Gapless tuning of quantum cascade laser frequency combs with external cavity optical feedback.

We present the operation of quantum cascade laser frequency combs in an external cavity configuration. Experimental observations show dependence of comb repetition rate and optical spectrum on the external cavity length. The low phase-noise comb regime is extended to a broader range of bias currents, enabling gapless frequency tuning of the comb modes. Dual-comb measurements also confirm improved comb stability in the presence of unwanted optical feedback when operating in an external cavity configuration. These observations indicate that aside from the continuing efforts to assure low and uniform dispersion characteristics of quantum cascade laser frequency combs, the proposed simple approach of adding a broadband external cavity can significantly enhance operation of sub-optimal devices for spectroscopic applications.

Computational coherent averaging for free-running dual-comb spectroscopy.

Dual-comb spectroscopy is a rapidly developing spectroscopic technique that does not require any opto-mechanical moving parts and enables broadband and high-resolution measurements with microsecond time resolution. However, for high sensitivity measurements and extended averaging times, high mutual coherence of the comb-sources is essential. To date, most dual-comb systems employ coherent averaging schemes that require additional electro-optical components, which increase system complexity and cost. More recently, computational phase correction approaches that enables coherent averaging of spectra generated by free-running systems have gained increasing interest. Here, we propose such an all-computational solution that is compatible with real-time data acquisition architectures for free-running systems. The efficacy of our coherent averaging algorithm is demonstrated using dual-comb spectrometers based on quantum cascade lasers, interband cascade lasers, mode-locked lasers, and optically-pumped microresonators.

Mid-infrared dual-comb spectroscopy with interband cascade lasers.

Two semiconductor optical frequency combs, consuming less than 1 W of electrical power, are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3-4 µm spectral region. The devices are 4 mm long by 4 µm wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with optical multi-pass cell sensitivity enhancement. The system provides a spectral coverage of 33 cm-1 (1 THz), 0.32 cm-1 (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ∼100 at 100 µs integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.

Cavity Attenuated Phase Shift Faraday Rotation Spectroscopy.

Cavity attenuated phase shift Faraday rotation spectroscopy has been developed and demonstrated by oxygen detection near 762 nm. The system incorporates a high-finesse cavity together with phase-sensitive balanced polarimetric detection for sensitivity enhancement and achieves a minimum detectable polarization rotation angle (1σ) of 5.6 × 10-9 rad/√Hz, which corresponds to an absorption sensitivity of 4.5 × 10-10 cm-1/√Hz without the need for high sampling rate data acquisition. The technique is insusceptible to spectral interferences, which makes it highly suitable for chemical trace gas detection of paramagnetic molecules such as nitric oxide, nitrogen dioxide, oxygen, and the hydroxyl/hydroperoxyl radicals.

Frequency-locked cavity ring-down Faraday rotation spectroscopy.

Cavity ring-down Faraday rotation spectroscopy (CRD-FRS) is a technique for trace gas measurements of paramagnetic species that retrieves the molecular concentration from the polarization rotation measured as the difference between simultaneously recorded ring-down times of two orthogonal polarization states. The differential measurement is inherently insensitive to nonabsorber related losses, which makes off-resonance measurements redundant. We exploit this unique property by actively line-locking to a molecular transition for calibration-free trace gas concentration retrieval. In addition, we enhance the effective duty-cycle of the system by implementing a Pound-Drever-Hall laser lock to the cavity resonance, which allows for ring-down rates of up to 9 kHz. The system performance is demonstrated by measurements of trace oxygen with a minimum detection limit at the ppmv/√Hz-level.

Dual-comb spectroscopy using plasmon-enhanced-waveguide dispersion-compensated quantum cascade lasers.

