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Biomedical subjects

P T Cahill

Publications and source records attributed to P T Cahill.

At least 19 recordsLinked to original sources
PubMed1998

Radiologist evaluation of a multispectral image compression algorithm for magnetic resonance images.

With the advent of teleradiology and picture archiving and communication systems (PACS), the expense and time required for image transmission and long term image archiving become important. The use of validated image compression algorithms can greatly reduce these costs. A lossy, multispectral image compression scheme at compression ratios (CR) of 25:1 and 32:1 was used for a set of 26 different patient MR exams. The original and compressed/decompressed (CD) images sets were evaluated in a blinded fashion by four radiologists in two phases. The main objective was to determine whether radiologic interpretation would vary between the two types of CD image sets and the corresponding originals. In general, the compression algorithm caused a slight decrease in image quality; however, the interpretation of pathology did not change between the original and CD image sets. In only one case at the maximum CR = 32 did one of four radiologists change the interpretation of pathology after CD. In this study, lossy multispectral image compression of MR images at CR = 25 maintained diagnostic integrity. This could play a significant role in image storage and communications.

Algorithms↗
PubMed1996

Quantitation of normal canine hippocampus formation volume: correlation of MRI with gross histology.

The hippocampal formation possesses an important role in the development and maintenance of short-term memory. In this study, magnetic resonance imaging (MRI) and gross histology were used to quantify the volume of the hippocampal formation in canines. High resolution MRI, using 1 mm thick slices and an intraplanar resolution of 0.35 mm was performed at 2.0 T both in vivo and in vitro following in situ fixation. The volumes of the hippocampal formations were determined from MR images and compared to those obtained from one mm thick gross histologic sections. The average volume of the canine hippocampal formation, measured from in vivo and in vitro MR images was 476.0 +/- 79.5 and 467.3 +/- 53.7 mm3, respectively. Determined from gross histology, the volume of the hippocampal formation was 463.6 +/- 24.1 mm3. Quantitation of the canine hippocampal formation using in vivo MRI showed good correlation with in vitro MRI and histology, verifying the reliability and reproducibility of in vivo MRI measurements. High resolution MRI using 1 mm thick slices through the whole canine hippocampal formation is necessary for accurate volume determination of a structure of this size.

Animals↗
PubMed1995

MRI evaluation of dexamethasone acetate therapy for osteoarthritis in the hand.

A patient with clinically diagnosed Stage I primary idiopathic osteoarthritis of the first metacarpal trapezium joint of the left hand received intra-articular administration of dexamethasone acetate. Compared to pretreatment T2- weighted MR images, greatly decreased MR signal intensity was observed in the affected joint space and adjacent soft tissues 2 days after dexamethasone acetate therapy. This was representative of reduced edema in the metacarpal trapezium joint and demonstrated the ability of MRI to follow pharmacological treatment for arthritis.

Anti-Inflammatory Agents↗
PubMed1994

Transmission line analysis of noncylindrical birdcage resonators.

Transmission line theory, validated for the standard cylindrical birdcage coil, has been employed for the analysis of a rectangular birdcage resonator which is useful for MR imaging of the hand. Due to lack of cylindrical symmetry in the rectangular coil, RF field uniformity was evaluated and found to be critically dependent upon the choice of column from which the coil was linearly driven. Effective L1 and L2 inductance elements were determined using known formulas for self and mutual inductance contributions, and compensation of the different inductance elements of the hand coil was performed to produce cylindrically symmetric birdcage current patterns. RF field mapping using the Biot-Savart law demonstrated a rectangular coil sensitivity 22% greater than a comparable cylindrical version. MR hand and wrist images were acquired using the rectangular birdcage coil. In addition, transmission line analysis was extended to other noncylindrical birdcage geometries.

Hand↗
PubMed1994

Direct in vivo observation of transventricular absorption in the hydrocephalic dog using magnetic resonance imaging.

