Biomedical subjects

Petros Martirosian

Publications and source records attributed to Petros Martirosian.

16 recordsLinked to original sources

PubMed2007

Quantitative assessment of glomerular filtration rate with MR gadolinium slope clearance measurements: a phase I trial.

PURPOSE: To prospectively demonstrate the feasibility of quantifying the glomerular filtration rate (GFR) by assessing the renal clearance of gadolinium-based contrast medium from the extracellular fluid volume in healthy volunteers. MATERIALS AND METHODS: The study was approved by the ethics committee and the governmental drug administration department (registration number 4030139, EudraCT number 2004-002969-20, study protocol number 318/2004). Informed consent was obtained from 16 healthy volunteers (six female, 10 male; mean age, 24.5 years +/- 2.8 [standard deviation]). Thirteen volunteers (four women, nine men; mean age, 24.8 years +/- 2.7; range, 23-30 years) successfully contributed to the study. The GFR was assessed by recording the renal clearance of gadobutrol (3.75 mL, approximately 0.05 mmol per kilogram of body weight) at navigator-gated turbo fast low-angle shot magnetic resonance (MR) imaging. Time-signal intensity curves were constructed from manually drawn regions of interest in the liver, spleen, and renal cortex, and the GFR was calculated by using exponential fitting. Simultaneously obtained iopromide clearance measurements were the reference standard. Statistical evaluations included Bland-Altman plotting and analysis of the relative deviation from iopromide clearance. RESULTS: Evaluation of liver regions of interest revealed the lowest mean of paired differences from the iopromide clearance measurements (-5.9 mL/min per 1.73 m(2) +/- 14.6), with a mean GFR of 109.0 mL/min per 1.73 m(2) +/- 17.1 (134.1 mL/min per 1.73 m(2) +/- 35.4 for spleen, 100.7 mL/min per 1.73 m(2) +/- 25.1 for renal cortex) compared with a mean GFR of 103.1 mL/min per 1.73 m(2) +/- 9.4 measured by using iopromide clearance. The maximum deviation of MR-determined gadobutrol clearance values from iopromide clearance values was 29.2%. The mean disposition half-life of gadobutrol measured in the liver was 83.0 minutes +/- 14.2 (72.4 minutes +/- 20.2 in spleen, 92.6 minutes +/- 23.7 in renal cortex). CONCLUSION: The described MR imaging method enables absolute quantification of the GFR after routine contrast material-enhanced MR imaging.

Adult↗

PubMed2007

Optimization of blood-myocardial contrast in 3D true FISP cardiac imaging at 1.5 T.

Three-dimensional (3D) steady-state free precession (SSFP) MRI sequences are often applied to visualize both intra- and extracardiac pathologies. In the present study the contrast behavior of 3D true fast imaging with steady precession (True-FISP) sequences for cardiac imaging was optimized in numerical simulations and compared with measurements obtained in eight healthy volunteers on a 1.5 T whole-body scanner. Two SS preparation schemes in combination with and without a T(2) preparation were assessed to improve contrast between blood and myocardium using a navigator-gated and ECG-triggered 3D True-FISP sequence. Numerical simulations and experimental studies in volunteers showed that an SS preparation using a constant flip angle (CFA) is preferable to a linear flip angle (LFA) preparation in terms of contrast between blood and myocardium. The optimized 3D True-FISP sequence provides a reliable, accurate, and time-efficient means of obtaining a morphological cardiac diagnosis.

Adult↗

PubMed2006

Three-dimensional morphological magnetic resonance imaging in infants and children with congenital heart disease.

