A second-order and slice-specific linear shimming technique to improve spinal cord fMRI

PURPOSE

To develop a second-order and slice-specific linear shimming technique and investigate its efficiency in the mitigation of signal loss and distortions, and the increase of temporal signal-to-noise ratio (tSNR) within the spinal cord during functional Magnetic Resonance Imaging (fMRI) of the human cervical spinal cord.

METHODS

All scans were performed on a General Electric Discovery MR750 3 T scanner, using a head, neck and spine coil and a neurovascular array. To improve B₀ homogeneity, a field map was acquired, and second-order shims (SOS) were optimized over manually defined regions of interest (ROIs). Signal loss from dephasing by susceptibility-induced gradients was reduced by optimizing slice-specific x-, y- and z-shims to maximize signal within the spinal cord. Spectral-spatial excitation pulses were used in both the slice-specific linear shimming calibration scan and fMRI acquisitions. The shimming technique's efficiency was initially tested on eight healthy volunteers by comparing tSNR between images acquired with the manufacturer's standard linear shimming and with our SOS and xyz-shimming technique. Subsequently, using an increased spatial resolution as needed for fMRI of the spinal cord, tSNR measurements were performed on resting-state fMRI images from 14 healthy participants.

RESULTS

Spinal fMRI images acquired with only the standard linear shimming suffered from severe signal loss below the C5 vertebral level. The developed shimming technique compensated for this loss especially at levels C6 and C7, while tSNR was significantly higher at all vertebral levels with SOS and xyz-shimming than without it.

CONCLUSION

A comprehensive shimming approach which includes the use of spectral-spatial excitation pulses along with both second-order and slice-specific linear shim optimization reduces regional signal loss and increases tSNR along the c-spine (C3-C7), improving the ability to record functional signals from the human spinal cord.

PET Counting Response Variability Depending on Tumor Location, Activity, and Patient Obesity: A Feasibility Study of Solitary Pulmonary Nodule Using Monte Carlo

We aim to investigate the counting response variations of positron emission tomography (PET) scanners with different detector configurations in the presence of solitary pulmonary nodule (SPN). Using experimentally validated Monte Carlo simulations, the counting performance of four different scanner models with varying tumor activity, location, and patient obesity is represented using a noise equivalent count rate (NECR). NECR is a well-established quantitative metric which has positive correlation with clinically perceived image quality. The combined effect of tumor displacement and increased activity shows a linear ascending trend for NECR with slope ranges of (12.5-18.2)*10^-3 (kBq/cm³)^-1 for three-ring (3R) scanners and (15.3-21.5)*10^-3 (kBq/cm³)^-1 for four-ring (4R). The trend for the combined effect of tumor displacement and patient obesity is exponential decay with 3R configurations weakly dependent on the patient obesity if the tumor is located at the center of the field of view with exponent's range of (6.6-33.8)*10^-2cm^-1. The dependence is stronger for 4R scanners (9.6-38.5)*10^-2cm^-1. The analysis indicates that quantitative PET data from the same SPN patient possibly examined in different time points (e.g., during staging or for the evaluation of treatment response) are affected by the different detector configurations and need to be normalized with patient weight, activity, and tumor location to reduce unwanted bias of the diagnosis. This paper provides also with a proof of concept for the ability of properly tuned simulations to provide additional insights into the counting response variability especially in tumor types where often borderline decisions have to be made regarding their characterization.

Contrast-enhanced and unenhanced diffusion-weighted imaging of the breast at 3 T

AIM

To evaluate the effect of intravenous gadolinium contrast agent on diffusion-weighted sequences and apparent diffusion coefficient (ADC) measurements at 3 T.