RATIONALE AND OBJECTIVES: A model of chronic noncommunicating hydrocephalus in canines was developed, and gadolinium-DTPA (Gd-DTPA)-enhanced magnetic resonance imaging, physiologic and morphologic studies were performed to investigate transventricular absorption of cerebrospinal fluid. METHODS: Chronic hydrocephalus was induced in 12 mongrel dogs by injection of a silastic mixture into the prepontine cisterns. Ventricular pressure was measured during the development of hydrocephalus, and lateral ventriculo-ventricular perfusions with Gd-DTPA were performed under controlled conditions during serial magnetic resonance imaging studies. RESULTS: Hydrocephalus developed over an average of 129 +/- 24 days after induction, and the intraventricular pressure increased from an initial level of 14 +/- 4 cm H2O to a stabilized plateau of 25 +/- 5 cm H2O. Increased signal intensity in the brain matter, as seen on magnetic resonance images of chronic hydrocephalic dogs perfused with Gd-DTPA in the lateral ventricles, was consistent with the presence of the contrast agent in the periventricular extracellular space. This increased signal intensity was not observed in control animals. CONCLUSIONS: These results provide direct evidence of transventricular absorption in chronic hydrocephalus.

Absorption↗
PubMed1993

Use of transmission line analysis for multi-tuning of birdcage resonators.

A general analysis for double- and higher order tuning of birdcage resonators is presented based on a lumped element transmission line model. Expressions were developed for the determination of the resonant frequencies of bandstop and bandpass birdcage coils and, with specific restrictions, for capacitor values required to obtain any two desired mode one resonant frequencies. Experimental measurements on three variants each of an eight-column bandstop and an eight-column bandpass were in excellent agreement with theory; the average absolute frequency difference and percent deviation were 1.51 +/- 1.57 MHz and 2.61 +/- 2.36%, respectively. In addition, 31P and 1H phantom images were obtained at 2.0 T using a representative bandstop coil.

Magnetic Resonance Imaging↗
PubMed1992

Quantitative MRI studies for assessment of multiple sclerosis.

Although magnetic resonance imaging (MRI) is a valuable aid in the initial diagnosis of multiple sclerosis (MS), quantitatively MRI has been disappointing in staging and evaluating therapy protocols by means of serial examinations. In this study, image processing algorithms were developed for the global analysis of MR images of the cerebrum. Limited three-dimensional segmentation was achieved through histogram analysis by these algorithms, which are essentially operator independent. The effects of coil response and tip angles, patient positioning, and interslice gap thicknesses were examined for 10 MS patients with repeated examinations for a total of 72 images. Effects of technique and instrumentation errors were approximately 6%, and agreement between two independent operators for measuring the total MR pixel sum from periventricular effusions and intense MS plaques was better than 97% with a standard deviation of 2.9%.

Adipose Tissue↗
PubMed1992

Experimental design and fabrication of birdcage resonators for magnetic resonance imaging.

Although the birdcage resonator has been theoretically described for single- and multinuclear operation, this study provides the basic experimental guidelines needed for the fabrication and testing of such coils for various geometries and resonant frequencies from 10 to 95 MHz. The correlation of coil dimensions and resonant frequencies with individual inductance elements, L1 and L2, is also shown. Experimentally derived algorithms are presented for the determination of the capacitance needed for low-pass resonators based on measurements of the coil's "global" inductance. Ten low-pass birdcage coils with eight legs were evaluated and their four resonant frequencies were within 4% of theoretical predictions.

Algorithms↗
PubMed1992

Explicit treatment of mutual inductance in eight-column birdcage resonators.

A formulation has been developed for the determination of self and mutual inductances in unloaded, eight-column symmetric birdcage coils using their expected resonant mode current patterns and well-known inductance formulas. The average frequency differences between theory and experiment for mode one resonances for nine low-pass coils were 0.66 (+/- 0.57) MHz and 2.14 (+/- 2.08) MHz using effective (self plus mutual) and self inductances, respectively, and similarly, for three high-pass coils, 1.19 (+/- 0.56) MHz and 2.79 (+/- 2.20) MHz. These frequency differences were more pronounced for the higher modes; for mode four, the differences neglecting mutual inductance were 14.30 (+/- 11.10) MHz and 10.42 (+/- 4.60) MHz for the low- and high-pass coils, respectively. This analysis provides the first explicit evaluation of the total end-ring and column inductances L1 and L2 within each birdcage section at resonance with resulting excellent agreement in resonant frequencies between theory and experiment.

