Small FOV Research Articles

SU‐E‐CAMPUS‐I‐04: Automatic Skin‐Dose Mapping for An Angiographic System with a Region‐Of‐Interest, High‐Resolution Detector

Purpose:Our real‐time skin dose tracking system (DTS) has been upgraded to monitor dose for the micro‐angiographic fluoroscope (MAF), a high‐resolution, small field‐of‐view x‐ray detector.Methods:The MAF has been mounted on a changer on a clinical C‐Arm gantry so it can be used interchangeably with the standard flat‐panel detector (FPD) during neuro‐interventional procedures when high resolution is needed in a region‐of‐interest. To monitor patient skin dose when using the MAF, our DTS has been modified to automatically account for the change in scatter for the very small MAF FOV and to provide separated dose distributions for each detector. The DTS is able to provide a color‐coded mapping of the cumulative skin dose on a 3D graphic model of the patient. To determine the correct entrance skin exposure to be applied by the DTS, a correction factor was determined by measuring the exposure at the entrance surface of a skull phantom with an ionization chamber as a function of entrance beam size for various beam filters and kVps. Entrance exposure measurements included primary radiation, patient backscatter and table forward scatter. To allow separation of the dose from each detector, a parameter log is kept that allows a replay of the procedure exposure events and recalculation of the dose components. The graphic display can then be constructed showing the dose distribution from the MAF and FPD separately or together.Results:The DTS is able to provide separate displays of dose for the MAF and FPD with field‐size specific scatter corrections. These measured corrections change from about 49% down to 10% when changing from the FPD to the MAF.Conclusion:The upgraded DTS allows identification of the patient skin dose delivered when using each detector in order to achieve improved dose management as well as to facilitate peak skin‐dose reduction through dose spreading.Research supported in part by Toshiba Medical Systems Corporation and NIH Grants R43FD0158401, R44FD0158402 and R01EB002873

Weightbearing CBCT, MDCT, and 2D imaging dosimetry of the foot and ankle

Aims: This study compares effective doses associated with 2D lateral, oblique, and AP radiographs with 3D MDCT and Cone Beam CT images of the foot and ankle. Methods: An anthropomorphic phantom of the foot and ankle was constructed from an adult human skeleton and soft tissue equivalent material. Optical stimulated dosimeters were placed at 21 locations within and on tissues and anatomy of interest. Effective dose was calculated following 2007 ICRP recommendations. Three projections were exposed to simulate conventional 2D imaging. Standard and optimized dose MDCT scans were exposed to simulate typical CT options. Ten, 20, and 30cm fields of view, 100 and 120kVp and 4.5 and 6.8mAs exposures were tested using a PedCAT® CBCT device designed for weightbearing imaging. Dose was calculated for an adult as well as for a 5 and 10-year-old child to assess the impact of age on risk estimation. Results: Standard adult effective doses for single foot imaging were 0.6µSv for 2D, 3.8µSv for CBCT, and 25µSv for MDCT (p = 0.0013). Dose differed significantly with age (p = 0.0185). For a 5-year-old, doses rise to 0.8µSv for 2D, 18µSv for CBCT, and 200µSv for MDCT. Small and medium CBCT fields produced adult doses of 2.3 mSv and 0.9 mSv respectively. Conclusions: The effective dose for small FOV CBCT or conventional 2D series examinations is comparable to a few hours of equivalent background dose. Such doses are negligible; therefore, the dose of radiation should not be a concern when considering the use of CBCT imaging for foot/ankle examination.

Correlation between MRI and double-balloon urethrography findings in the diagnosis of female periurethral lesions

