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Biomedical subjects

T G Walker

Publications and source records attributed to T G Walker.

At least 19 recordsLinked to original sources
PubMed2006

Limits to the polarization for spin-exchange optical pumping of 3He.

Based on measurements of the temperature dependence of 3He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed 3He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.

Journal Article↗
PubMed2006

Fast ground state manipulation of neutral atoms in microscopic optical traps.

We demonstrate Rabi flopping at MHz rates between ground hyperfine states of neutral 87Rb atoms that are trapped in two micron sized optical traps. Using tightly focused laser beams we demonstrate high fidelity, site specific Rabi rotations with cross talk on neighboring sites separated by 8 microm at the level of 10(-3). Ramsey spectroscopy is used to measure a dephasing time of 870 micros, which is approximately 5000 longer than the time for a pi/2 pulse.

Journal Article↗
PubMed2003

Dense atom clouds in a holographic atom trap.

We demonstrate the production of high-density cold 87Rb samples (2 x 10(14) atoms/cm3) in a simple optical lattice formed with YAG light that is diffracted from a holographic phase plate. A loading protocol is described that results in 10,000 atoms per 10 microm x 10 microm x 100 microm unit cell of the lattice site. Rapid free evaporation leads to a temperature of 50 microK and phase space densities of 1/150 within 50 ms. The resulting small, high-density atomic clouds are very attractive for a number of experiments, including ultracold Rydberg atom physics.

Journal Article↗
PubMed2002

129Xe-Xe molecular spin relaxation.

We identify the formation of bound 129Xe-Xe molecules as the primary fundamental spin-relaxation process at densities below 14 amagat. Low pressure Xe relaxation rate measurements as a function of gas composition show that Xe-Xe molecular relaxation contributes 1/T1 = 1/4.1 h to the total observed relaxation rate. The measured rate is consistent with theoretical estimates deduced from previously measured NMR chemical shifts. At atmospheric pressure the molecular relaxation is more than an order of magnitude stronger than binary relaxation. Confusion of molecular and wall relaxation mechanisms has historically caused wall relaxation rates to be overestimated.

Journal Article↗
PubMed2001

Nuclear polarization of hydrogen molecules from recombination of polarized atoms.

The nuclear polarization of H(2) molecules formed by recombination of polarized H atoms on a Cu surface was measured as a function of external magnetic field and of temperature of the surface. The proton polarization of the molecules was determined by scattering of a longitudinally polarized 203-MeV proton beam in the Indiana University Cyclotron Facility storage ring. The nuclear polarization of the molecules, relative to the polarization of the atoms before recombination, increased from near zero in a weak magnetic field to 0.42 +/- 0.02 in a 0.66 T field. A simple model of the relaxation accounts quantitatively for the observations.

Journal Article↗
PubMed1991

Cigarette smoking: an independent risk factor for atherosclerosis in the hypogastric-cavernous arterial bed of men with arteriogenic impotence.

We investigated the relationship between cigarette smoking and atherosclerosis of the hypogastric-cavernous arterial bed by evaluating arteriograms of young impotent men referred for selective pudendal angiography. Those patients with hemodynamically significant atherosclerosis had smoked more pack-years than had patients without arterial disease. These differences were statistically significant (p less than 0.05) for the common penile artery (32.8 pack-years, 40 patients versus 22.3 pack-years 57 patients) and the dorsal artery (31.3 pack-years, 48 patients versus 22.0 pack-years, 49 patients). The effect of cigarette smoking as an independent risk factor for atherosclerotic disease in the hypogastric-cavernous arterial bed was evaluated as well. When controlled for age, trauma history, hypertension and diabetes, cigarette smoking was independently associated with atherosclerosis in the internal pudendal artery (p less than 0.05). The relative risk (and 95% confidence interval) of developing internal pudendal artery atherosclerosis for each 10 pack-years smoked was 1.31 (1.05 to 1.64). A third analysis investigated the potential interactive effects of cigarette smoking and pelvic or perineal trauma. A significantly higher incidence (p less than 0.05) of cavernous artery atherosclerosis was found among smokers with a history of chronic perineal trauma (33 patients) compared to nonsmokers with a similar history (25 patients). The findings of this study indicate that cigarette smoking is an independent risk factor in the development of atherosclerotic lesions in the internal pudendal and common penile arteries of young impotent men. Cigarette smoking appears to predispose these patients to early atherosclerotic lesions in the cavernous artery following chronic perineal trauma.

Adolescent↗
PubMed1991

Role of lymphangiography in staging testicular seminoma.

Bipedal lymphangiography (LAG) and abdominal/pelvic computerized tomography (CT) are both useful in evaluating retroperitoneal lymph nodes in early-stage testicular seminoma. Fifty-four patients who had both radiologic studies performed between 1982 and 1986 were identified, and their films were reviewed. Four patients had evidence of retroperitoneal lymph node metastases on both CT and LAG. Of the 50 patients who had normal findings on CT scan, 39 (78%) had a normal LAG and 11 (22%) had a positive LAG. All 11 patients had architectural abnormalities within normal-sized nodes. No patient had positive findings on CT with negative LAG. In addition to aiding in staging, LAG is very useful in designing radiation treatment portals. Therefore, we believe that LAG can continue to play an important role in the accurate staging and treatment of patients with testicular seminoma.

Dysgerminoma↗
PubMed1990

Testicular seminoma: clinical and pathological features that may predict para-aortic lymph node metastases.