MATERIALS AND METHODS

Sixty-two biopsy-proven breast lesions were included in this prospective study. Magnetic resonance imaging (MRI) was performed at 3 T, using four echo-planar diffusion-weighted sequences (7,100 ms repetition time, 84 ms echo time) with b-values of 0 and 850, and 0 and 1,000 s/mm². The first pair of DWI sequences was taken before intravenous contrast medium injection. The second pair of sequences was taken 6.5 minutes after intravenous contrast medium administration (right after the dynamic T1 sequence). A freeform region of interest (ROI) was drawn inside the lesion excluding adjacent normal tissue, necrotic, or cystic components and ADC values were calculated. The paired samples t-test was used to assess differences between ADC measurements before and after intravenous contrast medium administration. Sensitivity, specificity, positive predictive value, negative predictive value, and area under the curve were calculated for each diffusion sequence.

RESULTS

Twenty-seven malignant and 35 benign lesions were analysed. Fifty-eight lesions were masses, and four lesions were non-mass-like enhancements (NMLEs). Two of the NMLEs were malignant, and two were benign lesions. The contrast-enhanced ADC measurements were lower than the unenhanced measurements on b=850 and 1,000 s/mm² (p<0.05). The receiver operating characteristic (ROC) analysis displayed similar area under the curve values between the different diffusion sequences.

CONCLUSION

The injection of intravenous contrast medium reduces ADC values; however, the effect of contrast medium is modest. Sensitivity and specificity are not significantly affected.

Spectroscopic evaluation of glioma grading at 3T: the combined role of short and long TE

Purpose. To evaluate the diagnostic value of 3T ¹H-MRS in grading cerebral gliomas using short and long echo times. Methods. 1H-MRS was performed on 71 patients with untreated cerebral gliomas. Metabolite ratios of NAA/Cr, Cho/Cr, Cho/NAA, and mI/Cr were calculated for short and long TE and compared between low and high grade gliomas. Lipids were qualitatively evaluated. ROC analysis was performed to obtain the cut-off values for the metabolic ratios presenting statistical difference between the two glioma grades. Results. Intratumoral Cho/Cr at both TEs and long TE Cho/NAA were significantly different between low and high grade gliomas. Peritumoral NAA/Cr of both TEs, as well as long TE Cho/Cr and Cho/NAA ratios, significantly differentiated the two tumor grades. Diagnostic sensitivity of peritumoral short TE NAA/Cr proved to be superior over the other metabolic ratios, whereas intratumoral short TE Cho/Cr reached the highest levels of specificity and accuracy. Overall, short TE 1H-MRS reached higher total sensitivity in predicting glioma grade, over long TE. Conclusion. An advantage was found in using short TE over long TE 1H-MRS in the discrimination of low versus high grade gliomas. Moreover, the results suggested that the peritumoral area of gliomas may be more valuable in predicting glioma grade than using only the intratumoral area.

Magnetic resonance spectroscopy of the breast at 3T: pre- and post-contrast evaluation for breast lesion characterization

PURPOSE

To determine whether in vivo proton magnetic resonance spectroscopy at 3T can provide accurate breast lesion characterization, and to determine the effect of gadolinium on the resonance of tCho.

METHODS

Twenty-four positive-mammogram patients were examined on a 3T MR scanner. 1H-MRS was performed before and after gadolinium administration. tCho peak was qualitatively evaluated before and after contrast injection.

RESULTS

Fourteen out of 27 lesions proved to be malignant after histopathological diagnosis. Using 1H-MRS, before contrast injection, 6/14 confirmed malignancies and 11/13 benign lesions were correctly classified; while, after contrast injection, 11/14 confirmed malignancies and 12/13 benign processes were correctly classified. Post gadolinium 1H-MRS proved useful in picking up tCho signal, improving the overall accuracy, sensitivity, and specificity by 35%, 83%, and 9%, respectively.

CONCLUSION

1H-MRS overall accuracy, sensitivity, and specificity in detecting breast lesion's malignancy were increased after gadolinium administration. It is prudent to perform 1H-MRS before contrast injection in large breast lesions to avoid choline underestimation. In cases of small or non-mass lesions, it is recommended to perform 1H-MRS after contrast injection for better voxel prescription to enable a reliable preoperative diagnosis.