This study aims to evaluate the correlation of MRI findings with double-balloon urethrography (DBU) in diagnosing female urethral diverticula and other periurethral lesions.In this retrospective study, females with clinically suspected periurethral lesions who underwent both MRI and DBU between 2008 and 2012 were evaluated. MRI was performed on a 1.5Tesla unit using a pelvic phased array coil. Protocol included small FOV pelvic images, multiplanar T2-w, T1-w with and without contrast injection. DBU was performed by a dedicated catheter. Images were evaluated in consensus by two readers. Diverticula were evaluated by, size, number, complexity, location and connection to urethra, and other periurethral lesions were evaluated by size, location and connection. Supplement clinical and surgical data were retrieved from medical records and telephone interviews.Seventeen females (mean age 44 years, range 20–69) were included in the study. Diverticula were diagnosed by both modalities (9 cases), by neither (6 cases, 88% correlation) by MRI alone (1 case) and by DBU alone (1 case). Among diverticula, correlation of number, complexity, location and demonstration of connection to urethra was 89%, 67%, 67%, and 56%, respectively. Alternative diagnosis solely by MRI included vaginal wall cysts (3 cases), endometriosis (1 case) and ectopic ureter (1 case). No periurethral lesion was found by either modality in 2 cases.The correlation between MRI and DBU in diagnosing female periurethral lesions is very good for anatomical delineation of diverticula. MRI, which does not involve radiation, may also indicate alternative diagnoses that can contribute to proper patient management.

Completion of a Truncated Attenuation Image From the Attenuated PET Emission Data

Positron emission tomographs (PETs) are currently almost exclusively designed as hybrid systems. The current standard is the PET/CT combination, while prototype PET/MRI systems are being studied by several research groups. One problem in these systems is that the transaxial field-of-view of the second system is smaller than that of the PET camera and does not provide complete attenuation data. Because this second system provides the image for PET attenuation and scatter correction, the smaller FOV causes truncation of the attenuation map, producing bias in the attenuation corrected activity image. In this paper, we propose a maximum-a-posteriori algorithm for estimating the missing part of the attenuation map from the PET emission data. The method is evaluated on five artificially truncated 18F-FDGPET/CT studies, where it reduced the error on the reconstructed PET activities from 20% to less than 7%. The results on a PET/MRI patient study with 18F-FDG are presented as well.

Wavelet Encoding Spectroscopic Imaging in Small FOV Regime: Comparison to Chemical Shift Imaging at 3 Tesla

We have recently proposed a new magnetic resonance spectroscopic imaging (MRSI) technique called wavelet encoding spectroscopic imaging (WE-SI), and described its implementation on a clinical 1.5 T scanner. This technique is proposed as an alternative to chemical shift imaging (CSI), to decrease acquisition time, and voxel contamination. The proposed method is implemented here on a clinical 3 T scanner. Phantom and in vivo studies are chosen to validate the technique at higher field, as well as to fully explore the usefulness of this technique, and find its niche of application in the chain of existing MRSI techniques. In wavelet encoding, a set of dilated and translated wavelets are used to span a localized space by dividing it into a set of sub-spaces with pre-determined sizes and locations. Due to their simple shapes, Haar wavelets are chosen. They are represented in the modified PRESS sequence by the selective excitation and refocusing radio-frequency (RF) pulses. The wavelets dilation and translation are achieved by changing the strength of the localization gradients and frequency shift of the RF pulses, respectively. Data acquisition time is reduced using the minimum recovery time when successive MR signals from adjacent sub-spaces are collected. The results obtained at 3 T confirm those obtained at 1.5 T, and demonstrate that despite the low signal-to-noise ratio, the proposed WE-SI provides accurate results and reduces both voxel contamination and acquisition time as compared to CSI. This applies especially in the small field-of-view regime where only a small number of voxels is required.

Collimator Interchange System for Adaptive Cardiac Imaging in C-SPECT

Compared to imaging the heart with conventional cameras, dedicated cardiac SPECT systems can achieve much higher performance through use of a small field of view. To realize this potential, however, the heart must be reliably placed in the appropriate small FOV prior to imaging, thus requiring a separate scout operation to locate the heart and estimate its size. Furthermore, to achieve high performance across the general population, a system should provide several imaging configurations optimized for different size and location of the heart and the size of the patient. Because of the critical role the collimator plays in SPECT, it would be ideal if a dedicated collimator could be used for each of the different patient groups, as well as for the scout imaging. The ability to exchange collimators without moving the patient can also enable serial studies with different imaging options while preserving anatomic registration.