BACKGROUND: Cardiac MRI has become a clinically useful supplement to ECHO and conventional X-ray angiography in the diagnostic work-up of patients with congenital heart disease (CHD). Three-dimensional (3D) sequences are capable of depicting both intracardiac and extracardiac structures with high accuracy in adults and adolescents. However, diagnostic image quality in infants and young children has not yet been reported. OBJECTIVE: To apply an optimized 3D steady-state free-precession (SSFP) MR sequence in infants and children with CHD. MATERIALS AND METHODS: In 20 patients (median age 1.8 years; ten male) with CHD, whole-chest imaging was performed with navigator-gated, isotropic 3D SSFP MRI at 1.5 T. Sequence parameters were adapted to special requirements in infancy. Measurements of intra- and extracardiac structures were performed by two independent observers and compared to spin-echo and cine gradient-recalled-echo sequences. RESULTS: Diagnostic image quality was achieved with the 3D SSFP technique in all patients, allowing the establishment of a diagnosis in all cases. Interobserver comparison of measurements from reformatted 3D SSFP datasets revealed only minor differences with standard deviations ranging from 0.3-1.3 mm for intracardiac and 0.3-0.7 mm for extracardiac anatomy (P = ns). CONCLUSION: Isotropic 3D SSFP MRI allows reliable and accurate assessment of CHD, even in free-breathing infants and young children.

Child↗

PubMed2006

Contrast-enhanced dynamic MR nephrography using the TurboFLASH navigator-gating technique in children.

The purpose of this work was to test the feasibility of an MR examination protocol for the comprehensive assessment of renal morphology, excretion and split renal function using a navigator-gated TurboFLASH sequence. A navigator-gated T1-weighted single-slice TurboFLASH sequence suitable for dynamic MR urography and nephrography was implemented. A protocol was developed allowing for assessment of urinary excretion and split renal function by recording the renal clearance of a gadolinium (Gd) diethylene-triamine-pentacetic-acid (DTPA) bolus. Ten patients aged between 14 months and 14 years (mean age 4.8+/-4.6 years) were evaluated with the following indications: pelvicalyceal dilatation (n=4), follow-up after pyeloplasty (n=1), duplex systems (n=3), large renal cyst (n=1), and renal insufficiency (n=1). Dynamic MR urography and MR split renal function were compared to MAG3 scintigraphy. Evaluation of morphology, excretion and function required 50-60 minutes examination time, plus 10 minutes for post-processing. The TurboFLASH sequence yielded image acquisition at nearly identical diaphragm positions allowing for accurate region-of-interest evaluation within the renal parenchyma and the urinary passage. Static and dynamic MR urography showed the morphology of the urinary tract and excretion with sufficient diagnostic imaging quality, and the results were in diagnostic compliance with scintigraphy. MRI and scintigraphy yielded similar results for split renal function with a correlation coefficient of R=0.968 determined by linear regression. Our conclusions were that the method is robust, easy to perform on a clinical 1.5 T MRI system, rapid to evaluate and post-process and, therefore, easy to incorporate into clinical routine. Compared to scintigraphy, the higher spatial resolution of the MR examination provides additional important information improving the management of the pediatric patients without the application of radioactive tracers.

Adolescent↗

PubMed2006

Morphological, contrast-enhanced and spin labeling perfusion imaging for monitoring of relapse after RF ablation of renal cell carcinomas.

MR perfusion imaging was applied for the assessment of completeness in the destruction of renal cell carcinomas by RF ablation (RFA) in a pilot study. An arterial spin labeling (ASL) approach was compared to conventional contrast-enhanced T1-weighted (CE-T1w) imaging. Ten patients suffering from renal cell carcinoma were treated by RFA. For the assessment of the extent of coagulation and for the detection of residual tumor, T1-weighted gradient-echo imaging, T2-weighted spin echo imaging and two different perfusion imaging techniques were performed before, 1 day and 6 weeks after RFA at 1.5 T. Perfusion imaging comprised CE-T1 weighted and FAIR-TrueFISP ASL imaging. Perfusion images recorded in the acute stage after RFA showed higher compliance to the definitive ablation volume reached after 6 weeks than T2-weighted images, which underestimated the true necrosis size. In the detection of residual tumor tissue, both modalities complimented each other. The exclusion of residual tumor tissue could more reliably be performed using perfusion-imaging methods. Both perfusion-imaging modalities showed sufficient imaging quality for post-interventional monitoring. Perfusion imaging provides a higher predictability of the completeness of tumor ablation and extent of coagulation than T2-weighted imaging alone. Since the results of the FAIR-TrueFISP sequence are promising, the administration of potentially nephrotoxic contrast media may be avoided in the respective patient cohort.

