Biomedical subjects

J H Maki

Publications and source records attributed to J H Maki.

15 recordsLinked to original sources

PubMed1998

Optimizing three-dimensional gadolinium-enhanced magnetic resonance angiography. Original investigation.

RATIONALE AND OBJECTIVES: This primarily theoretical work examines three-dimensional gadolinium-enhanced magnetic resonance angiography f8p4Gd-MRA) with the goal of understanding how to achieve the best possible images with respect to signal to noise ratio (SNR) and k-space induced artifacts. Patient variables, contrast injection schemes, and pulse sequence parameters are considered for this purpose. METHODS: A theoretical analysis, including computer simulation, describes how contrast material injection profiles influence 3D Gd-MRA images, both in terms of intravascular signal and resultant artifacts. Further theoretical analysis of the spoiled gradient refocused pulse sequence describes how to maximize SNR. Clinical imaging complements computer modeling. RESULTS: Equations were derived relating contrast injection parameters and pulse sequence variables to SNR and artifacts. For present imaging equipment, administering contrast material over a duration of 60% to 80% of the total imaging time and using fractional echo techniques gives the best SNR without significantly sacrificing image quality. CONCLUSIONS: Three-dimensional Gd-MRA can be tailored to a specific clinical situation and imaging system through the use of proper breath-holding, bolus timing, Gd administration, and pulse sequence design.

Artifacts↗

PubMed1998

Three-dimensional contrast-enhanced MR angiography of the extracranial carotid arteries: two techniques.

Two methods of contrast-enhanced dynamic 3-D MR angiography of the head and neck are demonstrated. In the first, arterial and arteriovenous phases are temporally resolved by timing contrast injection such that maximum arteriovenous signal intensity difference is matched to acquisition of central k-space. A second, faster sequence allows for temporal resolution of arterial, early arteriovenous, late arteriovenous, and washout arteriovenous phases without a need for precise injection timing.

Carotid Arteries↗

PubMed1997

Contrast-enhanced abdominal MR angiography: optimization of imaging delay time by automating the detection of contrast material arrival in the aorta.

PURPOSE: To improve gadolinium-enhanced magnetic resonance (MR) angiogram quality by automatically synchronizing acquisition of central k-space image data with the arterial phase of contrast material bolus infusion. MATERIALS AND METHODS: A spin-echo pulse sequence with orthogonal 90 degrees and 180 degrees pulses was used to monitor signal in a single 4 x 4 x 12-cm voxel that encompassed a segment of aorta. An increase in signal that corresponded to the arrival of gadolinium was used to trigger three-dimensional, spoiled gradient-echo abdominal MR angiography in 50 adult patients. RESULTS: Arterial signal intensity increased 28-fold with automatic compared to 19-fold with manual triggering (P < .05) at an approximate dose of 0.3 mmol/kg. Automatic triggering with a lower dose (approximately 0.2 mmol/kg) resulted in 20-fold arterial enhancement, which is comparable with enhancement after manual triggering at the high dose. In addition, venous enhancement was less (1.5-fold) with automatic than with manual (3.5-fold) triggering at the same dose (P < .05). CONCLUSION: Automatic triggering results in improved arterial-tovenous contrast. It increases arterial enhancement or enables MR angiograms to be obtained with less contrast material. The authors now routinely use this technique for aortorenal imaging with a gadolinium-based contrast material dose of 20 mmol (40 mL) in patients who weigh more than 50 kg and 10 mmol (20 mL) in patients who weigh less than 50 kg.

Abdomen↗

PubMed1996

Three-dimensional contrast-enhanced MR angiography.

Three-dimensional contrast-enhanced magnetic resonance (MR) angiography is a relatively new technique that uses the transient shortening in blood T1 following the intravenous injection of gadolinium chelates to image blood vessels irrespective of flow. For many applications, 3D contrast-enhanced MR angiography is developing into a safe, fast, and cost-effective alternative to conventional diagnostic angiography. One of its biggest advantages over other MR angiography techniques (and CT angiography) is the ability to image in a plane parallel to the vessel of interest. This feature, combined with the inherent properties of a 3D gradient refocused sequence, make 3D contrast-enhanced MR angiography intrinsically fast, high resolution and free from saturation and turbulence-related artifacts. This article is designed to familiarize the reader with the theory of 3D contrast-enhanced MR angiography and the application of the technique to different vascular territories. Contrast agents, relaxation effects, contrast bolus effects, pulse sequences, artifacts, and post-processing, as well as the present state of thinking with regard to optimal contrast injection timing/detection and Fourier space mapping are discussed. Patient preparation and techniques and imaging parameters for body applications of gadolinium-enhanced MR angiography, including aorta, renal arteries, mesenteric arteries, portal venous system, pelvis and legs, pulmonary arteries, and carotid arteries are included.

Fourier Analysis↗

PubMed1993

An incarcerated peristomal gastric hernia causing gastric outlet obstruction.

