Bo Nordell

Noise considerations in the determination of diffusion tensor anisotropy

In this study the noise sensitivity of various anisotropy indices has been investigated by Monte-Carlo computer simulations and magnetic resonance imaging (MRI) measurements in a phantom and 5 healthy volunteers. Particularly, we compared the noise performance of indices defined solely in terms of eigenvalues and those based on both the eigenvalues and eigenvectors. It is found that anisotropy indices based on both eigenvalues and eigenvectors are less sensitive to noise, and spatial averaging with neighboring pixels can further reduce the standard deviation. To reduce the partial volume effect caused by the spatial averaging with neighboring voxels, an averaging method in the time domain based on the orientation coherence of eigenvectors in repeated experiments has been proposed.

A method for MR quantification of flow velocities in blood and CSF using interleaved gradient-echo pulse sequences

The aim of this study was to establish a rapid method for in vivo quantification of a large range of flow velocities using phase information. A basic gradient-echo sequence was constructed, in which flow was encoded along the slice selection direction by variation of the amplitude of a bipolar gradient without changes in sequence timings. The influence of field inhomogeneities and eddy currents was studied in a 1.5 T interleaved sequences for calibration and in vivo flow determination were constructed, and flow information was obtained by pairwise subtraction of velocity-encoded from velocity non-encoded phase images. Calibration was performed in a nongated mode using flow phantoms, and the results were compared with theoretically calculated encoding efficiencies. In vivo flow was studied in healthy volunteers in three different areas using cardiac gating; central blood flow in the great thoracic vessels, peripheral blood flow in the popliteal vessels, and flow of cerebrospinal fluid (CSF) in the cerebral aqueduct. The results show good agreement with results obtained with other techniques. The proposed method for flow determination was shown to be rapid and flexible, and we thus conclude that it seems well suited for routine clinical MR examinations.

Three-dimensional dose mapping from gamma knife treatment using a dosimeter gel and MR-imaging

A new method has been investigated for the mapping of dose distributions in three dimensions delivered by the Leksell gamma knife. The irradiation unit is used to selectively treat small volumes in the brain with single high doses of ionising radiation--a treatment procedure known as radiosurgery. The dosimetry method investigated utilises a dosimeter gel consisting of ferrous sulphate solution and agarose which is, prior to irradiation, loaded into a cavity in a spherical phantom. Chemical changes induced in the gel by the radiation are measured by means of an MR-scanner. This imaging method permits rapid evaluation of the dose distribution in an irradiated volume. It thus offers a potential verification of individual radiation intracranial target treatment regimes as well as quality assurance measurements, assuming that the precision and accuracy of the dose mapping are adequate. The dose and its distribution registered by the gel dosimeter, in this initial experiment, are in good agreement with corresponding computed data obtained with the KULA treatment planning system of the gamma knife. The gel has thus the potential of being an attractive alternative dose mapping method to those used at present in radiosurgery, i.e. radiographic film and small ionisation chambers. The precision of the dosimeter gel is, however, not yet sufficient high to be used as a basic dosimetry system for the gamma knife.

A rotating phantom for the study of flow effects in MR imaging

A common type of phantom used for the study of flow effects in MR imaging is the tube phantom, where a liquid passes through a set of tubes placed in the main magnetic field of an MR scanner. Among the disadvantages with this type of phantom are that a distribution of velocities is present in each tube, and that quantifications of flow effects using tube phantoms may be very time-consuming. In this work, we describe the design and the properties of a rotating wheel flow phantom used for quantification of the effects of flow through the imaging plane as well as in the imaging plane. The proposed phantom is constructed as a rotating gel-filled wheel, surrounded by static volumes filled with the same gel, and the evaluation of the information from rotating and static parts is made with a specially designed computer program. The phantom can be used as a plug flow phantom covering simultaneously an interchangeable velocity interval, which at present has the range -52 mm/s, +52 mm/s. It is shown that the phantom gives adequate information on the dependence of pixel content on first-order motion in MR modulus and phase images. Among the fields of application are rapid calibration of MR imaging units for flow determination using phase information, as well as testing of pulse sequence characteristics and verification of theoretical predictions concerning the flow dependence in MR images.

Phantom and in vivo study of the look–locher T1 mapping method

This paper describes and tests the LL-EPI method for obtaining quantitative T1 estimates in a few seconds thereby allowing dynamic T1 studies. It is shown that the method works even when there is an inflow into the imaged volume, e.g., in a vessel. No calibration is needed. The method has been tested in a phantom study with several different scan parameter set-ups, with and without inflow. The method shows robustness and individual scan parameters and inflow rates do not influence the ability to calculate the Gd-DTPA concentration. Linearity prevail between the measured 1/T1 and the Gd-DTPA concentration in the range 150 < T1 < 2500 ms. In a dynamic Gd-DTPA phantom study, it was shown that the dynamic LL-EPI T1 mapping technique was three times more sensitive than the signal from a T2*-weighted EPI sequence. In an in vivo study, dynamic T1 mapping of the Gd-DTPA uptake in a meningioma was performed. Inspection of the uptake curves indicates that the method is feasible in clinical perfusion studies.

