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Biomedical subjects

P Y Poon

Publications and source records attributed to P Y Poon.

At least 19 recordsLinked to original sources
PubMed1992

Gastroepiploic veins: CT appearance in pancreatic disease.

The frequency with which gastroepiploic vein (GEV) enlargement was seen on CT and its relevance to disease of the portal venous system associated with pancreatic disease were studied. We performed a retrospective study of 50 patients with proved pancreatic disease and another 50 patients without such disease. The CT examinations were done in incremental dynamic fashion after a bolus injection of contrast medium. Scans were evaluated for collateral channel formation, including GEV enlargement, and for involvement of the portal venous system by pancreatic disease. Part of the GEV arcade was visible in 36 patients without pancreatic disease, and on average measured 3.2 mm in diameter (range, 1-5.5 mm). GEV enlargement was visible in 62% of the patients with disease; 16% demonstrated a vessel 6 mm or more in diameter. Thirty-four percent of the patients with disease had portal venous complications: 26% had isolated splenic vein involvement, 2% had isolated portal vein involvement, and 6% had a combination of splenic and portal vein involvement. Of the patients with splenic vein disease, 81% had collateral channel formation, 50% of them demonstrating isolated GEV enlargement. Patients with splenic vein disease due to acute pancreatic disease had a much higher instance of GEV enlargement (83.3%). Collateral vessels are commonly seen on CT scans of patients with splenic vein disease and most often occur via enlarged GEVs. Acute pancreatic disease is frequently associated with GEV enlargement, suggesting that the latter represents an early response to splenic vein disease. In contrast, multiple collateral pathways tend to develop in patients with chronic pancreatic disease.

Adult↗
PubMed1992

Medical audit of mammography: a simplified alternative.

Assessing radiologists' mammographic interpretation and performing a complete audit of a mammographic practice are notoriously difficult and time consuming. The authors propose a simplified method for accomplishing a satisfactory medical audit. A search of the 1987 pathological reports of St. Michael's Hospital in Toronto yielded 153 patients who underwent excisional biopsy of the breast and for whom preoperatively obtained mammograms were available. The medical records of the patients were reviewed to obtain demographic data, as well as information as to whether the excised lesions had been palpable and the stage of the tumour. The mammograms of the patients were categorized retrospectively by four independent observers. The findings were correlated with the results of the pathological examinations and analysed with receiver-operating characteristic (ROC) curves. Forty-five nonpalpable and 121 palpable lesions were identified, of which 70 were malignant and 96 benign. The positive predictive value of mammography for the nonpalpable lesions was 20%. Three of the 9 nonpalpable cancers and 35 of the 56 palpable ones had metastasized to the axillary lymph nodes. The area under the ROC curves for the four radiologists ranged from 0.84 to 0.89. This audit method in inexpensive and easily applied.

Adult↗
PubMed1991

Optimization of prostatic magnetic resonance imaging technique.

With a 1.5-T magnetic resonance imager the authors systematically varied a large number of technical factors to obtain an optimum balance between high image quality and reasonable imaging time for the prostate gland. Each parameter was adjusted relative to benchmark images of very high quality to achieve a reasonable acquisition time with as little loss of the signal-to-noise ratio (SNR) as possible. Image quality was judged subjectively by magnetic resonance radiologists and objectively by measurements of SNR for the prostate. The authors recommend multislice, multiecho spin-echo pulse sequences with dual surface coils, fat suppression, reduced bandwidth, a repetition time of 1500 ms, echo times of 30 and 60 ms, a flip angle of 60 degrees, two excitations, a slice thickness of 5 mm with a 1.5-mm gap and 192 phase-encoding steps. The acquisition time for one such series was 9.6 minutes.

Humans↗
PubMed1989

Myelolipoma: radiologic findings in seven patients.

Myelolipomas are rare, non-functioning, benign tumors which occur both in the adrenal gland and in various extra-adrenal locations, particularly presacrally. We report the computed tomographic (CT) and ultrasonographic (US) findings in seven patients with pathologically proven myelolipomas. Magnetic resonance (MR) imaging was also obtained in one. Our series includes one patient with bilateral myelolipomas, one with a presacral myelolipoma, and one with a calcified myelolipoma. Radiographic findings can suggest the diagnosis. A characteristic mass is lucent on plain films, is echogenic on US, shows fat attenuation on CT, is avascular at angiography, and has a signal intensity similar to that of fat on T1-weighted MR images. However, the absence of fat can render imaging findings nonspecific. Percutaneous needle biopsy is a simple, safe, effective way to establish the diagnosis when imaging features are not typical.

Adrenal Gland Neoplasms↗
PubMed1988

Magnetic resonance imaging for monitoring relapse of acute myeloid leukemia.

Magnetic resonance provides a non-invasive tool for monitoring normal and leukemic bone marrow. Measurements of the T1 relaxation times are elevated in acute myelogenous leukemia. However, interpatient variability diminishes the usefulness of MR measurements for diagnosing leukemia. In following the time course of individual patients, T1 relaxation time measurements appear to give an early and sensitive indication of leukemic relapse after remission and this may serve a clinical role in the management of leukemia by reducing the regularity of necessary marrow aspirates and biopsies.

