavernous sinus thrombosis: CT and MR findings.

PURPOSE: In autopsy reports of patients who died of septic cavernous sinus thrombosis, tributary venosinus occlusion has been a common finding related to intracranial inflammatory complications. The purpose of this article is to illustrate the MR and CT appearance of septic cavernous sinus thrombosis and tributary venous occlusion. METHODS: Over a period of 7 years, eight patients with septic cavernous sinus thrombosis were examined by contrast-enhanced thin-section CT. The CT scans of these eight patients and those of 30 healthy control subjects were assessed independently and subjectively by two blinded readers to ascertain the presence, size, and density of areas of nonopacification within the cavernous sinus and the presence of filling defects and dilation of tributary veins and venous sinuses. In six subjects, MR images supplemented by a contrast-enhanced spoiled gradient-recalled acquisition in the steady state (SPGR) sequence were assessed with respect to the presence of filling defects, expansion, and signal abnormalities within the cavernous sinus and tributary veins and sinuses. The MR and CT findings were compared. RESULTS: The CT studies of the eight patients were consistently differentiated from those of the control subjects by the two readers. Contrast-enhanced CT findings in patients included areas of nonopacification that were present within the cavernous sinus bilaterally in six cases and unilaterally in two. The size of the filling defects exceeded 7 mm in 76% of thrombosed cavernous sinuses compared with 9% of control subjects. The mean density of filling defects in patients differed significantly from those in control subjects. Comparison of the MR and CT findings in six cases showed the contrast-enhanced SPGR sequence to be equivalent to CT with respect to delineation of filling defects. CONCLUSION: Contrast-enhanced high-resolution CT findings indicate that venosinus thrombosis associated with septic cavernous sinus thrombosis is not restricted to the superior ophthalmic vein and is more common than previously assumed. A contrast-enhanced SPGR MR sequence may be used as a reliable alternative to establish the diagnosis of cavernous sinus and tributary venosinus thrombosis.

Spinal cord vascular disease: characterization with fast three-dimensional contrast-enhanced MR angiography.

BACKGROUND AND PURPOSE: Noninvasive characterization of spinal vascular lesions is essential for guiding clinical management, and several MR angiographic techniques have been applied in the past with variable results. The purpose of our study was to assess the potential of a dynamic 3D contrast-enhanced MR angiographic sequence to characterize spinal vascular lesions and to identify their arterial feeders and venous drainage. METHODS: A contrast-enhanced gradient-echo 3D pulse sequence providing angiographic information within 24 seconds was applied prospectively in 12 consecutive patients with a presumed spinal vascular lesion. The images were evaluated for visibility of the arterial feeder, and the results were compared with those of conventional angiography performed the next day. RESULTS: The MR angiographic findings proved that the lesions were correctly characterized as spinal arteriovenous malformations (AVMs) (n = 6), spinal dural arteriovenous fistulas (AVFs) (n = 3), a hemangioblastoma (n = 1), a teratoma (n = 1), and a vertebral hemangioma (n = 1). The arterial feeder was visible in all six AVMs and in the hemangioblastoma, corresponding to conventional angiographic findings. In two of three spinal dural AVFs, an enlarged draining medullary vein was seen within the neural foramen, providing correct localization. The third fistula could not be seen owing to reduced image quality from motion artifacts. CONCLUSION: Fast 3D contrast-enhanced MR angiography is a noninvasive technique with high accuracy in the characterization of spinal vascular disease. Visibility of the arterial pedicles corresponds well with that of digital subtraction angiography, facilitating the management of these patients.

Ramsay Hunt syndrome associated with brain stem enhancement.

Postcontrast T1-weighted MR images in a patient with Ramsay Hunt syndrome showed an enhancing lesion in the region of the nucleus of the pontine facial nerve and abnormal enhancement of the intrameatal, labyrinthine, and tympanic facial nerve segments and of the geniculate ganglion, as well as enhancement of the vestibulocochlear nerve and parts of the membranous labyrinth. This enhancement most probably resulted from a primary neuritis of the intrameatal nerve trunks of the seventh and eighth cranial nerves.

Contrast-enhanced MR of the facial nerve in patients with posttraumatic peripheral facial nerve palsy.

PURPOSE: To estimate the value of noncontrast and contrast-enhanced T1-weighted MR imaging in detecting the underlying mechanisms of injury and regeneration in immediate- or delayed-onset posttraumatic peripheral facial nerve palsy. METHODS: Twenty-four patients with posttraumatic peripheral facial nerve palsy were examined on a 1.5-T MR imaging unit with precontrast and postcontrast T1-weighted spin-echo and gradient-echo sequences. RESULTS: Abnormal enhancement of the distal intrameatal nerve segment was visible in 92% of the patients up to 2 years after their initial trauma. A hematoma within the geniculate ganglion was seen in 33% of the patients with a longitudinal fracture. The greater superficial petrosal nerve (in 32% of patients) and the geniculate ganglion (in 48% of patients) were thick and intensely enhancing. Hematoma within the cochlea/vestibule or enhancement of the cochlea/vestibule and the vestibulocochlear (eighth) nerve was observed in transverse fractures. CONCLUSION: MR images can show long-lasting abnormal nerve enhancement, especially in the distal intrameatal nerve segment, related to the long-lasting breakdown of the blood/peripheral nerve barrier associated with nerve degeneration and regeneration after traumatic stretching of the greater superficial petrosal nerve. Additionally, intraoperatively observed perineural and intraneural scar formation leads to thickening and intense enhancement of the affected nerve segments on MR images. A hematoma in the region of the geniculate ganglion can be seen in some but not all patients. Associated damage of the inner ear structures in patients with transverse fractures is also visible on MR images.