Biomedical subjects

Anne L Martel

Publications and source records attributed to Anne L Martel.

7 recordsLinked to original sources

PubMed2007

Improving functional magnetic resonance imaging motor studies through simultaneous electromyography recordings.

Specially designed optoelectronic and data postprocessing methods are described that permit electromyography (EMG) of muscle activity simultaneous with functional MRI (fMRI). Hardware characterization and validation included simultaneous EMG and event-related fMRI in 17 healthy participants during either ankle (n = 12), index finger (n = 3), or wrist (n = 2) contractions cued by visual stimuli. Principal component analysis (PCA) and independent component analysis (ICA) were evaluated for their ability to remove residual fMRI gradient-induced signal contamination in EMG data. Contractions of ankle tibialis anterior and index finger abductor were clearly distinguishable, although observing contractions from the wrist flexors proved more challenging. To demonstrate the potential utility of simultaneous EMG and fMRI, data from the ankle experiments were analyzed using two approaches: 1) assuming contractions coincided precisely with visual cues, and 2) using EMG to time the onset and offset of muscle contraction precisely for each participant. Both methods produced complementary activation maps, although the EMG-guided approach recovered more active brain voxels and revealed activity better in the basal ganglia and cerebellum. Furthermore, numerical simulations confirmed that precise knowledge of behavioral responses, such as those provided by EMG, are much more important for event-related experimental designs compared to block designs. This simultaneous EMG and fMRI methodology has important applications where the amplitude or timing of motor output is impaired, such as after stroke.

Adult↗

PubMed2003

Prevalence of complicated carotid atheroma as detected by magnetic resonance direct thrombus imaging in patients with suspected carotid artery stenosis and previous acute cerebral ischemia.

BACKGROUND: It is recognized that complicated plaque largely accounts for the morbidity and mortality from atherosclerosis. Ideally, investigation of symptomatic and asymptomatic patients would identify atheromatous plaques independently of stenosis. We have previously shown that a magnetic resonance direct thrombus imaging (MRDTI) technique demonstrates complicated atheroma as high signal within the carotid arterial wall. We used this technique to examine the prevalence of complicated carotid plaque in vivo in the ipsilateral arteries of recently symptomatic patients with suspected carotid artery stenosis and to compare this with their contralateral arteries and with those of healthy age- and sex-matched controls. METHODS AND RESULTS: The carotid arteries of 120 patients with suspected severe carotid artery stenosis and previous acute cerebral ischemia were imaged using MRDTI, as were 28 control arteries. High signal was not seen in any control artery. However, there was a 60% prevalence of high signal, suggestive of complicated plaque in the patients' ipsilateral arteries. The prevalence of high signal was significantly greater in the patients' ipsilateral vessels compared with the contralateral, asymptomatic side (60% versus 36%, chi2 P<0.001), particularly for vessels of only moderate stenosis. CONCLUSIONS: MRDTI high signal suggestive of complicated plaque is prevalent in the ipsilateral carotid arteries of patients with carotid stenosis and recent cerebral ischemic events. MRDTI has a potential role in identifying "at risk" plaque, studying atherogenesis and the effects of plaque-modifying strategies.

Brain Ischemia↗

PubMed2003

Characterization of complicated carotid plaque with magnetic resonance direct thrombus imaging in patients with cerebral ischemia.

BACKGROUND: Thromboembolic disease secondary to complicated carotid atherosclerotic plaque is a major cause of cerebral ischemia. Clinical management relies on the detection of significant (>70%) carotid stenosis. A large proportion of patients suffer irreversible cerebral ischemia as a result of lesser degrees of stenosis. Diagnostic techniques that can identify nonstenotic high-risk plaque would therefore be beneficial. High-risk plaque is defined histologically if it contains hemorrhage/thrombus. Magnetic resonance direct thrombus imaging (MRDTI) is capable of detecting methemoglobin within intraplaque hemorrhage. We assessed this as a marker of complicated plaque and compared its accuracy with histological examination of surgical endarterectomy specimens. METHODS AND RESULTS: Sixty-three patients underwent successful MRDTI and endarterectomy with histological examination. Of these, 44 were histologically defined as complicated (type VI plaque). MRDTI demonstrated 3 false-positive and 7 false-negative results, giving a sensitivity and specificity of 84%, negative predictive value of 70%, and positive predictive value of 93%. The interobserver (kappa=0.75) and intraobserver (kappa=0.9) agreement for reading MRDTI scans was good. CONCLUSIONS: MRDTI of the carotid vessels in patients with cerebral ischemia is an accurate means of identifying histologically confirmed complicated plaque. The high contrast generated by short T1 species within the plaque allows for ease of interpretation, making this technique highly applicable in the research and clinical setting for the investigation of carotid atherosclerotic disease.

Aged↗

PubMed2003

Deep venous thrombosis: diagnosis by using venous enhanced subtracted peak arterial MR venography versus conventional venography.