Aged↗

PubMed2006

Perfusion MR imaging with FAIR true FISP spin labeling in patients with and without renal artery stenosis: initial experience.

The purpose of this study was to prospectively evaluate an arterial spin-labeling technique, flow-sensitive alternating inversion-recovery (FAIR) true fast imaging with steady-state precession (FISP), for noninvasive quantification of renal perfusion in patients without a history of renal artery stenosis (RAS) and in patients with proved RAS. The study was approved by the local ethics committee, and all participants provided written informed consent. Six patients with hypertension but no history of renal artery disease and 12 patients with RAS underwent FAIR true FISP magnetic resonance (MR) imaging in a whole-body 1.5-T unit. RAS grade and scintigraphic perfusion data served as the reference standards. On the FAIR true FISP perfusion images, severe RAS (>70% luminal narrowing) could be clearly distinguished from no or mild RAS and moderate RAS (< or =70% luminal narrowing) (P < .005). Significant correlations between FAIR perfusion data and stenosis grade (r = -0.76) and between FAIR and single photon emission computed tomographic perfusion values (r = 0.83) were observed. FAIR true FISP was found to be suitable for quantitative perfusion imaging of the kidneys in patients with RAS.

Aged↗

PubMed2006

Whole-body magnetization transfer contrast imaging.

PURPOSE: To demonstrate the feasibility of whole-body magnetization transfer (MT) contrast imaging. MATERIALS AND METHODS: Whole-body MT imaging was performed on eight healthy volunteers and five patients (mean age=40.5+/-17.8 years) with diagnoses of dermatomyositis (N=1), B-symptoms with suspicion of paraneoplastic disease (N=1), metastatic malignant melanoma (N=1), and multiple sclerosis (MS) (N=2). Measurements were carried out on a 1.5-Tesla whole-body MR scanner capable of parallel signal reception. A three-dimensional (3D) gradient-echo sequence (TR=17 msec, TE=4.8 msec, flip angle=10 degrees) was applied in combination with a Gaussian off-resonance MT preparation pulse acting at an off-resonance of 1.500 Hz with a 500 degrees effective flip angle. Whole-body images were constructed from five different body regions. RESULTS: In all subjects, whole-body MT contrast images were obtained within less than 20 minutes of measuring time. The images showed sufficient diagnostic image quality to assess the patients' pathologies. The MT ratios (MTRs, in percent units) for the volunteers were as follows: white matter (WM) 51.1+/-1.0, gray matter (GM) 42.2+/-1.3, skeletal muscle (mean value of four muscle groups) 50.3+/-2.1, liver 39.4+/-3.2, spleen 31.8+/-2.6, renal cortex 30.4+/-1.9, and renal medulla 25.6+/-1.3. The MTRs for the pathologies were as follows: skeletal muscle in dermatomyositis approximately 30, metastases in malignant melanoma 30.7-36.0, uterus myoma 49.3, and MS lesions 30-40. CONCLUSION: Our preliminary data indicate that MT contrast in whole-body MRI is feasible, and may be useful for rapid whole-body assessment of diseases that exhibit high contrast in MT imaging, such as MS and muscular disorders.

Adult↗

PubMed2006

Quantitative lung perfusion mapping at 0.2 T using FAIR True-FISP MRI.