Bone Marrow↗
PubMed1988

Magnetic resonance imaging, computed tomography, and radionuclide scintigraphy in detection of liver metastases.

A series of 100 patients with suspected hepatic metastases was studied with magnetic resonance (MR), unenhanced computed tomography (CT), and radionuclide (RN) scintigraphy. Each set of images was read by three clinicians using a five-point scale to allow receiver operating characteristic (ROC) analysis using truth data derived from clinical review. Performance was measured by the areas under the ROC curves (0.940 +/- .018 for MR, 0.951 O +/- .013 for CT and 0.943 +/- .013 for RN) which were statistically not significantly different. We conclude that at their present level of development these three diagnostic examinations have equivalent performance and that MR is not superior in the detection of hepatic metastases.

Evaluation Studies as Topic↗
PubMed1988

MRI of periprostatic venous plexus in staging of early prostatic carcinoma.

The periprostatic venous plexus can be observed as a bright rim in coronal magnetic resonance (MR) images obtained by spin-echo (SE) 2060/60 technique. A carcinoma of the prostate which penetrates through the capsule into the periprostatic tissues interrupts or obliterates this rim, whereas an intact rim indicates that the tumor is confined within the prostatic capsule. One hundred patients with proven prostatic carcinoma were prospectively imaged by MR to detect periprostatic involvement. The results were compared with those obtained by cystoscopy and digital rectal examination under general anesthesia. The imaging and clinical methods agreed with each other in 76% of the patients.

Adult↗
PubMed1987

Acute radiation-induced pulmonary damage: a clinical study on the response to fractionated radiation therapy.

Acute radiation-induced pulmonary damage can be a significant cause of morbidity in radiation therapy of the thorax. A prospective, clinical study was conducted to obtain dose-response data on acute pulmonary damage caused by fractionated radiation therapy. The endpoint was a visible increase in lung density within the irradiated volume on a computed tomographic (CT) examination as observed independently by three diagnostic radiologists. Fifty-four patients with various malignancies of the thorax completed the study. CT chest scans were taken before and at preselected times following radiotherapy. To represent different fractionation schedules of equivalent biological effect, the estimated single dose (ED) model, ED = D X N-0.377 X T-0.058 was used in which D was the average lung dose within the high dose region in cGy, N was the number of fractions, and T was the overall treatment time in days. Patients were grouped according to ED and the percent incidence of pulmonary damage for each group was determined. Total average lung doses ranged from 29.8 Gy to 53.6 Gy given in 10 to 30 fractions over a range of 12 to 60 days. Five patient groups with incidence ranging from 30% (ED of 930) to 90% (ED of 1150) were obtained. The resulting dose-response curve predicted a 50% incidence level at an ED value (ED50) of 1000 +/- 40 ED units. This value represents fractionation schedules equivalent to a total average lung dose of 32.9 Gy given in 15 fractions over 19 days. Over the linear portion of the dose-response curve, a 5% increase in ED (or total dose if N and T remain constant), predicts a 12% increase in the incidence of acute radiation-induced pulmonary damage.

Adolescent↗
PubMed1987

Mediastinal lymph node metastases from bronchogenic carcinoma: detection with MR imaging and CT.

Magnetic resonance (MR) imaging and computed tomography (CT) were compared in a prospective study of 48 patients for the detection of metastatic mediastinal lymphadenopathy from bronchogenic carcinoma. The images were interpreted by three experienced radiologists using a five-point rating scale, enabling receiver operating characteristic (ROC) analysis. Imaging results were evaluated against "truth" data based on analysis of surgical specimens from mediastinoscopy and thoracotomy. All MR images were cardiac gated to reduce cardiac motion artifacts in the mediastinum. MR and CT both performed well, as indicated by similar areas under the ROC curves of 0.779 +/- 0.039 for MR imaging and 0.781 +/- 0.038 for CT scanning. No strong correlation between nodal size and metastatic involvement could be found for either MR or CT results. As long as nodal size remains the sole criterion in the detection of metastatic mediastinal lymphadenopathy, MR imaging is unlikely to enable better interpretations than CT scanning.

Adult↗
PubMed1987

Disappearing iliac vessels: an MR phase cancellation phenomenon.

Magnetic resonance images often fail to distinguish the iliac vessels on one side. Phantom studies and theoretical analysis demonstrated that this phenomenon arises from a loss of the flow void effect. At a certain angle, the additional phase accumulation that results from motion in the section-selection direction exactly cancels the additional phase accumulation from motion in the frequency-encoding direction. Thus, vessels that cut through the imaging plane near this critical angle have substantial signal intensity. This artifact can be eliminated by alternating the direction of the section-selection gradient on sequential phase encodings.

Humans↗
PubMed1987

Magnetic resonance imaging of retroperitoneal lymphadenopathy.