PURPOSE: To assess diagnostic accuracy and interobserver variability at venous enhanced subtracted peak arterial (VESPA) magnetic resonance (MR) venography compared with those at conventional venography for the diagnosis of femoral and iliac deep venous thrombosis (DVT). MATERIALS AND METHODS: A single anteroposterior maximum intensity projection (MIP) venogram of the femoral and iliac veins was constructed by using VESPA MR venography in 55 symptomatic patients suspected of having lower limb DVT. All patients also underwent conventional venography, results of which were used as the standard of reference. VESPA MR venograms were interpreted by two independent reviewers (reviewers A and B) who were unaware of other results. Sensitivity and specificity of VESPA MR venography for the diagnosis of thrombus in the femoral and iliac veins were calculated. Interobserver variability was calculated for these observations by using weighted kappa with equally spaced weights for positive, nondiagnostic, and negative studies. Nondiagnostic studies were reinterpreted separately by reviewer A on the basis of source data. RESULTS: Sensitivity of VESPA MR venography for the femoral veins (20 of 20) and iliac veins (seven of seven) was 100% for both reviewers. Specificity was 100% (39 of 39 for reviewer A, 40 of 40 for reviewer B) for the iliac veins and 97% (31 of 32) for the femoral veins for both reviewers. Segments in which the VESPA MR venograms were nondiagnostic were excluded from this analysis. Interobserver variability as calculated by using weighted kappa for positive, negative, and nondiagnostic studies was 0.85 for femoral veins and 0.97 for iliac veins. Interpretation of the source data led to correct diagnosis in six of six cases in which the VESPA MR venograms were nondiagnostic. CONCLUSION: VESPA MR venography yielded MIP venograms that were highly accurate for the diagnosis of DVT in femoral and iliac veins. Interpretation of the studies was also highly reproducible.

Adult↗

PubMed2003

Separating arterial and venous components from 3D dynamic contrast-enhanced MRI studies using factor analysis.

Dynamic contrast-enhanced MRI has been used extensively for angiography but in order to generate separate arterial and venous images some form of postprocessing is required. This typically involves the subtraction of one image in a dynamic sequence from another in order to suppress unwanted signal; however, this also has the effect of decreasing the signal-to-noise ratio (SNR) of the image. In this study, factor analysis, a technique related to eigenimage filtering, is used to separate arterial and venous components from dynamic contrast-enhanced images of the legs acquired with a temporal resolution of 30 sec. The SNR of the venous and arterial images extracted from a series of 20 patients using conventional single subtraction, a double subtraction method, and factor analysis were compared. Results show that the use of factor analysis improved the SNR in the venous images by a factor of 2.3 compared with the use of simple subtraction. A subjective comparison of the maximum intensity projection images generated from the venous images was also carried out and showed a significant preference for those generated using factor analysis over those generated using other subtraction methods.

Contrast Media↗

PubMed2002

Diagnosis of lower-limb deep venous thrombosis: a prospective blinded study of magnetic resonance direct thrombus imaging.

BACKGROUND: Current magnetic resonance techniques generate high signal from venous blood and show thrombi as filling defects. Magnetic resonance direct thrombus imaging (MRDTI) directly visualizes acute thrombus. OBJECTIVE: To determine the accuracy of MRDTI for diagnosis of acute symptomatic deep venous thrombosis (DVT) below and above the knee. DESIGN: Prospective, blinded study. SETTING: A 1355-bed university hospital. PATIENTS: 101 patients with suspected DVT who had had routine venography. Participants were recruited from a cohort of patients with suspected DVT. All patients with a positive venogram and one quarter of patients with a negative venogram were selected by using a random sequence. INTERVENTION: MRDTI was performed within 48 hours of venography and was interpreted by two reviewers. MEASUREMENTS: Diagnosis of DVT overall; isolated calf, femoropopliteal, and ileofemoral DVT; and thrombus in the calf, femoropopliteal, and iliac segments. RESULTS: The reports from two readers had sensitivities of 96% and 94% and specificities of 90% and 92% for diagnosis of DVT. Sensitivities were 92% and 83% for isolated calf DVT, 97% and 97% for femoropopliteal DVT, and 100% and 100% for ileofemoral DVT. Specificities were 94% and 96% for isolated calf DVT and 100% and 100% for both femoropopliteal and ileofemoral DVT. Similarly, sensitivity and specificity within each of the venous segments ranged from 91% to 100%. Interobserver variability measured by using a weighted kappa statistic ranged from 0.89 to 0.98 for these measures. CONCLUSION: Magnetic resonance direct thrombus imaging is an accurate noninvasive test for diagnosis of DVT, and its accuracy is maintained below the knee. Comparison of individual venous segments showed that results of MRDTI agreed strongly with findings on venography. Scanning was well tolerated, and interpretation was highly reproducible.

Adult↗

PubMed2002

The relationship between proximal pole blood flow and the subjective assessment of increased density of the proximal pole in acute scaphoid fractures.

The cause and importance of increased density of the proximal pole of the scaphoid seen in radiographs after an acute fracture were investigated. Blood flow to the proximal poles of 32 nonsurgically treated scaphoid fractures were assessed 1 to 2 weeks after injury by measuring their uptake of gadopentetate dimeglumine with dynamic magnetic resonance imaging and calculating its steepest slope and percentage of enhancement values. Initial (week 0) and week 8 scaphoid series radiographs were independently assessed by 2 observers for increased proximal pole density; both agreed that the density had increased during treatment in 9 fractures. All 9 fractures united, suggesting that increased proximal pole density is not a major determinant of fracture outcome. Increased density of the proximal pole was associated with low proximal pole blood flow, but this relationship was not absolute, suggesting that other factors contribute to the appearance of increased density.

Adolescent↗