Radiation doses to patients undergoing enteroclysis

Enteroclysis is a minimally invasive radiographic examination of the small intestine. During the procedure, considerable radiation dose is delivered to the patients. This study intends to: (a) evaluate the radiation dose to the patient using thermoluminescent dosemeters (TLDs, according to the protocol used at Radiology Department, University Hospital of Larissa, Greece; (b) estimate the thyroid surface dose (TSD) and doses to some radiosensitive organs located in the irradiation field. A total of 46 patients was examined. Patients were divided into two groups according to the digital X-ray machine used. The mean entrance surface dose (ESD) was 601.2 ± 96.2 mGy and the mean fluoroscopy time was 8.5 ± 3 min, while the mean TSD was 0.34 ± 0.6 mGy. The ESD for group A was lower by 20 % than group B due to fluoroscopic mode used. The dose values were higher than those in the literature. A local diagnostic reference level was introduced for further patient dose optimisation.

Radiation doses to paediatric patients and comforters undergoing chest X rays

Pneumonia is an important cause of hospital admission among children in the developed world and it is estimated to be responsible for 3-18 % of all paediatric admissions. Chest X ray is an important examination for pneumonia diagnosis and for evaluation of complications. This study aims to determine the entrance surface dose (ESD), organ, effective doses and propose a local diagnostic reference level. The study was carried out at the university hospital of Larissa, Greece. Patients were divided into three groups: organ and effective doses were estimated using National Radiological Protection Board software. The ESD was determined by thermoluminescent dosemeters for 132 children and 76 comforters. The average ESD value was 55 ± 8 µGy. The effective dose for patients was 11.2 ± 5 µSv. The mean radiation dose for comforter is 22 ± 3 µGy. The radiation dose to the patients is well within dose constraint, in the light of the current practice.

T2 FLAIR Increased Signal Intensity at the Posterior Limb of the Internal Capsule: Clinical Significance in ALS Patients

Amyotrophic lateral sclerosis (ALS) is characterized by progressive upper and lower motor neuron degeneration. A hyperintense signal on T2-weighted images along the corticospinal tract has been reported in patients with confirmed ALS. However, the specificity of this finding is under consideration, since it is also identified in healthy controls. Moreover, the correlation of this finding with disease progression has not yet been established. The purpose of our study is to evaluate the frequency with which this high signal appears in the posterior limb of the internal capsule (PLIC), compare visual with quantitative measurements, and correlate these with the progression of the disease. Our prospective clinical study included 24 patients and 51 healthy volunteers. In the ALS patient group, the diagnosis was established according to the criteria of El Escorial in the revised form of Airlee House. All patients were neurologically examined and underwent diagnostic procedures to exclude other diseases resembling ALS. The initial MRI was performed six months to two years after the onset of symptomatology. All ALS patients were clinically examined regarding their symptoms from the upper and lower motor neurons. Follow-up MRIs were performed in nine out of 24 patients over a period of six months. Signal changes in the PLIC are visually evaluated on FLAIR images, and are classified as distinct, mild or no signal change. Fractional anisotropy (FA) measurements are performed by placing a region of interest (ROI) in the PLIC bilaterally. Both findings are being compared. Mild signal changes were visualized in the PLIC in ten volunteers and seven patients. Distinct T2 FLAIR signal changes were visualized in the PLIC in seven ALS patients. No distinct signal change was visualized in the controls. Moreover this increased T2 FLAIR signal change became more accentuated with disease progress. FA measurements in patients were lower than in age-matched healthy subjects, with a further decrease with disease progression. Our findings indicate that although mild hyperintensity of the PLIC is not pathognomonic for ALS, detection of a distinct PLIC hyperintensity that gradually accentuates might actually be a sign of progressive ALS. This finding is supported by the progressively decreasing FA measurements. Larger numbers of patients need to be included and re-evaluated to obtain statistically significant results.