Detection of vertical root fracture using cone-beam computerized tomography: an in vitro assessment

This study aimed to compare 2 cone-beam CT units [NewTom 3G (small FOV), Iluma (ultra/low resolution)] and an intraoral CCD sensor in the detection of vertical root fracture (VRF). The VRFs were created in 30 teeth, and 30 intact teeth served as control samples. All images were evaluated twice by 4 observers. Kappa coefficients were calculated to assess intra- and interobserver agreement, and t tests were used to compare Az values (alpha = 0.05). Both intra- and interobserver agreement values were higher for the ultra-resolution Iluma and NewTom 3G images compared with the low-resolution Iluma and intraoral CCD images. Az values for the ultra-resolution Iluma and NewTom 3G images were also higher than for the Iluma low-resolution and intraoral CCD images. No significant differences (P > .05) in diagnostic accuracy were found between the Iluma ultra-resolution and NewTom 3G images, with the exception of the second reading of observer 2 (P = .036), and no significant differences (P > .05) were found between the intraoral digital and low-resolution Iluma images. Both ultra-resolution Iluma and NewTom 3G images performed better than low-resolution Iluma and intraoral CCD images in the detection of VRF.

Investigation on a small FoV gamma camera based on LaBr 3:Ce continuous crystal

Recently scintillating crystals with high light yield coupled to photodetectors with high quantum efficiency have been opening a new way to make gamma cameras with superior performances based on continuous crystals. In this work we propose the analysis of a gamma camera based on a continuous LaBr3:Ce crystal coupled to a multi-anodes photomultiplier tube (MA-PMT). In particular we take into account four detector configurations, different in crystal thicknesses and assembling. We utilize a new position algorithm to reduce the position non linearity affecting intrinsic spatial resolution of small FoV gamma cameras when standard Anger algorithm is applied. The experimental data are obtained scanning the detectors with 0.4 mm collimated 99 m Tc source, at 1.5 mm step. An improvement in position linearity and spatial resolution of about a factor two is obtained with the new algorithm. The best values in terms of spatial resolution were 0.90 mm, 0.95 mm and 1.80 mm for integral assembled, 4.0 mm thick and 10 mm thick LaBr3:Ce crystal respectively.

SU‐EE‐A4‐06: Impact of KVp and Bowtie Compensator On Cone‐Beam CT Image Quality: An Elekta Synergy XVI Experience

Purpose: To evaluate the impact of kVp and Bowtie compensator on cone‐beam CT image quality using Elekta Synergy XVI system. Materials and Methods: An XVI system was customarily calibrated at 80, 100, 120, and 138 kVp for various collimation settings. For each kVp, the beam quality and output of the x‐ray tube was measured using a 6‐cm3 ionchamber with a series of tube current (mA) with and without Bowtie filter. Catphan 500 phantom was used for CBCT imaging. For the small FOV and 20‐cm length collimation (S20), high‐resolution CBCT images were reconstructed with voxel size of .5×0.5×0.5 mm3. Spatial resolution, uniformity, CT‐number accuracy and linearity were measured on the Catphan images. Contrast‐to‐noise‐ratio (CNR) was measured using the means (S) and standard deviations (s) in 6‐mm diameter circular region‐of‐interest (ROI) within the polystyrene (PS) and LDPE by the formula of ( ) / (2 2) / 2 LDPE PS LDPE PS CNR = S − S s +s. Results: The exposure rate with Bowtie filter is linearly proportional to the mAs, increases at a rate of (kVp)2.85, and decreased to 73% of that without Bowtie at equivalent kVp and mAs settings. Without Bowtie, the spatial uniformity improves but the CNR deteriorates when increasing kVp at equivalent exposure rates. Application of Bowtie compensator improves the spatial uniformity by more than 11% and increases the accuracy of CT number for soft‐tissue materials at all tested kVps. Bowtie compensator also increases the CNR from 1.64 to 1.94 at 100 kVp, but reduces the CNR by ∼20% at 120kVp and 138kVp. Conclusion: Through this clinical investigation, we have found that Bowtie compensator improves the uniformity and CT‐number accuracy for soft‐tissue materials and may increase the CNR at the low kVp but decrease CNR at the high kVp. In image‐guided radiotherapy, using Bowtie with low kVp for small‐body and no Bowtie with high kVp for large‐body imaging appeared to be optimal settings.

