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Biomedical subjects

R E Sepponen

Publications and source records attributed to R E Sepponen.

At least 19 recordsLinked to original sources
PubMed2001

Multiple-slice spin lock imaging of head and neck tumors at 0.1 Tesla: exploring appropriate imaging parameters with reference to T2-weighted spin-echo technique.

RATIONALE AND OBJECTIVES: Spin lock imaging has been shown to be useful in characterizing head and neck tumors. The purposes of this study were to explore and develop multiple-slice spin lock gradient-echo (SL-GRE) sequences for head and neck imaging and to compare the tumor contrast on SL images to spin-echo (SE) T2-weighted images at 0.1 T. METHODS: On the basis of measured relaxation times of tumors and head and neck tissues, the authors evaluated with signal equations the effect of imaging parameters on tissue contrast produced by the SL-GRE sequence. In the clinical study, 34 patients with pathologically verified head and neck tumors were imaged with multiple-slice SL-GRE (repetition time 1500 ms/echo time 30 ms) out-of-phase fat/water sequences and compared with T2-weighted SE (repetition time 1500 ms/echo time 120 ms) sequences. The conspicuity of tumors was evaluated by calculating the contrast-to-noise ratios (CNRs). RESULTS: The combination of a short echo time of 30 ms and the length of locking pulses in the range of 10 to 35 ms produced optimal CNRs for head and neck tumor imaging. The measured CNRs and subjective evaluation for tumor detection were satisfactory with both imaging sequences. However, the CNRs between tumors and salivary gland tissues were significantly greater with the SL sequence than with the T2-weighted sequence. CONCLUSIONS: The multiple-slice SL-GRE technique provides image contrast comparable to that of SE T2-weighted imaging for head and neck tumors at 0.1 T. With short locking pulse lengths and echo times, wide anatomic coverage and reduced motion and susceptibility artifacts can be achieved. The out-of-phase SL technique is useful in imaging salivary gland tumors.

Head and Neck Neoplasms↗
PubMed2000

A shielded Overhauser marker for MR tracking of interventional devices.

Improvements to an active MR tracking technique are described. Real-time position monitoring of interventional procedures can be realized by incorporating a small marker that emits an NMR signal into the tip of an interventional device, and the marker's emitted NMR signal is enhanced by use of the Overhauser phenomenon. A significant advance over prior designs has achieved by making the marker have a cylindrical shape and by confining the saturation energy to the marker's interior. The performance of the improved active marker was verified in the laboratory and in vitro. The experiments demonstrated that the marker was visible in MR images when inserted in different excised tissues, and even in air, with positive contrast and with various imaging sequences. The tissue magnetization was minimally perturbed, and the marker emitted a variable but enhanced signal in all orientations in the magnetic field. The marker can potentially be used to mark locations on the body for frameless stereotaxy or to identify inserted devices.

Equipment Design↗
PubMed1998

High-accuracy MR tracking of interventional devices: the Overhauser marker enhancement (OMEN) technique.

A new technique for visualization of interventional devices in magnetic resonance imaging is presented. Determination of the position of an invasive device is made possible by incorporating into the device a small marker that emits the NMR signal. This signal is enhanced by the use of the Overhauser phenomenon. This technique differs from the earlier reported techniques for marking interventional instruments in the sense that the contrast between the marker and tissue is not based on different relaxation rates, but on NMR signal enhancement. A prototype marker was constructed and inserted into an inductively fed loop-gap resonator that couples saturation energy with the marker. Circuit analogies are presented that model the Overhauser phenomenon and the coupling circuit. In vitro experiments demonstrated that the marker is visible in MR images up to a slice thickness of 50 mm when inserted in excised animal liver and fat tissues.

Adipose Tissue↗
PubMed1998

Spin lock and magnetization transfer MR imaging of local liver lesions.

The present study was designed to evaluate tissue contrast characteristics obtained with the spin-lock (SL) technique by comparing the results with those generated with a magnetization transfer(MT)-weighted gradient echo [GRE, echo-time (TE)=40 ms] sequence. Twenty-eight patients with hepatic hemangiomas (n=14), or metastatic liver lesions (n=14) were imaged at 0.1 T by using identical imaging parameters. Gradient echo, single-slice off-resonance MT, and multiple-slice SL sequences were obtained. SL and MT-effects were measured from the focal liver lesions and from normal liver parenchyma. In addition, tissue contrast values for the liver lesions were determined. Statistically significant difference between the SL-effects of the hemangiomas and metastases, and also between the MT-effects of the lesions was observed (p < 0.02). Tissue contrast values for the lesions proved to be quite similar between the SL and MT techniques. Our results indicate that at 0.1 T multiple-slice SL imaging provides MT based tissue contrast characteristics in tissues rich in protein with good imaging efficiency and wide anatomical coverage, and with reduced motion and susceptibility artifacts.

Adult↗
PubMed1998

Determination of T1rho values for head and neck tissues at 0.1 T: a comparison to T1 and T2 relaxation times.