Magnetic Resonance Imaging↗
PubMed1991

Generalized electrical analysis of low-pass and high-pass birdcage resonators.

The radio-frequency 'birdcage resonator' has found wide use in MRI/MRS for its field homogeneity and signal-noise characteristics. This paper presents a general analysis, derived from lumped element transmission line theory, of the electrical behavior of unloaded, N-column birdcage resonators applicable to several versions of the basic design including low-pass and high-pass coils. Analytic expressions and computer results are presented for both types of coil describing resonant frequencies, input and characteristic impedances, dispersion relations, pass-bands, resonant peak bandwidth and Q. Theoretical expressions for normalized resonant frequency difference ratios independent of element values and resonator geometry have been developed for generic low- and high-pass coils. Experimental measurements of resonant frequencies were made for six coils, and the average agreement with theoretical predictions was approximately 4%.

Magnetic Resonance Imaging↗
PubMed1989

Computed tomography versus magnetic resonance imaging of the brain. A collaborative interinstitutional study.

A retrospective study (1983-1984) of magnetic resonance imaging (MRI) and computed tomography (CT) examinations in 471 patients with known pathology in the brain and craniocervical junction was conducted in order to determine the relative efficacy of MRI versus CT. All MRI examinations involved slice thickness greater than 10 mm, and only single-slice single-echo or multislice single-echo imaging techniques were available. These studies were evaluated independently by two neuroradiologists from a panel of six for anatomic abnormalities, lesion contrast, and radiologist's impression. Results, which excluded microadenomas of the pituitary and approximately 9% of studies in which consensus was not achieved by the readers, were as follows: (1) 14% of the studies were positive on MRI but normal on CT; (2) in 55% of the studies, MRI was better than CT; (3) MRI was equal or better than CT in 95% of the studies; and (4) CT was better than MRI in 5% (21/421) of the examinations. There were no patients in this series where CT was positive but MRI missed the abnormality.

Adult↗
PubMed1988

MR imaging of portal venous thrombosis: correlation with CT and sonography.

Fourteen patients with portal venous thrombosis (PVT) diagnosed by CT and/or sonography were studied with MR. Three of the 14 had portal hypertension. The MR findings were compared with those of eight patients with portal hypertension, but without CT or sonographic evidence of PVT. MR imaging showed portal venous thrombosis in all 14 PVT cases. Intraluminal thrombi of less than 5 weeks duration appeared markedly hyperintense relative to liver and muscle on both T1- and T2-weighted images. Older thrombi appeared hyperintense relative to liver and muscle in eight of 11 cases, but only on T2-weighted images. MR showed thrombi in 11% more portal vessels than did CT (MR = 30, CT = 27) and in 28% more vessels than did sonography (MR = 32, sonography = 25). MR also showed 24% more collateral vessels than did CT (MR = 31, CT = 25) and 50% more vessels than did sonography (MR = 33, sonography = 22). Third-echo images (echo time = 96 msec, repetition time = 1500-2150 msec) verified the presence of venous thrombi in 28 (93%) of 30 PVT vessels, and they differentiated flow-related intravascular signal from true thrombi in six (17%) of 36 portal hypertension vessels. We conclude that MR is a valuable tool for imaging portal vein thrombosis. MR is a good substitute for CT and can be more informative than sonography.

Female↗
PubMed1987

Segmental patterns of avascular necrosis of the femoral heads: early detection with MR imaging.