A 77-year-old woman had had several 3-day episodes of nausea and vomiting for 3 years. Upper endoscopy led to findings consistent with pyloric outlet obstruction, and barium studies demonstrated an incarcerated peristomal gastric hernia. This is taken to be the third description of this diagnosis. We discuss the pathophysiology of gastric hernias.

Aged↗

PubMed1991

The use of gradient flow compensation to separate diffusion and microcirculatory flow in MRI.

This paper describes a new MR imaging technique termed Modified Stejskal Tanner versus Flow Compensation (MST/FC) for the separation of diffusion and microcirculatory flow. The theory behind the sequence is explained, along with a five-component model of microcirculation applicable to any "perfusion" imaging technique. Phantom data is presented showing that (1) diffusion effects can be matched between MST and FC (suggesting the possibility of flow-compensated diffusion imaging), and (2) the technique is a quantitative method of separating diffusion and slow (less than 0.25 mm/s) tortuous flow through a Sephadex column. Furthermore, animal images show the technique to be feasible and quantitative in measuring rat brain microcirculation under normal, vasodilated (hypercarbia), and no-flow (post mortem) conditions.

Acetone↗

PubMed1991

Diffusion/microcirculation MRI in the rat brain.

The CO2 fraction of an anesthetized rat's breathing mixture was changed (from 0 to 10%) to attempt to change the brain microcirculation and observe these changes in diffusion measurements of the neural tissue. Brain apparent diffusion coefficients were measured to be (0.71 +/- 0.01) X 10(-3) mm2/s before sacrifice and (0.39 +/- 0.01) X 10(-3) mm2/s after sacrifice. Multiple diffusion components were observed, consistent with flowing material, but the extra components did not increase with increased CO2. It is proposed that the additional components may be due to extracellular, extravascular water such as CSF.

Animals↗

PubMed1991

Pre- and postmortem diffusion coefficients in rat neural and muscle tissues.

Pulsed gradient diffusion-weighted spin-echo images (7 to 11 gradient strengths) were obtained in a coronal slice through the midbrain for five normal adult white rats before and after sacrifice in a 2-T CSI system with air temperature control. The pulse sequence was cardiac gated and respiratory synchronized in order to minimize motion artifacts (Tr greater than 2 s. Te = 30 ms). Diffusion coefficients reflecting several tissue compartments (D*) in brain and muscle were calculated and referenced to simultaneously imaged tubes of water. In the living animals, brain cortical matter had a value of D* = (0.82 +/- 0.02) x 10(-3) mm2/s. deeper brain regions had a value of D* = (0.73 +/- 0.02) x 10(-3) mm2/s, and the muscle had a value of D* = (1.4 +/- 0.1) x 10(-3) mm2/s. Postmortem the values in brain dropped by approximately 30%, while remaining constant in muscle. Signal intensity in the spin-echo images for muscle tissue rose by 50% over a 1- to 2-h interval after sacrifice while that of brain tissue remained relatively stable.

Animals↗

PubMed1991

In vivo measurement of proton diffusion in the presence of coherent motion.

Measurement of the self-diffusion coefficient D of water in tissue has been performed traditionally using the technique proposed by Stejskal and Tanner. A variant of that technique is shown here, employing flow-compensated gradients that significantly reduce the sensitivity to small coherent motions that are common in body imaging. An interleaved sequence with four values of diffusion-sensitizing gradient (b) minimizes registration errors. Eddy currents and other systematic errors are reduced, permitting the measurement of standards in an imaging context within 5% of nonimaging values in the literature. The flow-compensated sequence permits the measure of D for tissues in the abdominal cavity of the rat. We present in vivo measurements of D for the following rat tissues; liver, kidney (cortex), kidney (medulla) muscle, brain, fat.

Acetone↗

PubMedDate not supplied

Maximization of contrast-to-noise ratio to distinguish diffusion and microcirculatory flow.

Optimization of the contrast-to-noise ratio (CNR) is described for microcirculation magnetic resonance (MR) imaging techniques based on flow-compensated/flow-dephased sequences, both with and without even-echo rephasing. The authors present the most advantageous manner of applying flow-dephased gradients, such that dephasing is maximal while diffusion losses are minimal. The theoretical considerations include phase, diffusion, echo time, and bandwidth in the determination of the optimal parameters for microcirculation imaging. Studies in phantoms consisting of stationary and flowing copper sulfate in Sephadex columns demonstrate the validity of the calculations. Optimized in vivo images of a rat stroke model demonstrate the potential of the flow-compensated/flow-dephased technique and the importance of optimizing CNR.

Animals↗

PubMedDate not supplied

MR imaging of microcirculation in rat brain: correlation with carbon dioxide-induced changes in blood flow.