Patients with clinical stage I testicular seminoma usually receive elective para-aortic lymph node radiation after orchiectomy, which is effective in controlling subclinical microscopic disease. However, the majority of patients with clinical stage I seminoma do not harbor occult metastases and, therefore, do not require elective nodal treatment. Vascular space invasion by the primary testis tumor recently has been shown to be an important predictor of metastases in nonseminomatous tumors but no such information exists to date in pure seminoma. Therefore, patients with clinical stage I testicular seminoma were compared to clinical stage II to IV cancer patients with respect to the presence of several features of the primary tumor. Vascular space invasion was identified significantly less frequently in stage I cancer patients (17%, 5 of 29) than in those with stage II or greater disease (39%, 11 of 28, p equals 0.03, 1-tailed t test). Microscopic invasion of the tunica and rete testis, and necrosis also were identified slightly more frequently in the higher stage cancer patients. Of the 12 patients with a maximum tumor dimension of more than 6 cm. 9 (75%) were in the stage II or higher group. Patient age, symptom duration and presenting complaint were similar in the 2 groups. Many higher stage cancer patients did not exhibit aggressive histological characteristics and, therefore, the absence of these features cannot be used to select patients for surveillance. On the other hand, patients with clinical stage I tumors that exhibit vascular space invasion may have an increased rate of occult para-aortic lymph node metastases. Therefore, the presence of vascular space invasion may be a useful criterion for exclusion of patients from surveillance protocols. Confirmatory data are needed before a final recommendation can be made.

Adult↗
PubMed1990

Arteriogenic impotence: findings in 195 impotent men examined with selective internal pudendal angiography. Young Investigator's Award.

Selective internal pudendal angiography was performed in 195 men (average age, 35.4 years +/- 10.3) who were suspected of having arteriogenic impotence. In the majority of patients, disease was localized to the cavernosal arteries. A previous series that involved older patients had demonstrated significant disease in the hypogastric and internal pudendal arteries. When controlled for trauma, the data revealed no significant difference (X2 test, P greater than .10) in the distribution of hemodynamically significant penile arterial disease. However, in patients who had sustained major pelvic trauma, the common penile artery was frequently hemodynamically compromised. There is a great deal of variation in the origin of the internal pudendal artery. An accessory pudendal artery was demonstrated in 7% of the patients. If a selective internal pudendal artery injection fails to demonstrate the penile arterial anatomy, a less selective injection should be performed. Bilateral injections should always be performed, as unilateral arterial disease was present in 15% of the patients.

Adolescent↗
PubMed1990

"High-flow" priapism: treatment with superselective transcatheter embolization.

Hemodynamically, most cases of priapism occur as a result of venous outflow obstruction producing engorgement of the corpora cavernosa. In a small number of patients, however, the cause is uncontrolled arterial inflow, often from direct arterial trauma. The authors report two cases of arterial or "high-flow" priapism that were successfully treated with selective transcatheter embolization with autologous clot.

Adult↗
PubMed1989

Regional infusion of urokinase into occluded lower-extremity bypass grafts: long-term clinical results.

The initial outcome, long-term patency rate, and rate of limb salvage were studied in patients after regional urokinase infusion for treatment of thrombosed lower-extremity grafts. Seventy-one infusions were performed in 53 patients. Complete clot lysis occurred in 75% of grafts, with establishment of antegrade blood flow in 66%. Variables that favorably influenced clot lysis and the reestablishment of antegrade blood flow through the graft were a short duration of occlusion and a suprainguinal graft position. The median duration of patency after infusion and adjunctive therapy was 162 days, with 75% limb salvage at 301 days. No statistically significant variables that influenced the length of patency were identified. These long-term patency results are inferior to the reported results in suprainguinal grafts after reoperation. They appear similar to reported results for occluded infrainguinal grafts treated with thrombectomy and patch grafting.

Female↗
PubMed1988

Arteriography and radiology of impotence.

Both arterial and venous dysfunction are now recognized as significant causes of organic impotence. Numerous therapeutic modalities are currently available, including surgical bypass, venous ligation, transluminal angioplasty, and transcatheter vessel occlusion. The specific nature of this patient population requires correlation of angiographic findings with the patient's history and physical examination to determine the appropriate intervention and obtain a satisfactory outcome.

Angiography↗
PubMed1988

Donor renal angiography: its influence on the decision to use the right or left kidney.

The left kidney from a living, related donor is preferred for kidney transplantation because it has a longer renal vein and thus is easier to implant in the recipient. However, anatomic variations in the renal arteries occasionally dictate the choice of the right kidney instead. To determine the influence of angiographic findings on the decision to use the right or the left kidney for transplantation, we assessed the role of preoperative angiography in the evaluation of living, related kidney donors. The records of 81 potential renal donors who underwent arteriography were reviewed. Three patients did not undergo donor nephrectomy, for reasons unrelated to the results of arteriography. Of the remaining 78 patients, two-thirds (52) underwent left nephrectomy as planned. A right nephrectomy was performed in the other 26 donors. In 19 of these 26 patients (24% of the 78 who underwent nephrectomy), the right kidney was chosen because of anatomic variations in the renal arteries seen on arteriography. Right donor nephrectomy was performed in the remaining seven of the 26 patients because of venous or collecting-system variants in the left kidney in three and because of better anatomic compatibility with the recipient in the other four. We conclude that donor angiography will dictate the choice of kidney in approximately one-fourth of prospective renal donors.

Adolescent↗