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Biomedical subjects

T H Lim

Publications and source records attributed to T H Lim.

At least 109 records · Page 6Linked to original sources
PubMed1991

Noninvasive determination of left ventricular output and wall stress in volume overload and in myocardial disease by cine magnetic resonance imaging.

The current study used cine magnetic resonance imaging to determine the effect of increasing severity of valvular regurgitation on systolic wall stress and to demonstrate that wall stress was disproportionately increased in relation to the severity of regurgitation in patients with myocardial disease. A total of 39 patients with predominantly mitral (n = 22) or aortic (n = 17) regurgitation with (n = 13) and without (n = 26) myocardial disease and 10 normal volunteers were examined with cine magnetic resonance imaging (MRI) at 1.5 T. Left ventricular (LV) cardiac output (CO) and peak systolic (PS) wall stress (WS) and end-systolic (ES) WS were calculated from blood pressure recordings, carotid pulse tracings, and wall thickness (h) and diameter (D) measurements obtained from cine MRI. Patients were classified into three degrees of severity according to their LV regurgitant volume (RV). Myocardial disease was defined by an ejection fraction (EF) of less than 40%. Mean LV EF was 61 +/- 3% in normal volunteers, 64 +/- 3% in patients with regurgitation, and 25 +/- 2% in patients with myocardial disease. LV CO was directly related to RV in patients without myocardial disease, whereas it was disproportionately low in relation to RV in patients with myocardial disease. PS WS was significantly higher in severe mitral and/or aortic regurgitation compared with moderate, mild, and no mitral and/or aortic regurgitation. Compared with the degree of regurgitation, PS WS was disproportionately higher in patients with myocardial disease. Thus LV CO and WS rise progressively with increasing severity of regurgitation. Disproportionately high systolic WS relative to RV indicates the presence of myocardial disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

In vitro comparative biomechanical analysis of transpedicular screw instrumentations in the lumbar region of the human spine.

An analysis of the load-displacement behavior of stabilized spines in comparison with intact spines was undertaken using fresh human cadaveric spines (T12/L1--sacrum). The three-dimensional load-displacement data of the five vertebral bodies of an intact specimen in clinically relevant loading cases were recorded using the Selspot II motion measuring system. After testing the intact specimen, an instability was created at the L4-5 level. The unstable motion segment was stabilized sequentially with three transpedicular screw instrumentations. The stabilized specimens were tested, and the data for the stabilized tests were normalized with respect to the intact data to determine the degree of stabilization achieved in various loading modes as a function of the three devices. The results showed that the three transpedicular devices included in this study were effective in imparting stability to the injured ligamentous spinal segment at a p less than 0.01 level of significance. The differences among the devices were not significant.

Aged↗
PubMed1991

Effect of disc degeneration at one level on the adjacent level in axial mode.

Nonlinear three-dimensional finite element models of a ligamentous two motion segments spine specimen (L3-L4-L5) were developed to investigate the effects of disc degeneration, simulated at the L4-L5 level, on the biomechanical behavior of the adjacent intact L3-L4 motion segment. The disc degeneration was simulated by removing the hydrostatic capabilities of the nucleus and making the L4-L5 disc stiffer than a normal disc. The results of the degenerated model were compared with the predictions for a model in which the L4-L5 disc was left intact. The loads on the facets decreased, and intradiscal pressure in the intact L3-L4 disc increased as a result of disc degeneration compared with the intact model. The predicted increase in the intradiscal pressure and the associated increase in the disc bulge in the posterior region over time may trigger the degenerative process at the L3-L4 motion segment. This is in accordance with the Wolff's law; living tissue responds to chronic changes in stresses and strains. The limitations of the present two motion segments model and the potentials of multisegmental models are discussed.

Cadaver↗
PubMed1991

Effects of rigidity of an internal fixation device. A comprehensive biomechanical investigation.

Internal fixation with instrumentation often accompanies surgical fusion to augment spinal stability, provide temporary fixation while the surgical fusion mass unites, and enhance postoperative mobilization of a patient. Some surgeons, however, feel that the existing plate-screw designs are too rigid and are the primary cause of "iatrogenic" adverse effects clinically observed. A three-part study, involving in vitro experimental protocol, analytical finite-element-based models, and an in vivo canine investigation, was undertaken to study the role of decreasing rigidity of a device on the biomechanical response of the stabilized segments. Two alternatives--the use of one variable screw placement (Steffee plate [unilateral, 1VSP model]) as opposed to two VSP plates (bilateral, 2VSP model) and two VSP plates with polymer washers placed in between the integral nut and plate (2MVSP model)--were considered for achieving a reduction in the rigidity of the conventional VSP system. The load-displacement data obtained from the in vitro experiments and the stress distributions within the stabilized and intact models predicted by the finite-element models revealed that the unilateral VSP system is less rigid and is likely to reduce stress shielding of the vertebral bodies compared with the 2VSP model. The undesirable effects associated with the use of the 1VSP plate system are the presence of coupled motions due to the inherent asymmetry and the likely inability to provide enough rigidity for decompression procedures requiring a complete excision of the disc. The use of two MVSP plates overcomes these deficiencies.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

MR anatomy of ventricular septal defect in double-outlet right ventricle with situs solitus and atrioventricular concordance.