Gamma-ray bursts from the early Universe: predictions for present-day and future instruments

Long Gamma Ray Bursts (GRBs) constitute an important tool to study the Universe near and beyond the epoch of reionization. We delineate here the characteristics of an 'ideal' instrument for the search of GRBs at z>6-10. We find that the detection of these objects requires soft band detectors with a high sensitivity and moderately large FOV. In the light of these results, we compare available and planned GRB missions, deriving conservative predictions on the number of high-z GRBs detectable by these instruments along with the maximum accessible redshift. We show that the Swift satellite will be able to detect various GRBs at z>6, and likely at z>10 if the trigger threshold is decreased by a factor of ~2. Furthermore, we find that INTEGRAL and GLAST are not the best tool to detect bursts at z>6: the former being limited by the small FOV, and the latter by its hard energy band and relatively low sensitivity. Finally, future missions (SVOM, EDGE, but in particular EXIST) will provide a good sample of GRBs at z>6 in a few years of operation.

TU‐EE‐A4‐04: A New Antiscatter Grid for High‐Resolution Region‐Of‐Interest (ROI) X‐Ray Imaging

Purpose: A new custom‐designed antiscatter grid for high‐resolution angiographic detectors is presented that would improve the image quality without introducing substantial grid‐line artifacts. Method and Materials: The new antiscatter grid is a custom‐made, parallel‐focus, crisscross cellular grid that employs gold septa material (CREATV MicroTech, Potomac, MD). The prototype 4.25 cm × 4.25 cm field‐of‐view grid has 20 μm thick septa with interspace distance of 380 μm, height 1.95 mm, and grid‐ratio of 5. This study was performed with the Microangiographic detector (43 μm pixel, 1024×1024 pixel matrix, 4.5 cm × 4.5 cm field‐of‐view, 250‐μm‐thick structured CsI(Tl) scintillator coupled to a CCD camera via minifying taper) in simulated neurovascular angiographic conditions, where a uniform head‐equivalent phantom was used as scattering media. The air‐gap between the phantom and the detector‐with‐grid was kept at 2.5 cm as used in clinical conditions for minimal blurring, but increased scatter. The standard lead‐beam‐stop technique was employed to determine the scatter‐fraction with and without the grid. In order to evaluate the low‐contrast imaging performance of the grid, phantoms with three different bone patterns of varying thickness and three different simulated iodinated vessel inserts in acrylic were imaged with and without‐grid at 70 kVp.Results: The grid demonstrated approximately 59% scatter reduction at 70 kVp for the uniform head‐equivalent phantom without introducing substantial grid‐line artifacts following flat‐field correction. The average contrast‐improvement‐factor for the low‐contrast vessel‐phantom was found to be 1.75, whereas for the relatively higher contrast bone‐phantom it was 1.5. The primary transmission factor was measured to be 66%. Conclusion: The grid demonstrated significant scatter reduction when used in simulated neurovascular angiographic conditions even for the small FOV. Use of this grid with the Micro‐angiographic detector with reduced air‐gap can provide substantially improved image quality.(Partial support: NIH R01‐NS43924, R01‐EB002873, Toshiba Medical System Corp.)

SU‐FF‐I‐40: A Novel Gantry‐Free DBT System Using a Stationary Multi‐Beam Field Emission X‐Ray Source Array Based On Carbon Nanotubes (CNTs)

Purpose: To test the feasibility of a novel gantry‐free digital breast tomosynthesis (DBT) system using a stationary multi‐beam field emission X‐ray source array based on carbon nanotubes (CNTs) and to compare the system performance with the conventional devices. Method and Materials: Two tomosynthesis imaging systems have been built: a compact model and a full scale model, which contain 9 and 25 individual X‐ray pixels, respectively. The compact model can only image a partial breast phantom due to the limited span of the X‐ray source array and the small FOV of the detector. The full scale system is capable of full field digital mammography by utilizing a detector with 20‐cm FOV. The geometry of the full scale system is also comparable to the conventional DBT devices (refer to the supporting material). The system geometry, such as the source to detector distance (SDD) and X‐ray source position, is calibrated. The slice images at different depths are reconstructed using ordered subset convex (maximum likelihood) method. The system performance is evaluated by measuring parameters such as MTF and SNR. Results: By eliminating the rotary gantry, the system design is simplified and the issue if image blurring due to x‐ray source motion is removed. The total scan time can potentially be further shortened with a faster detector readout speed. Conclusion: By eliminating the rotary gantry, the system noise and equipment cost of the tomosynthesis imaging system are reduced. The total scan time can be further shortened with faster detector readout speed. The novel stationary tomosysthesis system shows great potential in clinical imaging.