In order to optimize head and neck magnetic resonance (MR) imaging with the spin-lock (SL) technique, the T1rho relaxation times for normal tissues were determined. Furthermore, T1rho was compared to T1 and T2 relaxation times. Ten healthy volunteers were studied with a 0.1 T clinical MR imager. T1rho values were determined by first measuring the tissue signal intensities with different locking pulse durations (TL), and then by fitting the signal intensity values to the equation with the least-squares method. The T1rho relaxation times were shortest for the muscle and tongue, intermediate for lymphatic and parotid gland tissue and longest for fat. T1rho demonstrated statistically significant differences (p < 0.05) between all tissues, except between muscle and tongue. T1rho values measured at locking field strength (B1L) of 35 microT were close to T2 values, the only exception being fat tissue, which showed T1rho values much longer than T2 values. Determination of tissue relaxation times may be utilized to optimize image contrast, and also to achieve better tissue discrimination potential, by choosing appropriate imaging parameters for the head and neck spin-lock sequences.

Adipose Tissue↗
PubMed1997

Interventional MR imaging. Demonstration of the feasibility of the Overhauser marker enhancement (OMEN) technique.

PURPOSE: The poor localization facility of interventional instruments in MR imaging has been one of the major obstacles to the popularization of interventional MR imaging. It has been suggested that the Overhauser enhancement be used to generate markers of small size and high visibility. This article studies the feasibility of a localization marker based on this method. MATERIAL AND METHODS: A small Overhauser marker was constructed on the tip of a coaxial cable and comparative images were taken by a 0.23 T imager with and without electron spin irradiation. RESULTS: During irradiation an enhanced signal intensity from the marker was observed. The signal from the marker also exceeded the signal from a 0.25 mmol MnCl2 reference phantom. CONCLUSION: Its small size and high signal-to-noise ratio, together with immunity to most system nonlinearities and imaging errors, makes the Overhauser marker a promising localization method for the accurate positioning of interventional devices. The method may be applied at any field strength, and markers are visible in images obtained with any practical imaging sequence.

Chlorides↗
PubMed1996

Spin lock and magnetization transfer imaging of head and neck tumors.

PURPOSE: To evaluate and compare the spin lock and magnetization transfer techniques in the differentiation of benign and malignant head and neck tumors at magnetic resonance (MR) imaging. MATERIALS AND METHODS: Forty consecutive patients with histologically verified head and neck tumors (20 malignant and 20 benign tumors, including five infections) were studied with a 0.1-T MR unit. The spin lock and magnetization transfer effects were defined as 1-(signal intensity with stronger preparation pulse/signal intensity with weaker preparation pulse). RESULTS: A strong correlation between the spin lock and magnetization transfer effects was found (r = 85, P < .001). With a spin lock effect of 0.48 and a magnetization transfer effect of 0.32 as the thresholds, sensitivity for detecting a malignant tumor was 95% and 94%, respectively, and specificity was 60% and 65%. CONCLUSION: Low spin lock and magnetization transfer effects are characteristic of benign tumors. High spin lock and magnetization transfer effects were associated with malignancy, but there were overlapping values for salivary gland infections, some benign tumors, and malignancies. The spin lock technique seems to be an effective method for generating magnetization transfer-based contrast in the head and neck tumors.

Diagnosis, Differential↗
PubMed1996

Low-field MR imaging--development in Finland.

The development project for application of MR imaging to diagnosis of internal hemorrhages was initiated by the Instrumentarium Corporation in 1978. The goal was to develop a diagnostic tool for emergency clinics. Due to the rapid development of imaging technology, the goal was changed to a cost-effective MR unit. During the past 16 years, several generations of low-field units have been introduced. Consequently, a vast amount of clinical and technical knowledge about low-field MR has been gained. The interest in low-field units is rapidly increasing. A part of this may be explained by the pressure to reduce the cost of health care. There are some features which make the low-field approach clinically interesting. These include the feasibility of open magnet configurations, and the availability of unique contrast parameters such as magnetization transfer and T1p. One important aspect is the inherent safety of a low-field MR unit. This article reviews the methods and devices introduced through the development of low-field technology in Finland.

Equipment Design↗
PubMed1995

Spin lock magnetic resonance imaging in the differentiation of hepatic haemangiomas and metastases.

Spin lock (SL) imaging technique, generating T1 rho-weighted images, was applied to the differentiation of hepatic haemangiomas from metastatic focal liver lesions. 17 haemangiomas and 16 metastases in 32 patients were imaged at the field-strength of 0.1 T using a multiple slice SL technique and a conventional gradient-echo (GRE) sequence with identical timing parametres. Spin lock effects of the hepatic lesions and different abdominal tissues were calculated. Images with adequate coverage of the liver and of good quality with few motion induced artefacts were acquired. A definite, statistically significant, difference was found between the SL-effects of hepatic haemangiomas and a liver metastases. Haemangiomas showed an SL effect of 46.6 +/- 3.4% and metastases of 56.2 +/- 5.8% (mean +/- SD, p < 0.0001). The multiple slice SL technique showed potential in distinguishing haemangiomas from metastatic liver lesions and should be considered as an alternative to the conventional T2 and magnetization transfer (MT) based methods.

