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Biomedical subjects

A L Martel

Publications and source records attributed to A L Martel.

12 recordsLinked to original sources
PubMed2003

Differences in the diagnostic accuracy of acute stroke clinical subtypes defined by multimodal magnetic resonance imaging.

BACKGROUND: Despite its importance for acute stroke management, little is known about the underlying pathophysiology when patients with acute stroke are classified using clinical methods. OBJECTIVE: To examine the relation between the magnetic resonance defined stroke subtype and clinical stroke classifications using diffusion weighted imaging (DWI), perfusion weighted imaging (PWI), and angiographic magnetic resonance techniques. METHODS: Consecutive patients with clinical syndromes consistent with acute anterior circulation stroke were assessed clinically within six hours of onset and scanned as soon as possible using multimodal magnetic resonance imaging (MRI). Patients were classified clinically into total or partial anterior circulation syndromes using the Oxford classification, or according the severity of the National Institutes of Health stroke scale (NIHSS) (severe > 15; mild/moderate 15). There were 42 with partial anterior circulation syndromes (PACS) and 42 with total anterior circulation syndromes (TACS). Patients with TACS or severe stroke were more likely to have actually suffered a stroke (Fischer's exact test, p = 0.01), to have a correctly classified stroke (chi(2) 28.2, p < 0.01), to have persisting occlusion (chi(2) 30.6, p < 0.01), and to have a large DWI-PWI mismatch (chi(2) 17.1, p < 0.01). CONCLUSIONS: There is more inaccuracy in patients presenting with acute PACS or clinically mild to moderate anterior circulation stroke than in those with TACS or severe acute stroke syndromes. The latter appear more likely to be the targets for acute stroke interventions, as they include a significantly higher proportion of patients with persisting occlusion and diffusion/perfusion mismatch.

Acute Disease↗
PubMed2002

Segmentation of MR images for computer-assisted surgery of the lumbar spine.

This paper describes a segmentation algorithm designed to separate bone from soft tissue in magnetic resonance (MR) images developed for computer-assisted surgery of the spine. The algorithm was applied to MR images of the spine of healthy volunteers. Registration experiments were carried out on a physical model of a spine generated from computed tomography (CT) data of a surgical patient. Segmented CT, manually segmented MR and MR images segmented using the developed algorithm were compared. The algorithm performed well at segmenting bone from soft tissue on images taken of healthy volunteers. Registration experiments showed similar results between the CT and MR data. The MR data, which were manually segmented, performed worse on visual verification experiments than both the CT and semi-automatic segmented data. The algorithm developed performs well at segmenting bone from soft tissue in MR images of the spine as measured using registration experiments.

Algorithms↗
PubMed2001

Extracting parametric images from dynamic contrast-enhanced MRI studies of the brain using factor analysis.

Factor analysis of dynamic studies (FADS) is a technique that allows structures with different temporal characteristics to be extracted from dynamic contrast enhanced studies without making any a priori assumptions about physiology. These dynamic structures may correspond to different tissue types or different organs or they may simply be a useful way of characterising the data. This paper describes a method of automatically extracting factor images and curves from contrast enhanced MRI studies of the brain. This method has been applied to 107 studies carried out on patients with acute stroke. The results show that FADS is able to extract factor curves correlated to arterial and venous signal intensity curves and that the corresponding factor images allow a distinction to be made between areas of the brain with normal and abnormal perfusion. The method is robust and can be applied routinely to dynamic studies of the brain. The constraints described are sufficiently general to be applicable to other dynamic MRI contrast enhanced studies where an increase in contrast concentration produces an increase in signal intensity.

Cerebrovascular Circulation↗
PubMed2001

A 3D MRI sequence for computer assisted surgery of the lumbar spine.

The aim of this research was to develop a magnetic resonance (MR) sequence capable of producing images suitable for use with computer assisted surgery (CAS) of the lumbar spine. These images needed good tissue contrast between bone and soft tissue to allow for image segmentation and generation of a 3D-surface model of the bone for surface registration. A 3D double echo fast gradient echo sequence was designed. Images were filtered for noise and non-uniformity and combined into a single data set. Registration experiments were carried out to directly compare segmentation of MR and computed tomography (CT) images using a physical model of a spine. These experiments showed the MR data produced adequate surface registration in 90% of the experiments compared to 100% with CT data. The MR images acquired using the sequence and processing described in this article are suitable to be used with CAS of the spine.

Humans↗
PubMed2001

Perfusion MRI of infarcted and noninfarcted brain tissue in stroke: a comparison of conventional hemodynamic imaging and factor analysis of dynamic studies.

RATIONALE AND OBJECTIVES: To investigate the hemodynamics of infarcted and noninfarcted regions of the brain in patients with stroke secondary to a complete middle cerebral artery occlusion. Also, to compare factor analysis, a novel method of analyzing perfusion-weighted images, with more conventional techniques. METHODS: Twenty-two patients with complete unilateral occlusion of the middle cerebral artery were examined by T1-weighted, contrast-enhanced, perfusion-weighted imaging, diffusion-weighted imaging, and magnetic resonance angiography. Quantitative cerebral blood volume (CBV), cerebral blood flow (CBF), and time-to-peak-intensity (TTP) images were generated. Factor analysis of dynamic studies (FADS) was used to generate "early" and "late" images. The hemodynamic parameters for the infarcted and noninfarcted regions of the occluded territory were compared with those for the brain territory on the nonoccluded side. RESULTS: Three regions were shown: (1) Normal tissue on the unaffected side; (2) an infarcted region, which was characterized by reduced CBV, CBF, and early FADS values with increased TTP values; and (3) a noninfarcted region with reduced early FADS and increased late FADS and TTP values compared with the normal region. Cerebral blood volume and CBF values were not reduced significantly in the noninfarcted region. CONCLUSIONS: The differences in parameters such as TTP, CBV, and CBF are significant, and it is necessary to use more than one parameter when interpreting magnetic resonance imaging perfusion data. Factor analysis of dynamic studies provides additional information to conventional methods of analyzing perfusion data.