Surgical treatment of patients with double-outlet right ventricle (DORV) depends on the spatial relation between the associated ventricular septal defect (VSD) and the arterial valves. This study was performed to refine the magnetic resonance (MR) imaging characteristics of VSD in DORV. Ten patients with DORV underwent MR imaging. Coronal, transverse, and oblique images were obtained with an electrocardiographic-gated multiple-section spin-echo technique. The transverse plane was the most valuable in defining the spatial relation of the VSD to the arterial valves. The site of fusion of the outlet septum with the VSD margin was diagnostically the most important feature in differentiating subaortic from subpulmonary VSD. The outlet septum fused with the anterior margin in subaortic VSD and the posterior margin in subpulmonary VSD; it was absent in doubly committed VSD. This study suggests that MR imaging can demonstrate the pertinent features of VSD and may be an indispensable adjunct to echocardiography in the preoperative evaluation of patients with DORV.

Angiocardiography↗
PubMed1990

Possible role of stresses in inducing spinal stenosis--a long term complication following disk excision.

A three-dimensional finite element model of an intact ligamentous lumbar motion segment (L3-4) was used to predict stresses in the pars interarticularis regions of the modeled vertebral bodies. The changes in stresses following disk excision, as compared to the intact model, also were computed. The predicted results show an increase in stresses in the posterior bony elements following disk excision. In some patients over a long period of time this increase in stresses, in association with other clinical factors, may lead to bony hypertrophy of the structures that surround the nerve roots. Ultimately, over a long period of time the increase in pressure on the entrapped nerve root may induce recurrent pain and other complications reported in the literature.

Biomechanical Phenomena↗
PubMed1989

Diagnostic accuracy and estimation of the severity of valvular regurgitation from the signal void on cine magnetic resonance images.

Anomalous blood flow caused by valvular regurgitation can be visualized as a narrowly defined area of signal loss in contrast to the high intensity of the blood pool on repetitive gradient refocused magnetic resonance (MRI) (cine MRI) views. In order to assess the accuracy of identification and measurements of the signal void in the evaluation of mitral (MR) and aortic regurgitation (AR), the cine MRI studies of 62 patients with regurgitation and of 20 normal volunteers were retrospectively analyzed in a blinded study of three independent reviewers. Accuracy for detection of MR was greater than 0.96 and for AR was greater than 0.91 for all three reviewers. The total volume of the signal void associated with regurgitation was measured on cine MRI images of 58 patients by two independent observers. The patients were divided into three groups of severity based upon assessment of severity of regurgitation by two-dimensional echocardiography and/or angiography. The total volume of signal loss was 9.0 ml (+/- 0.94 SEE; n = 18) for mild MR, 30.7 ml (+/- 5.3 SEE; n = 11) for moderate MR, and 83.4 ml (+/- 17.4 SEE; n = 5) for severe MR. In patients with AR, the calculated signal void was 27.3 ml (+/- 3.2 SEE; n = 24) in mild, 49.7 ml (+/- 2.9 SEE; n = 7) in moderate, and 75.8 ml (+/- 5.7; n = 6) in severe cases. The differences in the volume of the signal void were significant among the three groups of severity (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Aortic Valve Insufficiency↗
PubMed1989

Current applications of magnetic resonance vascular imaging.

A wide variety of MRI techniques is available for vascular imaging, each exploiting a different property of flowing blood to achieve contrast. These include spin-echo, which has been used for the diagnosis of aortic dissection and of great vessel anomalies, as well as for the evaluation of pulmonary flow in patients with pulmonary hypertension and pulmonary embolism. Spin echo excels in detecting infection and hematoma in the tissues around grafts and vessels. Phase display imaging has proven useful in differentiating signal of slow flow from that of intravascular thrombus. Imaging of peripheral vessels can be achieved with gradient refocused sequences, which provide bright intravascular signal over a wide range of flow velocities. These sequences may be combined with subtraction strategies to eliminate the signal from stationary tissues in order to generate an angiographic image. The advent of three-dimensional MR angiographic imaging techniques provides an effective way to display peripheral vessels. Early experience implies that MR angiography will play an important role in vascular imaging in the future, provided that the signal loss from turbulent flow can be minimized.