3D time of flight MR angiography: aquisition with small field of view and low phase encodes

The aim of the study was to evaluate and compare the image quality of the 3D TOF MRA acquired with a small FOV and low phase encodes with those MR angiographic images acquired with standard pulse sequence parameters. Twenty patients who were referred to our institution for MR imaging of the brain and strictly satisfied the selection criteria were included in this study. Apart from the routine protocol for MR imaging of the brain, 3D TOF MRA of the circle of Willis with a small FOV and a standard FOV were performed. The image quality of all MRA was evaluated by two independent observers who were blind to the pulse sequence parameters. From the standard FOV MRA, 22.5, 12.5, and 5% of the patients were graded as mild, moderate, and severe stenosis of the internal carotid artery, respectively. On the contrary, no apparent stenosis was observed from the small FOV MRA with low phase encodes. Regarding the reduction in MR artifacts and acquisition time achieved with the small FOV 3D TOF MRA with low phase encodes, this might be a useful MR angiographic technique to be used in routine clinical practice.

Factors affecting cancer detectability in 99Tc MIBI scintimammography

Abstract Scintimammography shows strong potential in detecting and differentiating breast cancer. This scintigraphic technique, using a standard gamma camera, allows high sensitivity and specificity values (>95%) for detected tumors more than 1 cm size. However, the sensitivity of scintimammography using conventional gamma cameras is considerably less (40–50%) for tumors with smaller size. Recently, the authors demonstrated how the use of a small FOV dedicated gamma camera (Single Photon Emission Mammography, or SPEM camera), with very high intrinsic spatial resolution (1.7 mm FWHM), working with breast moderately compressed and positioned close to the breast tumor (i.e., analogously to X-ray mammography) increased sensitivity up to 80% for tumors sized between 0.5 and 1 cm (T1b). The aim of this paper is to demonstrate how the reduced breast thickness can play a primary role in small cancer detection. Five different methods were taken into account: clinical measurements, comparing tumor SNR values obtained from the same patients in prone scintimammography and in SPEM, comparing SNR values between compressed and uncompressed breast in craniocaudal projection, breast phantom measurements, Monte Carlo simulations and simplified theoretical model. Results confirm that the mechanism for the improvement in visualizing sub-centimeter lesions due to compression is a reduction of lesion–detector distance. As a result of this reduced distance there is a less reabsorption of signal by interposed breast tissue, and improved detector intrinsic spatial resolution.