In this Letter, we report on sub-millisecond response time mid-infrared dual-comb spectroscopy using a balanced asymmetric (dispersive) dual-comb setup with a matched pair of plasmon-enhanced-waveguide dispersion-compensated quantum cascade lasers. The system performance is demonstrated by measuring spectra of Bromomethane (CH3Br) and Freon 134a (CH2FCF3) at approximately 7.8 µm. A purely computational phase and timing-correction procedure is used to validate the coherence of the quantum cascade lasers frequency combs and to enable coherent averaging over the time scales investigated. The system achieves a noise-equivalent absorption better than 1×10-3 Hz-1/2, with a resolution of 9.8 GHz (0.326 cm-1) and an optical bandwidth of 1 THz (32 cm-1), with an average optical power of more than 1 mW per spectral element.

DBCG hypo trial validation of radiotherapy parameters from a national data bank versus manual reporting.

INTRODUCTION: The current study evaluates the data quality achievable using a national data bank for reporting radiotherapy parameters relative to the classical manual reporting method of selected parameters. METHODS: The data comparison is based on 1522 Danish patients of the DBCG hypo trial with data stored in the Danish national radiotherapy data bank. In line with standard DBCG trial practice selected parameters were also reported manually to the DBCG database. Categorical variables are compared using contingency tables, and comparison of continuous parameters is presented in scatter plots. RESULTS: For categorical variables 25 differences between the data bank and manual values were located. Of these 23 were related to mistakes in the manual reported value whilst the remaining two were a wrong classification in the data bank. The wrong classification in the data bank was related to lack of dose information, since the two patients had been treated with an electron boost based on a manual calculation, thus data was not exported to the data bank, and this was not detected prior to comparison with the manual data. For a few database fields in the manual data an ambiguity of the parameter definition of the specific field is seen in the data. This was not the case for the data bank, which extract all data consistently. CONCLUSIONS: In terms of data quality the data bank is superior to manually reported values. However, there is a need to allocate resources for checking the validity of the available data as well as ensuring that all relevant data is present. The data bank contains more detailed information, and thus facilitates research related to the actual dose distribution in the patients.

Molecular dispersion spectroscopy based on Fabry-Perot quantum cascade lasers.

Two Fabry-Perot quantum cascade lasers are used in a differential dual comb configuration to perform rapidly swept dispersion spectroscopy of low-pressure nitrous oxide with <1 ms acquisition time. Active feedback control of the laser injection current enables simultaneous wavelength modulation of both lasers at kilohertz rates. The system demonstrates similar performance in both absorption and dispersion spectroscopy modes and achieves a noise-equivalent absorption figure of merit in the low 10-4/Hz range.

Wavelength modulated multiheterodyne spectroscopy using Fabry-Pérot quantum cascade lasers.

Multiheterodyne spectroscopy implemented with semiconductor Fabry-Pérot lasers is a method for broadband (> 20 cm-1), high spectral resolution (~1 MHz) and high time resolution (< 1 µs/spectrum) spectroscopy with no moving parts utilizing off-the-shelf laser sources. The laser stabilization approach demonstrated here enables continuous frequency tuning (at 12.5 Hz repetition rate) while allowing for multiheterodyne wavelength modulation spectroscopy (WMS). Spectroscopic detection of N2O around 1185 cm-1 is experimentally realized, which shows a direct absorption sensitivity limit of ~1.5⨯10-3/√Hz fractional absorption per mode. This can be lowered using WMS down to 5⨯10-4/√Hz per mode, limited by optical fringes. This approaches the range of sensitivities of standard single-mode laser based spectrometers, which demonstrates that the multiheterodyne method is well-suited for chemical sensing of spectrally broadened absorption features or for multi-species measurements.

Impact of 4D image quality on the accuracy of target definition.