Analysis of the Look-Locker T1 mapping sequence in dynamic contrast uptake studies: simulation and in vivo validation

An alternative to the pulse sequences at present used in dynamic contrast uptake MRI is the dynamic LL-EPI T(1) mapping method. This method generates T(1) estimates in a few seconds, thereby allowing dynamic studies. A particular advantage of the LL-EPI technique is that it provides the opportunity to generate spatial and temporal information about the paramagnetic contrast agent concentration independently of the inflow rate. This paper illustrates, by computer simulations, the accuracy of the estimated 1/T(1) value when using the LL-EPI technique in situations that are not supported by the model. The simulated situations not supported by the model are those in which the longitudinal and transversal relaxation rates change during the T(1) mapping. The most critical moment occurs during a bolus passage of contrast agent when the concentration gradient is large. The computer simulations of the LL-EPI T(1) mapping method in non-supported situations show that in normal perfused capillary tissue the error in the estimated 1/T(1) value is within the absolute error of 0.1 s(-1) in most simulated situations, although in a typical vessel the simulations do indicate that the stated absolute error tolerance of 0.5 s(-1) is exceeded relatively easily. However, this transgression can be rectified by a non-bolus injection of the contrast agent media.

Computer assisted dosimetry of scanned electron and photon beams for radiation therapy

A computer controlled beam forming system for energies up to 50 MeV has been developed in order to produce high quality electron and photon beams for radiation therapy. The desired radiation field shape and dose distribution are achieved by programming the scanning pattern of a narrow and unfiltered electron or photon beam. The computer that controls the scanning pattern also performs dosimetric analyses in the resultant radiation beams. The system allows real time display of the measured dose distributions at a rate of up to five discrete dose values per second for a 15 cm square field. Measurements in scanned as well as in stationary electron and photon beams at energies of 10, 20 and 50 MeV are presented. Finally, the consequences of photon generated electrons in the very broad high energy photon beams that can be produced by a scanning system are illustrated and discussed.

Production of 123I by photonuclear reactions on Xenon

Abstract Carrierfree 123 I has been produced with a 40 MeV electron beam by the photonuclear reaction 124 Xe (γ, n ) 123 Xe, which decays to 123 I. Condensed natural Xenon has been used as a target. The yield of 123 I is 22 MBq (0.6 mCi) per g 124 Xe using a liquid target of natural Xenon and a mean electron current of 100 μA during an irradiation time of 2 h. The produced 123 I activity has been used for labelling of human serum albumin and the radiochemical yield is 4–40%.

MRI of the brain and thorax during extracorporeal membrane oxygenation: preliminary report from a pig model.

Early diagnosis of cerebral hypoxic ischemic complications during extracorporeal membrane oxygenation (ECMO) is important to guide further treatment. However, diagnostic methods available during ECMO are limited, especially in adults and older children. Magnetic resonance imaging (MRI) is a sensitive and noninvasive method for assessment of vessel patency and brain parenchymal changes, and for measurement of brain perfusion. The use of MRI during ECMO has, to our knowledge, never been reported. We report the first animal experiment with MRI examination during ECMO. After a preliminary test with the mobile ECMO system in the MRI environment, a healthy pig was put on venoarterial ECMO, transported to the MRI department, and examined with sequences for anatomy and function of the brain and thorax. The results showed that the ECMO system was not adversely affected by the magnetic field at a distance from the camera where positioning and examination of the animal was possible. High-quality anatomical and functional images of the brain, heart, and thoracic vessels were acquired. The results suggest that MRI may be used for early diagnosis of cranial complications in patients on ECMO. MRI may also provide a useful tool for further research on flow dynamics and brain perfusion during ECMO.

Production of 11C by photonuclear reactions

Abstract 11 C was produced from activated charcoal in an oxygen environment, by the 12 C (γ,n) 11 C reaction, using the bremsstrahlung of a 50 MeV racetrack mutron. The optimal size and form of the activation vessel were derived by a combination of measurements and calculations. The activity produced consisted of 11 CO (76%) and 11 CO 2 (24%). After passing the gas over CuO heated to 973 K the 11 CO is oxidized to 11 CO 2 and trapped in sodium hydroxide. The maximum 11 C activity trapped after a 40 min irradiation with a mean electron current of 5 μA, was 370 MBq.