Perfusion measurements in lung tissue using arterial spin labeling (ASL) techniques are hampered by strong microscopic field gradients induced by susceptibility differences between the alveolar air and the lung parenchyma. A true fast imaging with steady precession (True-FISP) sequence was adapted for applications in flow-sensitive alternating inversion recovery (FAIR) lung perfusion imaging at 0.2 Tesla and 1.5 Tesla. Conditions of microscopic static field distribution were assessed in four healthy volunteers at both field strengths using multiecho gradient-echo sequences. The full width at half maximum (FWHM) values of the frequency distribution for 180-277 Hz at 1.5 Tesla were more than threefold higher compared to 39-109 Hz at 0.2 Tesla. The influence of microscopic field inhomogeneities on the True-FISP signal yield was simulated numerically. Conditions allowed for the development of a FAIR True-FISP sequence for lung perfusion measurement at 0.2 Tesla, whereas at 1.5 Tesla microscopic field inhomogeneities appeared too distinct. Perfusion measurements of lung tissue were performed on eight healthy volunteers and two patients at 0.2 Tesla using the optimized FAIR True-FISP sequence. The average perfusion rates in peripheral lung regions in transverse, sagittal, and coronal slices of the left/right lung were 418/400, 398/416, and 370/368 ml/100 g/min, respectively. This work suggests that FAIR True-FISP sequences can be considered appropriate for noninvasive lung perfusion examinations at low field strength.

Adolescent↗

PubMed2006

Quantitative ASL muscle perfusion imaging using a FAIR-TrueFISP technique at 3.0 T.

The feasibility of muscle perfusion imaging with diagnostic image quality was demonstrated using the FAIR-TrueFISP arterial spin labeling technique on a clinical 3.0 T whole-body scanner. In eight healthy volunteers (24 to 42 years old), quantitative perfusion maps of the forearm musculature were acquired before and after intense exercise. All measurements were carried out in a 3.0 T whole-body MR unit in combination with an eight-channel head coil. Pulsed arterial spin labeling and data recording were performed with an adapted FAIR-TrueFISP technique and quantitative perfusion maps were calculated on a pixel-by-pixel basis by means of the extended Bloch equations. Perfusion images with an in-plane resolution of 1 mm showed no significant distortions or blurring. Perfusion-time curves could be recorded with a temporal resolution of 6.4 s. Maximum perfusion in the musculature was found approximately 2 min after exercise, reaching values of up to 220 mL/min per 100 g of tissue with good delineation between the active muscles and the musculature not involved in the exercise. In conclusion, the TrueFISP pulsed arterial spin labeling technique allows patient-friendly assessment of muscular perfusion in a clinical whole-body scanner.

Adult↗

PubMed2006

Relaxivity of Gadopentetate Dimeglumine (Magnevist), Gadobutrol (Gadovist), and Gadobenate Dimeglumine (MultiHance) in human blood plasma at 0.2, 1.5, and 3 Tesla.

OBJECTIVES: We sought to determine the relaxivity and accurate relaxation rates of Gd-DTPA, Gd-BT-DO3A, and Gd-BOPTA at 0.2, 1.5, and 3 T in human blood plasma. MATERIALS AND METHODS: Contrast media concentrations between 0.01 and 16 mM in human plasma were used for relaxation measurements. The R1 and R2 relaxation rates and r1 and r2 relaxivities were determined. RESULTS: Gd-BOPTA produced the highest relaxation rates and relaxivities at all field strengths. The r1 and r2 values for Gd-BOPTA were 107-131% and 91-244% higher than for Gd-DTPA, respectively, and 72-98% and 82-166% higher than for Gd-BT-DO3A. Higher field strengths resulted in lower values of R1, R2, and r1 for all contrast agents tested and of r2 for Gd-DTPA and Gd-BT-DO3A. A linear dependence of R1 and R2 on concentration was found for Gd-DTPA and Gd-BT-DO3A and a nonlinear dependence for Gd-BOPTA for concentrations larger than 1 mM. The r1 and r2 relaxivity of Gd-BOPTA increased with decreasing concentration. CONCLUSIONS: Gd-BOPTA demonstrates the highest longitudinal r1 at all field strengths, which is ascribable to weak protein interaction. The R2/R1 ratio increases at higher field strength only for Gd-BOPTA, hence very short echo times are required for Gd-BOPTA to benefit from the higher longitudinal relaxivity.

Contrast Media↗

PubMed2006

Effects on MRI due to altered rf polarization near conductive implants or instruments.