Delineation accuracy of target shape and position depends on the image quality. This study investigates whether the image quality on standard 4D systems has an influence comparable to the overall delineation uncertainty. A moving lung target was imaged using a dynamic thorax phantom on three different 4D computed tomography (CT) systems and a 4D cone beam CT (CBCT) system using pre-defined clinical scanning protocols. Peak-to-peak motion and target volume were registered using rigid registration and automatic delineation, respectively. A spatial distribution of the imaging uncertainty was calculated as the distance deviation between the imaged target and the true target shape. The measured motions were smaller than actual motions. There were volume differences of the imaged target between respiration phases. Imaging uncertainties of >0.4 cm were measured in the motion direction which showed that there was a large distortion of the imaged target shape. Imaging uncertainties of standard 4D systems are of similar size as typical GTV-CTV expansions (0.5-1 cm) and contribute considerably to the target definition uncertainty. Optimising and validating 4D systems is recommended in order to obtain the most optimal imaged target shape.

A dual centre study of setup accuracy for thoracic patients based on cone-beam CT data.

BACKGROUND AND PURPOSE: To compare setup uncertainties at two different institutions by using identical imaging and analysis techniques for thoracic patients with different fixation equipments. METHODS AND MATERIALS: Patient registration results from Cone-Beam CT (CBCT) scans of 174 patients were evaluated (1068 CBCT scans). Patients were fixated using a standard or custom made fixation at Royal Marsden Hospital and Odense University Hospital, respectively. Five imaging protocols were retrospectively simulated to compare the fixation equipments. Systematic and random setup uncertainties were calculated to estimate sufficient treatment margins. RESULTS: The setup uncertainties are of similar sizes at the two institutions and there is no observable drift in the precision of the fixation equipments during the treatment course. When a correcting imaging protocol is performed there is a significant increase of the systematic setup uncertainties in between imaging fractions. A margin reduction of ≥0.2 cm can be achieved for patients with peak-to-peak respiration amplitudes of ≥1.2 cm when changing from 4D-CT to Active Breathing Coordinator™ (ABC). CONCLUSIONS: The setup uncertainties at the two institutions are the same despite different fixation equipments. Hence margins cannot be reduced by changing fixating equipment.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: I. Theory.

We give a simple two-transition model of Faraday modulation spectrometry (FAMOS) addressing the electronic X(2)Π(ν('') = 0) - A(2)Σ(+)(ν(') = 0) band in nitric oxide. The model is given in terms of the integrated line strength, S, and first Fourier coefficients for the magnetic-field-modulated dispersive line shape function. Although the two states addressed respond differently to the magnetic field (they adhere to the dissimilar Hund coupling cases), it is shown that the technique shares some properties with FAMOS when rotational-vibrational Q-transitions are targeted: the line shapes have a similar form and the signal strength has an analogous magnetic field and pressure dependence. The differences are that the maximum signal appears for larger magnetic field amplitudes and pressures, ∼1500 G and ∼200 Torr, respectively.

Faraday modulation spectrometry of nitric oxide addressing its electronic X2Π - A2Σ+ band: II. Experiment.

A first demonstration of Faraday modulation spectrometry (FAMOS) of nitric oxide (NO) addressing its strong electronic X(2)Π(ν″ = 0) - A(2)Σ(+)(ν(') = 0) band is presented. The instrumentation was constructed around a fully diode-laser-based laser system producing mW powers of ultraviolet light targeting the overlapping Q(22)(21/2) and R(12)Q(21/2) transitions at ∼226.6 nm. The work verifies a new two-transition model of FAMOS addressing the electronic transitions in NO given in an accompanying work. Although the experimental instrumentation could address neither the parameter space of the theory nor the optimum conditions, the line shapes and the pressure dependence could be verified under low-field conditions. NO could be detected down to a partial pressure of 13 µTorr, roughly corresponding to 10 ppb·m for an atmospheric pressure sample, which demonstrates the feasibility of FAMOS for sensitive detection of NO addressing its strong electronic band.

Investigation of respiration induced intra- and inter-fractional tumour motion using a standard Cone Beam CT.