Adult↗
PubMed1993

Magnetization transfer imaging of the abdomen at 0.1 T: detection of hepatic neoplasms.

Magnetization transfer (MT) techniques have been proposed as a method of increasing contrast in MR images. To evaluate the feasibility of MT imaging of the abdomen at 0.1 T and to assess the clinical utility of this technique, the authors studied tissue contrast with a gradient-echo pulse sequence and an MT sequence in four normal volunteers, and in 17 patients with known primary or secondary neoplasms of the liver. The MT technique increased contrast between the liver and other tissues such as spleen, skeletal muscle and subcutaneous fat. The technique also produced increased contrast between hepatic tumors and normal liver parenchyma in gradient-echo images.

Abdomen↗
PubMed1993

T1 rho dispersion imaging of diseased muscle tissue.

T1 rho dispersion, or the frequency dependence of T1 relaxation in the rotating frame, was used for in vivo muscle tissue characterization in 13 patients with primary skeletal muscle disease and in eight normal subjects for comparison. T1 rho dispersion measurements represent a new approach to magnetic resonance tissue characterization, possibly reflecting the macromolecular constituents of tissue. A definite, statistically significant, difference was found between the relative T1 rho dispersion values of normal and diseased muscle tissue. T1 rho dispersion measurements and images may increase the accuracy of identification of diseased muscles. Early identification of affected muscles is important for accurate diagnosis by muscle biopsy.

Adolescent↗
PubMed1992

Magnetization transfer contrast imaging of the human leg at 0.1 T: a preliminary study.

Magnetization transfer contrast imaging is an MR technique that capitalizes on interactions between the protons of mobile and macromolecularly bound water molecules. Studies to date, conducted primarily on 4.7 T and 1.5 T MR systems, have yielded results unique from conventional T1- and T2-weighted imaging studies. In this study, performed on a 0.1 T device, a section of lower leg was imaged in 20 normal human subjects and one patient with muscular dystrophy, using both a standard 500/22 gradient-echo sequence and a 500/22 gradient-echo sequence combined with off-resonance radio frequency irradiation designed to elicit magnetization transfer contrast. Results of the two techniques were compared. Our findings suggest that magnetization transfer contrast imaging is feasible at 0.1 T, and that this technique allows reproducible tissue characterization and improves contrast between certain tissues.

Adipose Tissue↗
PubMed1990

Magnetic resonance of diseased skeletal muscle: combined T1 measurement and chemical shift imaging.

Magnetic resonance examinations of skeletal muscle with differential T1 relaxation time measurements were performed in 19 patients with muscular dystrophies and congenital myopathies, and in eight control subjects. A field echo chemical shift imaging technique was used. T1 values of muscular tissue were measured from the primary composite images, and differential T1 values were calculated separately from water and fat images. Longitudinal relaxation times of skeletal muscle were significantly increased in both dystrophies and myopathies. The results of differential relaxation time measurements suggest that intramuscular fat reduces the abnormal increase in T1 of diseased muscle tissue. When characterizing diseases of skeletal muscle by T1 relaxation time measurements, the contribution of secondary fatty infiltration must be considered.

Adipose Tissue↗
PubMed1985

A method for imaging of chemical shift or magnetic field distributions.

A phase encoding method for imaging of chemical shift or magnetic field distributions is described. The method utilizes the spin-echo principle and the time period between signal collection and excitation is constant but the time period between excitation and the 180 degrees pulse is varied by constant steps. The method is relatively easy to apply with the Fourier or projection reconstruction methods.

Humans↗
PubMed1984

Cerebral NMR and CT imaging in dementia.

Thirteen patients with different types of dementia were studied with nuclear magnetic resonance (NMR) and CT imaging of the brain. The overall correlation between CT and NMR findings was good. Areas of hypodensity in cerebral white matter on CT were especially well visualized in the T2-weighted NMR images. Hypodense areas were seen in the white matter of patients with vascular dementias but not in those with primary degenerative dementias. Thus, the presence or absence of white matter changes may be an important factor in the differential diagnosis of vascular dementia and primary degenerative dementia. On the basis of NMR and CT findings, it was difficult to distinguish between the lacunar state type and Binswanger type of vascular dementia. It is suggested, therefore, that these two subgroups of vascular dementia be commonly named angiopathic dementia.

Aged↗
PubMed1984

Cerebral MR and CT imaging in polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy.

Cerebral magnetic resonance (MR) and CT imaging of two patients with polycystic lipomembranous osteodysplasia with sclerosing (PLO-SL) showed widespread progressive central and cortical atrophy. Computed tomography revealed calcifications in the area of the caudate nuclei and putamina in both patients. A reduced volume of brain white matter with deep cortical sulci was the most striking MR finding. Rather short relaxation times in periventricular areas adjacent to ventricular bodies were also observed.

Adult↗