Arterial Occlusive Diseases↗
PubMed1988

An analytical investigation of the mechanics of spinal instrumentation.

Three-dimensional nonlinear finite element models of the intact L4-5 one motion segment/two-vertebrae and L3-5 two motion segments/three-vertebrae were developed using computed tomography (CT) films. The finite element mesh of the L4-5 motion segment model was modified to simulate bilateral decompression surgery. The mesh was further altered to achieve stabilization, using an interbody bone graft and a set of Steffee plates and screws. The model behavior of the intact specimen in all loading modes and of the stabilized model in compression, flexion, and extension modes were studied. The stresses in the cancellous bone region were found to decrease. The interbody bone graft, due to an overall decrease in stresses in the bone below the screw, transmits about 80% of the axial load as compared with 96% transmitted by an intact disc in an intact model. Thus, the use of a fixation device induces a stress shielding effect in the vertebral body. The results indicate that although the bone graft transmits lesser loads than the intact disc, it is active in transmitting loads. The presence of low stresses in the cancellous bone region and high localized stresses in the cortical pedicle region surrounding the screw, compared with the intact case, suggests that the screws are likely to become loose over time. The use of an interbody bone graft alone or in combination with any existing fixation device also induces higher stresses at the adjacent levels. This may be responsible for the adverse iatrogenic effects seen clinically.

Biomechanical Phenomena↗
PubMed1983

Kinetics of trace element chromium(III) in the human body.

Chromium(III) has been reported to be an essential trace element involved in the control of glucose metabolism by insulin. We have studied the distribution and kinetics of intravenous [51Cr]chromium(III) in six human subjects using a whole-body scintillation scanner, a whole-body counter, and plasma counting. Principal concentrations were found in the liver, spleen, soft tissue, and bone. The data were fit to a model consisting of a plasma pool in equilibrium with fast (T1/2 = 0.5-12 h), medium (1-14 days), and slow (3-12 mo) compartments, and transfer rates were calculated for exchanges between compartments. Each of the imaged organs appeared to contain varying proportions of each compartment. Two patients with hemochromatosis, in which iron overload is postulated to exclude chromium transport, were found to have altered rate constants. The results should be useful in assessing the metabolism and nutritional requirement of chromium(III) in humans and its importance in disease processes such as diabetes.

Autoradiography↗
PubMed1979

Reduced chromium retention in patients with hemochromatosis, a possible basis of hemochromatotic diabetes.

Chromium (III) has recently been shown to be an essential trace mineral in rats, being required for normal function of insulin in controlling glucose metabolism. Chromium is transported in the body bound to transferrin, where it binds competitively with iron. Hemochromatosis is an iron storage disease in humans characterized by highly saturated transferrin levels and sometimes by diabetes. We postulated that the diabetes may be due to exclusion of chromium by iron at metabolic binding sites. 51Cr(III) was administered i.v. to 5 normal males, 6 patients with hemochromatosis prior to therapeutic removal of iron, and 5 patients with varying levels of iron loading. The retention of 51Cr was measured with a whole-body counter for 8 mo and blood levels were measured for 40--80 days. Analysis of the whole-body retention curves revealed 3 exponential components with T1/2s of .56 days, 12.7 days, and 192 days; the blood curves had 4 components with T1/2s of 13 min; 6.3 hr, 1.9 days, and 8.3 days. The T1/2s were not significantly different between the normals and patients. The coefficients of these components however, were significantly lower for the long T1/2 components in the iron-loaded patients, demonstrating reduced retention of 51Cr as postulated. Whether this reduced retention of chromium is causally related to diabetes in hemochromatosis and whether abnormal chromium metabolism is involved in endogenous diabetes, thus, becomes an important question for future study.

Adult↗
PubMedDate not supplied

Novel application of breath-hold turbo spin-echo T2 MRI for detection of acute myocardial infarction.

To assess the clinical utility of the breath-hold turbo spin-echo T2-weighted MRI in patients with acute myocardial infarction, the results of MRI were compared with those of electrocardiography, coronary angiography, and thallium-201 single photon emission tomography (SPECT) in 23 patients and 5 healthy volunteers. To compare MRI and thallium-SPECT, the left ventricle was divided into five segments, and the presence of myocardial infarction was determined in each segment. MRI demonstrated an abnormally bright signal in 49 of 140 segments (five segments each from 23 patients and 5 volunteers); thallium-SPECT showed a fixed perfusion defect in 52 segments, for an 85% diagnostic concordance rate. The size of the myocardial infarction measured on MRI corresponded well to that measured on thallium-SPECT (r = .70, P < .01). Breath-hold turbo spin-echo T2 MRI can be used for detection of acute myocardial infarction in conjunction with thallium-SPECT, especially when accurate localization of lesion, increased spatial resolution, and anatomic landmarks are needed.