Considerable interest has been shown in developing a magnetic resonance (MR) imaging technique with quantitative capability in the evaluation of tissue microcirculation ("perfusion"). In the present study, the flow-dephased/flow-compensated (FD/FC) technique is evaluated for measuring rat cerebral blood flow (CBF) under nearly optimal laboratory conditions. Imaging was performed on a 2.0-T system equipped with shielded gradient coils. Rat CBF was varied by manipulating arterial carbon dioxide pressure (PaCO2). In parallel experiments, optimized MR imaging studies (seven rats) were compared with laser Doppler flowmetry (LDF) studies (nine rats). LDF values showed a high degree of correlation between CBF and PaCO2, agreeing with results in the literature. MR imaging values, while correlating with PaCO2, showed considerable scatter. The most likely explanation is unavoidable rat motion during the requisite long imaging times. Because of this motion sensitivity, the FD/FC technique cannot provide a quantitative measure of CBF. It can, however, provide a qualitative picture.

Animals↗

PubMedDate not supplied

The effects of time varying intravascular signal intensity and k-space acquisition order on three-dimensional MR angiography image quality.

The optimum infusion timing and k-space ordering for obtaining gadolinium-enhanced three-dimensional MR angiograms was determined through computer modeling using temporal contrast characteristics obtained from patient gadolinium infusion data. The effects of bolus timing were evaluated by varying the relationship between peak intravascular gadolinium concentration and the time at which the center of k space was acquired (tck) for sequential and centric acquisition techniques. Flow phantom experiments were performed to validate the theoretical computations. Gadolinium concentration at the time of central k-space acquisition determines intravascular signal intensity. Artifacts, including vessel broadening and edge ringing, depend on the order in which k space is collected and on how rapidly the gadolinium concentration changes. Artifacts are greatest when the center of k space is acquired before the intravascular gadolinium peak. Application of the optimal infusion timing results in preferential arterial enhancement with a minimum of artifacts in patients undergoing MR angiography.

Algorithms↗

PubMedDate not supplied

MR angiography with an ultrasmall superparamagnetic iron oxide blood pool agent.

The purpose of the study was to investigate the use of a dextran-coated ultrasmall superparamagnetic iron oxide (USPIO) as a blood pool contrast agent for thoracic and abdominal MR angiography. Abdominal and thoracic MR angiography was performed in six healthy volunteers using two-dimensional and three-dimensional spoiled gradient echo (SPGR) sequences before and after intravenous administration of USPIO. Doses ranged from 1.1 to 2.6 mg Fe/kg. Flip angle was varied from 20 to 60 degrees. Subjective image quality, analysis of signal-to-noise ratio (SNR), and blood T1 relaxation times were measured. USPIO significantly lowered the T1 of blood (from 1,210 ms precontrast to 159 ms postcontrast at a dose of 2.6 mg Fe/kg) (P < .01). Image quality on coronal fast three-dimensional breath-hold SPGR images of the abdomen increased with increasing dose and was maximum at the highest dose, producing an aortic SNR of 9.6 compared to 1.8 precontrast. Axial two-dimensional time-of-flight (TOF) aortic SNR was reduced significantly from 13 on precontrast to 6 on the postcontrast images at the highest dose (P < .05) due to T2* shortening effects. There was little flip angle dependence on image quality. Due to the T1 shortening effect and long intravascular half-life, USPIO improved visualization of vascular anatomy using three-dimensional fast SPGR imaging. The echo time must be minimized to minimize signal loss from T2* shortening effects. The blood pool distribution of USPIO is useful for equilibrium-phase MR angiography.

Abdomen↗

PubMedDate not supplied

The effects of incomplete breath-holding on 3D MR image quality.

The purpose of this study was to investigate how fast three-dimensional (3D) MR image quality is affected by breath-holding and to develop an optimal breath-holding strategy that minimizes artifact in the event of an incomplete breath-hold. A computer model was developed to study variable-duration breath-holds during fast 3D imaging. Modeling was validated by 3D gradient-echo imaging performed on 10 volunteers. Signal-to-noise ratio (SNR) and image blur were measured for both simulated and clinical images. Insights gained were applied to clinical 3D gadolinium-enhanced MR angiography. Breath-holding significantly improved abdominal 3D MR image quality. Most of this benefit could be achieved with a breath-hold fraction of 50% if it occurred during acquisition of central k space. Breath-holding during peripheral k-space acquisition, however, had no significant benefit. Respiratory motion artifact on fast 3D MRI occurring when a patient fails to suspend respiration for the entire scan duration can be minimized by collecting central k space first (centric acquisition) so that premature breathing affects only the acquisition of peripheral k space.

Adult↗

PubMedDate not supplied

Contrast-enhanced MR angiography.

Gadolinium-enhanced MRA in the abdomen provides a safe, fast, and cost-efficient alternative to conventional diagnostic angiography. Numerous recent advances allow for high-resolution breath-hold imaging with optimal arterial-phase gadolinium bolus timing. As the technique is refined further, greater spatial and temporal resolution will become possible, thus increasing the usefulness of Gd-MRA.

Abdomen↗