Thirty-two patients (64 hips) in whom avascular necrosis (AVN) of the femoral heads was highly suspected clinically were studied by magnetic resonance (MR) imaging, radionuclide bone scintigraphy, and conventional radiography. MR studies were positive for AVN in 37 hips, compared with 30 positive scintigraphic studies. In all cases in which scintigraphy and radiography were positive, MR imaging demonstrated decreased signal from the affected femoral heads, indicative of bone marrow disease. Imaging results were confirmed by biopsy or subsequent imaging appearances. In patients with negative initial scintigraphic and radiographic studies, the MR imaging criterion for a positive study was a moderately decreased bone marrow signal displaying segmental patterns within an otherwise normal-appearing femoral head on relatively T1-weighted images. In this series of high-risk patients, radionuclide scintigraphy had a sensitivity of 81%, compared with 100% for MR imaging. MR imaging should be the imaging modality of choice for early evaluation of bone marrow changes indicative of AVN.

Femur Head Necrosis↗
PubMed1986

Effect of magnetic resonance imaging on DDD pacemakers.

A previous study suggested the safety of exposing patients with certain pacemakers models to magnetic resonance imaging (MRI). However, the function of a variety of more advanced DDD pacemakers and the effect of higher magnetic and radio-frequency (rf) field strengths has not been reported. In the present study 4 different DDD pacemakers (Cordis 233F, Intermedic 283-01, Medtronic 7000A, and Pacesetter 283) were tested in a saline phantom under several conditions and with various imaging sequences. Pacemaker output was monitored using electrocardiographic telemetry. All units paced normally in the static magnetic field. However, during imaging, all units malfunctioned, with total inhibition of atrial and ventricular output in 3 of the pacemakers. In the fourth pacemaker, ventricular backup pacing was activated at high rf pulse repetition rates. However, the MRI scanner could trigger atrial output in this pacemaker at rates of up to 800/minute. All malfunctions were a result of rf interference, whereas gradient and static magnetic fields had no effect. Thus, despite magnetic fields had no effect. Thus, despite magnetic field strengths adequate to close pacemaker reed switches, rf interference during MRI may cause total inhibition of atrial and ventricular output in DDD pacemakers, and can also cause dangerous atrial pacing at high rates. MRI should be avoided in patients with these DDD pacemakers.

Electrocardiography↗
PubMed1985

Multi-section multi-echo pulse magnetic resonance techniques: optimization in a clinical setting.

The user-selectable magnetic resonance imaging parameters involved in a multi-section multi-echo pulse sequence are described and their interrelationships explained. These parameters include repetition time (TR), echo delay time (TE), the number of sections (S), the number of echoes (E'), the time from the center of the last echo to the end of data acquisition (15 msec on the authors' system), and the time for T1 recovery. A simple mathematical relationship among these quantities, permitting easy calculation of the remaining parameters once a sufficient number have been specified, is given. In addition, a nomogram that permits rapid visual estimation of these parameters is presented. The effects of these choices and others is explained with respect to imaging time, region covered, and image contrast. Finally, the inherent trade-offs are illustrated in a clinical example involving the pelvis.

Humans↗
PubMed1984

Optimal injection technique for intravenous digital subtraction angiography. Animal and clinical studies of right atrial injection using small volumes (25 ml) at a high rate (35 ml/sec).

Small-volume right atrial injections of contrast material (25 ml) delivered at a high rate (30-35 ml) were routinely performed for digital subtraction angiography in 232 studies (698 injections). Excellent opacification of the aorta and peripheral arteries was consistently obtained. Superior opacification following right atrial injections compared with peripheral or superior vena cava boluses was confirmed in animal experiments.

Angiography↗
PubMed1984

Posterior fossa lesions: magnetic resonance imaging.

Studies of 40 patients with abnormalities of the posterior fossa shown on magnetic resonance (MR) imaging were reviewed and compared with CT scans. Thirteen lesions were demonstrated on MR only. Twenty-four lesions were shown on CT but MR provided more data. Three lesions were better shown on CT than on MR. MR is superior to CT because of the lack of streak artifacts and better contrast discrimination. At least two imaging sequences are required for evaluation of the lesions.

Brain Diseases↗