BACKGROUND: To investigate whether a standard Cone beam CT (CBCT) scan can be used to determined the intra- and inter-fractional tumour motion for lung tumours that have infiltrated the mediastinum. MATERIAL AND METHODS: This study includes 23 patients with non small cell lung cancer (NSCLC). The intra-fractional tumour motion was analysed for each patient on a 4D-CT scan as well as on three 4D-CBCT (fraction 3, 10 and 20). The 4D-CBCT was reconstructed from a standard 3D-CBCT using in-house developed software. The tumour (GTV) was delineated in the first phase of the 4D-CT. Registration of phase one from the 4D-CT and 4D-CBCT was used to copy the GTV to the CBCT scans. Hereafter the motion of the outlined GTV was tracked in the planning 4D-CT and the three 4D-CBCT using Pinnacle(®) version 8.1w (research version). Additionally, the inter-fractional tumour movement, relative to the bony structure, was obtained from the difference in tumour position between the 3D-CT and the standard 3D-CBCT. RESULTS: It is possible to track a lung tumour with mediastinal infiltration in the 4D-CBCT scan based on a standard 3D-CBCT. The respiration motion in the 4D-CBCT is not significantly different from the result found from the initial 4D-CT. Likewise, no differences in respiration motion was found between fractions 3, 10 and 20. CONCLUSION: This study shows that it is possible to track tumour motion for NSCLC patients with mediastinal infiltration using a standard 3D-CBCT. No change in the intra-fractional tumour motion of clinically relevance was observed during the fractionated treatment course. The inter-fractional tumour motion found underlines the importance of using daily IGRT with online match on soft tissue in order to be able to reduce treatment margins.

Reduction of Cone-Beam CT scan time without compromising the accuracy of the image registration in IGRT.

BACKGROUND: In modern radiotherapy accelerators are equipped with 3D cone-beam CT (CBCT) which is used to verify patient position before treatment. The verification is based on an image registration between the CBCT acquired just before treatment and the CT scan made for the treatment planning. The purpose of this study is to minimise the scan time of the CBCT without compromising the accuracy of the image registration in IGRT. MATERIAL AND METHODS: Fast scans were simulated by reducing the number of acquired projection images, i.e. new reconstructions based on a subset of the original projections were made. The deviation between the registrations of these new reconstructions and the original registration was measured as function of the amount of reduction. RESULTS AND DISCUSSION: Twenty nine head and neck (H&N) and 11 stereotactic lung patients were included in the study. The mean of the registration deviation did not differ significantly from zero independently of the number of projections included in the reconstruction. Except for the smallest subset of reconstructions (10% and 25% of the original projection for the lung and H&N patients, respectively) the standard deviation of the registration differences was constant. The standard deviations were approximately 0.1 mm and 0.2 mm for the H&N and lung group, respectively. Based on these results an in-house developed solution, able to reduce the Cone-Beam CT scan time, has been implemented clinically.

Cone beam CT evaluation of patient set-up accuracy as a QA tool.

PURPOSE: To quantify by means of cone beam CT the random and systematic uncertainty involved in radiotherapy, and to determine if this information can be used for e.g. technical quality assurance, evaluation of patient immobilization and determination of margins for the treatment planning. PATIENTS AND METHODS: Eighty four cancer patients have been cone beam CT scanned at treatment sessions 1, 2, 3, 10 and 20. Translational and rotational errors are analyzed. RESULTS AND CONCLUSIONS: For the first three treatment sessions the mean translational error in the AP direction is 1 mm; this indicates a small error in the calibration of coronal isocentric laser. The observed SD of the systematic error in each direction is 1 mm if a correction is made after the third fraction with an action limit of 4 mm. The SD of the random errors of the patient group is approximately 1 mm in each direction. The rotational errors have a vanishing mean and a systematic error of 0.5-1.2 degrees and a random error of 0.4-0.7 degrees. The uncertainties from the first three treatment sessions (disregarding rotations) lead to a margin of 4 mm from ITV to PTV for Head-and-Neck patients (all directions) and Thorax patients (AP and lateral directions). In the CC direction, the margin has to be 5 mm for the Thorax patients. The total uncertainty on the patient position grows during the treatment course, especially in the CC direction for patients receiving thoracical irradiation. This may stem from problems in the immobilization of these patients. Consequently, it may be necessary to increase the margins in the CC direction. Once the CBCT scans have been made, the information is available for off-line analysis without any extra workload. Thus, the CBCT data can supplement scheduled QA checks.