Coronary Angiography↗
PubMedDate not supplied

The relationship between disc degeneration and flexibility of the lumbar spine.

BACKGROUND CONTEXT: A relationship between degenerative changes of the intervertebral disc and biomechanical functions of the lumbar spine has been suggested. However, the exact relationship between the grade of disc degeneration and the flexibility of the motion segment is not known. PURPOSE: To investigate the relationship between degenerative grades of the intervertebral disc and three-dimensional (3-D) biomechanical characteristics of the motion segment under multidirectional loading conditions. STUDY DESIGN/SETTING: A biomechanical and imaging study of human cadaveric spinal motion segments. METHODS: One hundred fourteen lumbar motion segments from T12-L1 to L5-S1 taken from 47 fresh cadaver spines (average age at death, 68 years; range, 39 to 87 years) were used in this study. The severity of degeneration (grades I to V according to Thomson's system) was determined using magnetic resonance (MR) images and cryomicrotome sections. Pure unconstrained moments with dead weights were applied to the motion segments in six load steps. The directions of loading included flexion, extension, right and left axial rotation, and right and left lateral bending. RESULTS: When the MR images were graded, 2 segments had grade I disc degeneration; 45, grade II; 20, grade III; 26, grade IV; and 21, grade V. When the cryomicrotome sections were graded, 14 segments had grade I disc degeneration; 31, grade II; 22, grade III; 26, grade IV; and 21, grade V. Segments from the upper lumbar levels (T12-L1 to L3-4) tended to have greater rotational movement in flexion, extension, and axial rotation with disc degeneration up to grade IV, whereas the motion decreased when the disc degenerated to grade V. In the lower lumbar spine at L4-5 and L5-S1, motion in axial rotation and lateral bending was increased in grade III. CONCLUSIONS: These results suggest that kinematic properties of the lumbar spine are related to disc degeneration. Greater motion generally was found with disc degeneration, particularly in grades III and IV, in which radial tears of the annulus fibrosus are found. Disc space collapse and osteophyte formation as found in grade V resulted in stabilization of the motion segments.

Adult↗
PubMedDate not supplied

An explanation of renal hemodynamics in acute renal failure based on sequential CT in patients with Korean hemorrhagic fever.

The pattern of renal enhancement and washout of contrast medium was observed on sequential follow-up CT in 12 patients with Korean hemorrhagic fever, in which acute renal failure is one of the most important clinical features. Renal contrast enhancement and contrast medium washout were delayed longer in patients with severe oliguric renal failure. The delayed washout peaked at 4-5 days and did not return to normal until 8-9 days in the patients with severe oliguria; in the patients without severe oliguria the times were 1-2 days and 3-4 days, respectively. A characteristic "cart-wheel" pattern was observed during the washout stage in patients without severe oliguria. This "cart-wheel" pattern of washout is thought to result from relief of vasoconstriction and repair of tubular function. Multifocal "wedge-shaped" nonenhanced areas of the kidney, seen on the 2 week follow-up postcontrast CT, are thought to be ischemic zones due to persistent vasoconstriction. On the 6 week follow-up postcontrast CT in one patient, scarring of the kidney was detected in the same area that did not enhance on the 2 week CT. This scarring is thought to be a result of permanent vasoconstriction.

Acute Kidney Injury↗
PubMedDate not supplied

Localized amyloidosis of the respiratory system: CT features.

PURPOSE: Amyloidosis includes a spectrum of diseases associated with an abnormal extracellular deposition of amyloid. The respiratory system is involved in approximately 50% of patients with amyloidosis. The purpose of this review is to present the CT findings of localized amyloidosis of the respiratory system. METHODS: We reviewed the CT findings of localized amyloidosis of the respiratory system from the previous reports and our experiences. RESULTS: Three patterns of involvement in respiratory system are presented: tracheobronchial nodular and diffuse parenchymal. CT demonstrates nodules, plaques, or diffuse thickening of the airways with calcification. Postobstructive collapse is associated in case of diffuse airway involvement. Parenchymal nodules have sharp and lobulated margins with peripheral or subpleural locations. High-resolution CT shows reticular opacities, interlobular septal thickening, and small, well-defined nodules of 2 to 4-mm in diameter predominantly in the subpleural regions. CONCLUSION: Amyloidosis of respiratory system has variable findings and CT scans clearly depict extent and distribution of the disease, its postobstructive pulmonary complication, and other associated findings such as calcification and lymphadenopathy.

Aged↗