Brain↗
PubMed2001

Scaphoid blood flow and acute fracture healing. A dynamic MRI study with enhancement with gadolinium.

We have investigated whether assessment of blood flow to the proximal scaphoid can be used to predict nonunion in acute fractures of the scaphoid. We studied 32 fractures of the scaphoid one to two weeks after injury, by dynamic fat-suppressed T1-weighted gradient-echo MRI after the intravenous administration of gadopentetate dimeglumine (0.1 mmol/kg body-weight). Steepest slope values (SSV) and percentage enhancement values (%E) were calculated for the distal and proximal fragments and poles. All the fractures were treated by immobilisation in a cast, and union was assessed by CT at 12 weeks. Nonunion occurred in four fractures (12%), and there was no statistically significant difference between the proximal fragment SSV and %E values for the fractures which united and those with nonunion. The difference between the proximal pole SSV and %E values for the union and nonunion groups reached statistical significance (p < 0.05), but with higher enhancement parameters for the nonunion group. Our results suggest that poor proximal vascularity is not an important determinant of union in fractures of the scaphoid.

Adolescent↗
PubMed1999

Limitations of clinical diagnosis in acute stroke.

Trials in acute stroke have recruited on the basis of clinical diagnosis. Using MRI we have shown that clinical diagnosis is more limited than previously appreciated, thus trials may have been underpowered or confounded.

Clinical Trials as Topic↗
PubMed1999

The use of image registration in the analysis of bone scans for the assessment of temporomandibular joint uptake.

Bone scanning is a commonly used technique in the assessment of patients with facial asymmetry. Uptake in the temporomandibular joints can be assessed quantitatively, either as a right-to-left ratio or as the uptake in the joint relative to some other bony structure. We used image registration techniques in the development of an automatic method of quantitative analysis and compared this with a manual region of interest method. Both image warping and region warping were studied. Normal ranges were established in a series of 25 subjects with no evidence of temporomandibular disorder. The results of this study indicate that, by using image registration and standard regions of interest, the results from normal subjects show a smaller standard deviation and improved correlation between observers when compared with manual analysis.

Adolescent↗
PubMed1999

Magnetic resonance imaging virtual endoscopy of the labyrinth.

OBJECTIVE: To explore the feasibility of applying virtual endoscopic (VE) techniques to examine the normal anatomy of the labyrinth using magnetic resonance imaging (MRI) data. STUDY DESIGN: Feasibility study using data from a normal subject. MATERIALS AND METHODS: MRI data was acquired in a single normal subject using a three-dimensional (3D) CISS (Constructive Interference in the Steady State) sequence. Perspective volume rendering (PVR) techniques were used to produce virtual endoscopic visualization of the fluid filled structures of the inner ear. RESULTS: The reconstructive algorithms enabled generation of a "fly-through" simulation of the labyrinth. The cochlea, the oval window, the vestibule, the common crus, and the semicircular canals were successfully visualized. CONCLUSIONS: Virtual endoscopic techniques were successfully applied to MRI data enabling a 3D virtual display of the internal anatomy of the normal labyrinth.

Algorithms↗
PubMed1998

Assessment of 3-dimensional magnetic resonance imaging fast low angle shot images for computer assisted spinal surgery.

The objective of this research was to determine whether a fast 3-dimensional (3-D) gradient echo magnetic resonance imaging (MRI) sequence could be used to acquire images suitable for image guided surgery of the spine. The main difficulty with MRI is that inhomogeneities in the static magnetic field lead to geometric distortions in the images. We used a very fast 3-D MRI sequence with a wide bandwidth and short echo time (TE) to minimize these distortions. Fiducial markers that could be localized in MRI and computed tomography (CT) images and in physical space were attached to a phantom in order to assess the accuracy of a landmark based registration method. The effect of varying the MRI parameters on image contrast was also investigated. The results demonstrate that the registration can be undertaken with an accuracy of 0.4 mm using the 3-D MRI. This is comparable to the accuracy of 0.3 mm obtained with CT and is a significant improvement over the accuracy of the 2-D MRI techniques (> 1.0 mm). In vivo images demonstrating good contrast between the spine and surrounding soft tissues such as fat, intervertebral disks, and cerebrospinal fluid were obtained. The MRI acquired using the sequence described in this article shows promise for use in computer assisted surgery of the spine.

Algorithms↗
PubMed1994

Background subtraction in 99Tcm-DTPA renography using multiple background regions: a comparison of methods.

In order to obtain quantitative information from a 99Tcm-diethylenetriaminepentaacetate (DTPA) renogram it is first necessary to correct for the presence of background activity in the kidney time-activity curve. Three different methods of background subtraction which have been previously described are compared. Method 1 is a linear regression technique which uses separate tissue and heart time-activity curves to represent the background. Method 2 also uses multiple background regions and involves multiple regression. Method 3 uses a single heart region to represent the background. The three techniques are assessed using both simulation studies and patient data. The results show that method 3, which does not correct for extravascular activity, overestimates renal function particularly in poorly functioning kidneys. Method 1 gives the most accurate and reproducible results for both the simulation studies and patient data.

Computer Simulation↗