Strengths and weaknesses of 1.5T and 3T MRS data in brain glioma classification

Although magnetic resonance spectroscopy (MRS) methods of 1.5Tesla (T) and 3T have been widely applied during the last decade for noninvasive diagnostic purposes, only a few studies have been reported on the value of the information extracted in brain cancer discrimination. The purpose of this study is threefold. First, to show that the diagnostic value of the information extracted from two different MRS scanners of 1.5T and 3T is significantly influenced in terms of brain gliomas discrimination. Second, to statistically evaluate the discriminative potential of publicly known metabolic ratio markers, obtained from these two types of scanners in classifying low-, intermediate-, and high-grade gliomas. Finally, to examine the diagnostic value of new metabolic ratios in the discrimination of complex glioma cases where the diagnosis is both challenging and critical. Our analysis has shown that although the information extracted from 3T MRS scanner is expected to provide better brain gliomas discrimination; some factors like the features selected, the pulse-sequence parameters, and the spectroscopic data acquisition methods can influence the discrimination efficiency. Finally, it is shown that apart from the bibliographical known, new metabolic ratio features such as N-acetyl aspartate/ S, Choline/ S, Creatine/ S , and myo-Inositol/ S play significant role in gliomas grade discrimination.

Impact of dopamine transporter single photon emission computed tomography imaging using I-123 ioflupane on diagnoses of patients with parkinsonian syndromes

To assess the impact of I-123 ioflupane single photon emission computed tomography (SPECT) imaging on classifying patients with striatal dopaminergic deficits. Sixty-one patients with an initial diagnosis of parkinsonism or uncertain tremor disorder were screened and followed-up for one year. All patients were re-examined by two neurologists at our centre and were classified as having neurodegenerative or non-neurodegenerative disorders. Patients underwent I-123 ioflupane SPECT imaging. SPECT studies were blindly evaluated and classified as normal or abnormal (indicative of neurodegenerative disorders). The overall agreement of the SPECT imaging results with the initial classification was 65.6% (kappa=0.229, p=0.074) but was 90.2% (kappa=0.782, p<0.001) with the classification of the neurologists at our centre. I-123 ioflupane SPECT imaging is a valuable method in the evaluation of patients presenting clinically with uncertain parkinsonian syndromes or for whom diagnostic doubt exists.

A comparison of Monte Carlo simulation with experimental dosimetric techniques for a 6 MV stereotactic radiotherapy unit

PURPOSE

To compare Monte Carlo simulation with conventional dosimetry techniques for stereotactic radiotherapy (SRT), since accurate dosimetry of narrow photon beams is very complicated and has often been questioned, mainly due to the lack of lateral electronic equilibrium and uncertainty in beam energy in terms of steep dose gradients.

MATERIALS AND METHODS

In this work a Monte Carlo (MC, EGS4) simulation for dosimerty study was performed for the 6MV home made SRT unit of the University Hospital of Patras (Hellas). The results were compared with conventional small field dosimetry techniques such as ionization chamber, TLD's, and films (conventional and radiochromic). Hence, a comparison of many of the dosimetric techniques currently being used in small field dosimetry was attempted.

RESULTS

It was shown that all techniques are in reasonable agreement (within -/+ 2%) and that Monte Carlo can be used as a reliable reference for the dosimetry of the SRT beams, especially where lateral electronic equilibrium does not exist, as long as accurate simulation can be achieved.

CONCLUSION

This study is only limited by the insurance of accurate simulation of the linear accelerator, which can be a difficult task since it is limited by the availability of the manufacturer's designs and the availability of computers and computer time for adequate runs, but it could become a useful tool for Monte Carlo simulations, as it contains detailed analysis of the run parameters and component modules selection.