Characteristics and clinical application of a treatment simulator with Ct-option

Background and purpose: The integration of a scanner for computed tomography (CT) and a treatment simulator (Sim-CT, Elekta Oncology Systems, Crawley, UK) has been studied in a clinical situation. Image quality, hounsfield units (HU) and linearity have been evaluated as well as the implications for treatment planning. The additional dose to the patient has also been highlighted. Material and methods: Image data is acquired using an array of solid state X-ray detectors attached externally to the simulator's image intensifier. Three different fields of view (FOV: 25.0 cm, 35.0 cm and 50.0 cm) with 0.2 cm, 0.5 cm and 1.0 cm slice thickness can be selected and the system allows for an aperture diameter of 92.0 cm at standard isocentric height. The CT performance has been characterized with several criteria: spatial resolution, contrast sensitivity, geometric accuracy, reliability of hounsfield units and the radiation output level. The spatial resolution gauge of the nuclear associates quality phantom (NAQP) as well as modulation transfer functions (MTF) have been applied to evaluate the spatial resolution. Contrast sensitivity and HU measurements have been performed by means of the NAQP and a HU conversion phantom that allows inserts with different electron densities. The computed tomography dose index (CTDI) of the CT-option has been monitored with a pencil shaped ionization chamber. Treatment planning and dose calculations for heterogeneity correction based on the Sim-CT images generated from an anthropomorphic phantom as well as from ten patients have been compared with similar treatment plans based on identical, yet diagnostic CT (DCT) images. Results: The last row of holes that are resolved in the spatial resolution gauge of the NAQP are either 0.150 cm or 0.175 cm depending on the FOV and the applied reconstruction filter. These are consistent with the MTF curves showing cut-off frequencies ranging from 5.3 lp/cm to 7.1 lp/cm. Linear regression analysis of HU versus electron densities revealed a correlation coefficient of 0.99. Contrast, pixel size and geometric accuracy are within specifications. Computed tomography dose index values of 0.204 Gy/As and 0.069 Gy/As have been observed with dose measurements in the center of a 16 cm diameter and 32 cm diameter phantom, respectively for large FOV. Small FOV yields CTDI values of 0.925 Gy/As and 0.358 Gy/As which is a factor ten higher than the results obtained from a DCT under similar acquisition conditions. The phantom studies showed excellent agreement between dose distributions generated with the Sim-CT and DCT HU. The deviations between the calculated settings of monitor units as well as the maximum dose in three dimensions were less than 1% for the treatment plans based on either of these HU both for pelvic as well as thoracic simulations. The patient studies confirmed these results. Conclusions: The CT-option can be considered as an added value to the simulation process and the images acquired on the Sim-CT system are adequate for dose calculation with tissue heterogeneity correction. The good image quality, however, is compromised by the relative high dose values to the patient. The considerable load to the conventional X-ray tube currently limits the Sim-CT to seven image acquisitions per patient and therefore the system is limited in its capability to perform full three-dimensional reconstruction.

A cylindrical geometry for cardiac SPECT imaging

In SPECT imaging, a cylindrical detector geometry offers many advantages. Because the detector completely surrounds the patient, the cylindrical system geometry should provide more detector coverage than the rotating camera SPECT systems. This increased coverage means increased sensitivity, or improved resolution, and better image quality. Because the cylindrical detector system is stationary and the projection data is acquired through collimator rotation, this geometry simplifies system design and allows very stable imaging performance. The authors propose a cylindrical system geometry that features a large number of projections for a small FOV located at the center of the gantry. This geometry has potential for cardiac SPECT imaging, provided that the patient's body can be offset properly and reliably to place his heart in the specified FOV. General design issues and potential configurations for such a cardiac SPECT system are presented.

1010-66 Comparison of Supine and Upright SPECT Myocardial Perfusion Imaging

Cardiac tomography can be performed in the upright position using a stationary small FOV multicrystal camera (MCC) and a rotating chair. The chair both rotates and jogs to improve spatial resolution of the MCC. To investigate the diagnostic accuracy of this imaging technique compared to conventional supine SPECT, 25 patients with CAD (23 M, 2 F, mean age = 58 ± 10 years, 11 with prior MI) and 18 subjects with low pretest probability for having disease (9 M, 9 F, mean age 49 ± 11 years) underwent a one day rest/stress, low dose/high dose Tc-99m sestamibi study with sequential supine and upright SPECT imaging performed following rest and stress injections. Mean treadmill time for patients = 7.8 ± 2.1 mins, peak HR = 143 ± 17, and 11 were electrically positive. Mean treadmill time for subjects was 8.9 ± 3.4 mins, peak HR = 166 ± 17. Recent coronary angiography was performed in 17/23 CAD patients with 34 lesions >50%. Each tomogram was divided into 7 segments and each segment scored as normal, reversible, or fixed. The agreement between the 2 sets of scans was 98% and the segment agreement with regard to reversibility was 90%. Supine SPECT detected 21/23 (91%) of patients with CAD, the chair detected 20/23% (87%). The chair detected 18/34 (53%) CA lesions, supine detected 20/34 (59%). Sensitivity and specificity for the chair was 88%, and 100%, and for supine was 92%, and 100%. In conclusion, in this small population, there were no significant differences between supine and upright SPECT imaging for diagnosing CAD, identifying individual stenoses, or identifying defect reversibility.

302 0.2TMIR装置を用いたSSFP小視野撮影によるMRA画像の向上

302 0.2TMIR装置を用いたSSFP小視野撮影によるMRA画像の向上