In magnetic resonance imaging near metal parts variations in radio frequency (rf)-amplitude and of receive sensitivity must be considered. For loop structures, e.g., vascular stents, B1 produces rf eddy currents in accordance to Faraday's law; the B1-related electrical rf field E1 injects directly to elongated structures (e.g., wires). Locally, the rf magnetic field Bl,ind (induced B1) is superimposed onto the rf field from the transmitter coil, which near the metal can dominate spin excitation. Geometry and arrangement of the parts determine the polarization of B(1,ind). Components parallel to B0 are of special interest. A copper sheet (100 mm x 15 mm, 3 mm thick) and a 27 cm long copper wire were examined in a water phantom using the spin-echo (SE) technique. In addition to rf-amplitude amplification, rf-phase shift due to z components of B(1,ind) could be detected near the metallic objects. Periodic rf-amplitude instabilities had an amplified effect for phase-shifted regions. Phase-encoding artifacts occurred as distinct ghosts (TR=200 ms) or band-like smearing (TR=201 ms) from affected spin ensembles. SE phase imaging can potentially be used in interventional magnetic resonance imaging for background-free localization of metallic markers.

Algorithms↗

PubMed2005

Metal artifacts caused by gradient switching.

In metal parts, e.g., implants or instruments, eddy currents can be induced from gradient switching if positioned off-center inside the MR scanner. For the first time, a systematic analysis of related artifacts was performed. Current strength increases in conjunction with increasing size of the part, increasing electrical conductivity, distance from isocenter, and increasing gradient strengths. A xy-plane oriented copper ring (d(o) = 20 mm, d(i) = 15 mm, 2 mm thick) was examined at isocenter and at x = 15 cm, y = z = 0. Comparisons of xy-, xz-, and yz-slices, recorded for both possibilities to select encoding directions, revealed effects from ramp-down of the slice-selection and ramp-up of the read-out gradient. Near the metal part, temporary inhomogeneities were superimposed to the static field and spin-dephasing signal loss resulted, despite using spin-echo technique. Artifacts depended on excitation and read-out bandwidth. For an equivalent titanium ring, conductivity related effects could not be ascertained but distinct susceptibility effects occurred. MR compatibility of implants/instruments therefore requires both low susceptibility and low conductivity.

Artifacts↗

PubMed2005

Functional characterization of prostate cancer by integrated magnetic resonance imaging and oxygenation changes during carbogen breathing.

OBJECTIVE: The objective of this study was to evaluate the changes in oxygenation of prostate cancer induced by carbogen breathing using blood oxygen level-dependent (BOLD) magnetic resonance image (MRI) with an endorectal coil (eMRI). MATERIALS AND METHODS: In 32 patients with biopsy-proven prostate cancer, endorectal MRI was performed at 1.5 Tesla using the BOLD method. Images were acquired during 4 x 4-minute episodes alternating between room air and carbogen (95% O2/5% CO2) breathing. In each episode, 40 images were acquired (T2*-weighted EPI sequence, 12-14 slices, 3-mm thickness). All patients underwent radical prostatectomy; BOLD-MRI findings were correlated with the histopathologic results. RESULTS: BOLD-MRI could be evaluated in 29 patients, and revealed heterogeneous signal changes of normal prostate and cancer tissue similar to the heterogeneity of prostate tissue in anatomic/pathologic preparation. A significant signal intensity increase (P = 0.004) was found in normal central gland and peripheral zone during carbogen breathing. Signal enhancement in carcinoma was significantly lower (P = 0.004) compared with the contralateral normal side. CONCLUSION: Intrinsic blood-tissue contrast-functional MRI during carbogen breathing may help detect and characterize prostate carcinoma from normal tissue, particularly in small 1-sided carcinomas. This may be useful for identifying candidates for radiotherapy and monitoring noninvasive therapeutic approaches.

Aged↗

PubMed2005

[Positive contrast in the detection of magnetically labeled cells by MRI--in vitro experiments].