A prospective study was performed to determine the value of magnetic resonance imaging (MRI) in the detection of retroperitoneal lymphadenopathy. Fifty patients with known malignancy, who demonstrated enlarged lymph nodes on computed tomography (CT), were entered into the study. The best visualization of retroperitoneal lymphadenopathy was found with a multislice spin-echo pulse sequence having a repetition time of 2000 msec, and echo delay of 30 msec. Nodes had a signal intensity lower than fat and greater than muscle with this pulse sequence. More than 90% of the lymphadenopathies as shown by CT could be demonstrated by MRI. Due to the flow-void phenomenon, blood vessels did not have any signal and were easily visualized without the use of intravenous contrast medium. Sagittal and coronal images added to an appreciation of the size of nodes and the extent of retroperitoneal lymphadenopathy. Ten normal volunteers were also imaged by MRI and their retroperitoneal nodes were well demonstrated.

Humans↗
PubMed1987

Magnetic resonance imaging of intracranial epidermoids: report of two cases.

Intracranial epidermoids are uncommon benign tumors in the subarachnoid spaces. We have studied two patients with these tumors originating in the basal cisterns using magnetic resonance (MR) imaging, computed tomography (CT), and cerebral angiography. Both CT and MR showed the extraaxial position of these tumors and their extension into the posterior fossa well. Angiography did not show any abnormal vasculature but displacement of vessels by the tumor mass was well demonstrated. MR was found to be relatively specific in characterizing these tumors.

Adult↗
PubMed1987

A prospective evaluation of magnetic resonance imaging, computed tomography, and mediastinoscopy in the preoperative assessment of mediastinal node status in bronchogenic carcinoma.

Computed tomography, magnetic resonance imaging, chest roentgenography, and mediastinoscopy were compared prospectively as staging modalities to assess mediastinal node status in 84 patients with presumed operable bronchogenic carcinoma. Computed tomography was associated with a sensitivity of 71.0%, a specificity of 87.7% and an overall accuracy of 82.1%. Magnetic resonance imaging did not provide any advantage over computed tomography in the assessment of mediastinal node status. The accuracy of computed tomography was not dependent on cell type of the primary tumor. Although chest roentgenography had a sensitivity of 80.7%, the overall accuracy of 57.1% was unacceptably low. The sensitivity (87.1%), specificity (100%), positive (100%) and negative (93.0%) predictive values, and accuracy (95.2%) of mediastinoscopy exceeded those observed with all other modalities. We continue to recommend mediastinoscopy as the most accurate staging investigation in the routine management of patients with bronchogenic carcinoma.

Carcinoma, Bronchogenic↗
PubMed1986

Optimal pulse sequence for imaging hepatic metastases.

For magnetic resonance (MR) imaging studies in which the diagnosis is dependent on image contrast, it is essential that an optimized imaging technique be used. Using detection of hepatic metastases as an example, the authors describe a rational strategy for optimizing MR imaging technique. First, for a single patient with proved hepatic metastases, a variety of imaging sequences is discussed and evaluated, leading to characterization of the patient's hepatic tissues. Then the characteristics of the tissues of a representative patient population are presented. These are used to determine two optimal pulse sequences that maximize the achievable signal difference-to-noise ratio achievable in a fixed imaging time. The recommended imaging sequence for detection of hepatic metastases at 0.15 T is either a three-dimensional volume spin-echo (SE) sequence with echo time (TE) = 12 msec and repetition time (TR) = 184 msec or a multisection inversion recovery sequence with TE = 22 msec, inversion time = 250 msec, and TR = 1,375 msec. The variation of this optimum pulse sequence with field strength is also presented.

Humans↗
PubMed1986

Magnetic resonance imaging of the mediastinum.

Seven normal volunteers and 69 patients with known disease in either the mediastinum or hila or both were imaged using a prototype magnetic resonance imager operating at 0.15T. Normal mediastinal and hilar structures were readily identified and mediastinal diseases were well demonstrated by magnetic resonance imaging (MRI). The value of MRI lies in the morphological demonstration of the presence and extent of disease.

Adult↗
PubMed1985

Nuclear magnetic resonance study of iron overload in liver tissue.

Whole-tissue and homogenized samples of human liver were studied in a NMR spectrometer, T1 and T2 relaxation times were measured as a function of added inorganic or organic iron. When inorganic iron (Fe+3) was added, pronounced T1 and T2 shortening was noted. However, when organic iron, in the form of ferritin, was added, the amount of T1 and T2 relaxation enhancement was much reduced for the same amount of added iron. The in vitro ferritin results model the situation found in clinical studies of hemochromatosis. Only in cases of severe iron overload were significant decreases in relaxation times observed. The T2 relaxation time was the more reliable indicator of excessive levels of iron in the liver. The large range of T1 and T2 values encountered in normal volunteers precludes the use of MR to quantitatively measure iron levels in the liver. The T1 and T2 relaxation times measured at intervals for one individual tend to fluctuate as well, making the use of MR to follow the course of treatment of iron overload disorders unreliable.

Hemochromatosis↗