The representitativeness of patient position during the first treatment fractions.

BACKGROUND: During external radiotherapy daily or even weekly image verification of the patient position might be problematic due to the resulting workload. Therefore it has been customary to perform image verification only at the first treatment fraction. In this study it is investigated whether the patient position uncertainty at the initial three treatment fractions is representative for the uncertainty throughout the treatment course. METHODS: Seventy seven patients were treated using Elekta Synergy accelerators. The patients were immobilized during treatment by use of a customized VacFix bag and a mask of AquaPlast. Cone beam CT (CBCT) scans were performed at fractions 1, 2, and 3 and at the 10th and 20th treatment fractions. Displacements in patient position, translational and rotational, have been measured by an image registration of the CBCT and the planning CT scan. The displacements data are evaluated retrospectively and the effect of Action Level (AL) image verification protocols based on sessions 1, 2, 3 are simulated. The resulting overall patient position uncertainties of the different protocols are evaluated at the 10th and 20th fractions. RESULTS AND CONCLUSIONS: The differences between the addressed protocols are shown to be very small compared to the overall increase in patient position uncertainty during the treatment course. Thus the main problem in achieving the smallest possible uncertainty for the overall treatment is not the selection of 'the best' image verification protocol for the initial three fractions. The main challenge is that the overall patient position uncertainty increases during the treatment course. Information about the patient position during the first three fractions is therefore not representative for the overall patient position. For these types of patients and immobilization equipment it would consequently be an advantage to reduce the number of image verification sessions during the initial fractions and then compensate with additional imaging sessions during the remaining treatment course.

Set-up errors in patients undergoing image guided radiation treatment. Relationship to body mass index and weight loss.

BACKGROUND: The purpose of this study was to quantify the set-up errors of patient positioning during IGRT and to correlate set-up errors to patient-specific factors such as weight, height, BMI, and weight loss. PATIENTS AND METHODS: Thirty four consecutively treated head-and-neck cancer patients (H&N) and 20 lung cancer patients were investigated. Patients were positioned using customized immobilization devices consisting of vacuum cushions and thermoplastic shells. Treatment was given on an Elekta Synergy accelerator. Cone-beam acquisitions were obtained according to a standardized Action Limit protocol and compared to pre-treatment CT images. The average 3D deviation from three initial cone beam scans was compared to deviations at the 10th and 20th treatment session and correlated by linear regression analysis to height, weight, and BMI, and in H&N to weight loss as expressed by the relative weight change over time. RESULTS: The SD of the translational and rotational random set-up errors during the first three sessions for H&N were 0.9 mm (Left-Right), 1.1mm (Anterior-Posterior), 0.7 mm (Cranio-Caudal) and 0.7 degrees (LR-axis), 0.5 degrees (AP-axis), and 0.7 degrees (CC-axis). The equivalent data for lung cancer patients were 1.1 mm (LR), 1.1mm (AP), 1.5 mm (CC) and 0.5 degrees (LR-axis), 0.6 degrees (AP-axis), and 0.4 degrees (CC-axis). The median BMI for H&N and lung was 25.8 (17.6-39.7) and 23.7 (17.4-38.8), respectively. The median weekly weight change for H&N was -0.3% (-2.0 to 1.1%). With H&N and lung cancer analyzed separately, no statistically significant correlation was observed between set-up errors and height, weight, BMI, or weight change during treatment, irrespectively whether the 3D deviations from the initial three cone beam scans or scans from the 10th or 20th treatment sessions were used. CONCLUSION: This IGRT study did not support the hypothesis that set-up errors during radiotherapy are correlated to patient height, weight, BMI, or weight loss.