PURPOSE: Magnetically labeled cells (MLC) cause local field inhomogenities within the single voxels as well as on a macroscopic scale. The related Larmor frequency shift near MLC was exploited to obtain bright visualization applying spectral selective saturation (SSS). METHODS: SK-Mel28 cells were labeled with the superparamagnetic iron oxide contrast agent SHU 555A. Low cell concentrations (0, 5, 20, 30, 50, 75, and 150 MLC/microl) and high cell concentrations (10 x 10(3), 30 x 10(3), 60 x 10(3), and 100 x 10(3) MLC/10 microl) were examined at 3 Tesla. Shimming and frequency adjustment to spectrometer reference frequency v0 was performed with the built in routine of the scanner. A 2D spin echo sequence with broadband excitation and refocusing pulses was used (BWex = 1.000 Hz). Prior to each TR, a non-selective saturation sinc pulse centred at v0 was applied. Bandwidth (BWsat) of this pulse was varied from 100 Hz to 800 Hz in logarithmic steps. RESULTS: Without SSS the highest value of Crel (i.e. relative MR contrast between labeled to unlabeled samples) was found for 150 MLC/microl and was given by 10%. Applying SSS led to positive contrast of the complete labeled volumes and to remarkable improvements in Crel. With increasing cell concentrations Crel raised to maximum, that was given by 52% (BWsat=100 Hz) and 28% (BWsat = 200 Hz) found for 75 MLC/microl. For 150 MLC/microl Crel decreased. A contrast clarification could also be detected near cell aggregations despite saturation. CONCLUSION: Using SSS positive contrast can be achieved for voxels containing MLC and voxels close to cell clusters. Under in vitro conditions positive contrast improved the sensitivity to detect MLC as compared to negative contrast imaging techniques. It seems reasonable, that positive contrast approaches can be applied in vivo as the underlying physical mechanism are comparable.

Cell Line, Tumor↗

PubMed2004

FAIR true-FISP perfusion imaging of the kidneys.

Most arterial spin labeling (ASL) techniques apply echoplanar imaging (EPI) because this strategy provides relatively high SNR in short measuring times. Unfortunately, those techniques are very susceptible to static magnetic field inhomogeneities and perfusion signals from organs with fast transverse relaxation might decrease due to the exchange of water molecules in capillaries and organ tissue combined with relatively long echo times of EPI sequences. To overcome these problems a novel imaging technique, FAIR True-FISP, was developed. It combines a FAIR (flow-sensitive alternating inversion recovery) perfusion preparation and a true fast imaging with steady precession (True-FISP) data acquisition strategy. True-FISP was chosen since this sequence type does not show the mentioned disadvantages of EPI, but provides a similar SNR per measuring time. An important problem of this approach is that True-FISP sequences usually work in a steady state which is independent of a previous preparation of magnetization. For this reason a sequence structure had to be developed which keeps the advantages of True-FISP and makes the signal intensity sensitive to the FAIR preparation. Breathhold and nonbreathhold examinations of kidneys are presented and possible strategies to quantitative flow measurements are reported. It is shown that correction of spatially inhomogeneous receiver coil characteristics is easily feasible and leads to clinically valuable perfusion examinations of kidneys without application of potentially nephrotoxic contrast media.

Adult↗

PubMed2003

Inductively coupled rf coils for examinations of small animals and objects in standard whole-body MR scanners.

Inductively coupled solenoid coils fitting to objects in the size of mice or rats were developed to adapt modem whole-body MR scanners featuring sufficient gradient strength for animal examinations with high spatial resolution. Homogenous receiver characteristics is achievable over almost the whole inner region of the solenoid coils. The SNR can be increased by a factor 2 to 6 with the adapting coils for examinations using the head coil as connected receiver. Standard sequences on clinical 1.5 T scanners can be applied with adapted transmitter voltages. For example, a SNR value of about 30 is achievable in a mouse liver after 10 minutes measuring time using a 2-D spin echo imaging sequence and a size of 0.3 x 0.3 x 0.8 mm3 for the picture elements.

Animals↗