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J A Malko

Publications and source records attributed to J A Malko.

At least 19 recordsLinked to original sources
PubMed1991

Magnetic resonance artifact in the postoperative cervical spine. A potential pitfall.

An magnetic resonance imaging artifact that stimulates hypertrophic bone formation is described in patients who have had an anterior cervical discectomy. The magnetic resonance images of 26 patients with anterior cervical discectomy were retrospectively reviewed. Comparison was made to the available concurrent computed tomographic scans, computed tomographic myelograms, and operative reports. A bovine spine was drilled with a drill only at one level and with a metal suction tip in close proximity to the drill at another level, and magnetic resonance images were obtained. Artifact was present in 12 patients and absent in 14; this was confirmed in the 8 patients with comparison studies. Close correlation was seen with the prospective reading of the presence of artifact and operative drill use in the seven patients with available operative reports. The bovine spine model showed no artifact at the drill-only level and significant artifact at the level where the metal suction tip was positioned next to the drill. Small metal flecks were seen grossly at the second level, but not on plain roentgenograms. The metallic magnetic resonance artifact seen in postoperative cervical spines is probably from small bits of metal from the metal suction tip as it occasionally hits the drill. Bone abnormalities seen on magnetic resonance imaging at the level of a previous anterior cervical discectomy may need a cervical computed tomogram to confirm the findings.

Adult↗
PubMed1991

Manganese dipyridoxyl diphosphate. Effect of dose, time, and pulse sequence on hepatic enhancement in rats.

We used an animal model to investigate the hepatic enhancement characteristics of manganese dipyridoxyl diphosphate (MnDPDP) related to time, dose, and pulse sequence. The contrast doses selected were in the human tolerance range. Using an SE 300/15 pulse sequence, maximum mean hepatic enhancement of 45% (8 mumols/kg) and 58% (12 mumols/kg) over baseline was seen during a plateau maintained between 5 and 50 minutes postinjection in the 8 mumols/kg group, and between 10 and 90 minutes in the 12 mumols/kg group. This plateau was followed by a very gradual decline in hepatic enhancement. Using either 4 or 8 mumols/kg, there was a significant increase in postcontrast hepatic intensity on all relatively T1-weighted pulse sequences (spin echo [SE] 300/15, inversion recovery [IR] 1400/20/400, gradient echo [GE] 47/13/80 degrees, and GE 60/20/30 degrees) except GE 47/13/80 degrees at 4 mumols/kg. At 8 mumols/kg there was superior enhancement, with IR 1400/20/400 and SE 300/15, but at 4 mumols/kg there was no consistently superior sequence. None of the relatively T2-weighted pulse sequences (SE 2000/50, SE 2000/100, or GE 100/30/20 degrees) demonstrated a significant change in hepatic intensity using either dose of contrast. The data suggest that the best combination of dose, pulse sequence, and time for hepatic imaging with MnDPDP is 8 mumols/kg using heavily T1-weighted sequences 5 to 60 minutes following contrast administration.

Animals↗
PubMed1990

Hepatic iron overload: diagnosis and quantification by noninvasive imaging.

The diagnostic efficacy of magnetic resonance (MR) and computed tomography (CT) for detection and quantification of hepatic iron was assessed in a series of patients under investigation for clinical or biochemical evidence of hepatic iron overload. Thirty patients underwent MR imaging (SE 30,60/1000 or SE 30,60/2000) at 0.5 Tesla with calculation of hepatic T2 and liver to paraspinous muscle signal intensity ratios. Twenty-nine patients also had measurement of hepatic attenuation on noncontrast CT images. Results of these imaging studies were correlated in all patients with quantitative iron determination from liver biopsy specimens. The best predictor of liver iron among parameters studied was the ratio of the signal intensities of liver and paraspinous muscle (L/M) on a SE 60/1000 sequence. Both MR using L/M ratios and CT were sensitive methods for detection of severe degrees of hepatic iron overload with 100% of patients with hepatic iron on biopsy greater than 600 micrograms/100 mg liver dry weight detected on the basis of L/M less than 0.6 or CT attenuation greater than 70 Hounsfield units (HU). The MR parameter, however, was more specific than CT (100 vs 50%) and showed a higher degree of correlation with quantitated hepatic iron from biopsy. T2 measurements showed poor correlation with hepatic iron, due to difficulty in obtaining precise T2 measurements in vivo when the signal intensity is low. None of the parameters utilized was sensitive for detecting mild or moderate degrees of hepatic iron overload. We conclude that MR and CT are sensitive techniques for noninvasive detection of severe hepatic iron overload, with MR providing greater specificity than CT.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1989

Hand-held electronic teaching-aid for demonstrating MRI concepts.

We describe a hand-held electronic teaching-aid for magnetic resonance imaging (MRI), which can be used to demonstrate the dependence of signal intensity on changing pulse sequence parameters (TR and TE), the dependence of intensity on changing tissue parameters (T1 and T2), and the ideas of "T1-weighting" and "T2-weighting" as they relate to image contrast. The device was specifically designed to be easy to use and readily accessible to residents-in-training and MR neophytes.

Electronics↗
PubMed1989

Eddy-current-induced artifacts caused by an "MR-compatible" halo device.

Traction or halo devices thought to be compatible with magnetic resonance (MR) imaging and made of nonferromagnetic metals may actually cause MR image artifacts. The authors have observed such artifacts caused by eddy currents generated in the conducting frame of the device by the radio-frequency excitation field. The severity of the artifact depends on the orientation of the device in the MR unit and can be effectively eliminated by disrupting the electrical continuity of the supporting frame of the device.

Diagnostic Errors↗
PubMed1989

Renal corticomedullary junction. Performance of T1-weighted MR pulse sequences.

Inability to demonstrate the renal corticomedullary junction (CMJ) on magnetic resonance (MR) images has been reported in connection with several medical renal diseases. T1-weighted spin echo pulse sequences have been advocated to demonstrate a signal intensity difference between cortex and medulla. This study was undertaken to determine which of several T1-weighted spin echo (SE) and gradient echo (GE) sequences are better for delineation of the CMJ. The MR studies were performed at 0.5 Tesla on 27 normal volunteers. Multi-slice axial images of both kidneys were obtained in all subjects at each of the following five pulse sequences: SE 250/20, SE 500/30, SE 900/30, and GE 300/15 with 80 degrees and 64 degrees flip angles. Contrast/noise ratios were calculated for the signal intensity differences between cortex and medulla; the average standardized contrast/noise ratios ranked as follows: GE 300/15/80 degrees = 3.01 +/- 0.74, GE 300/15/64 degrees = 2.72 +/- 0.74, SE 250/20 = 2.02 +/- 0.33, SE 500/30 = 1.96 +/- 0.51, and SE 900/30 = 1.71 +/- 0.39. In addition, the five sequences for each patient were randomized and the images were independently ranked for delineation of CMJ by three MR radiologists. The cumulative subjective ranking for all observers from best to worst is as follows: SE 500/30, GE 300/15/80 degrees, GE 300/15/64 degrees, SE 900/30, SE 250/20. Although better contrast/noise ratios are achieved with the GE sequences and the more T1-weighted SE sequences, as a practical matter this does not seem to be the only significant factor when compared with the visual image evaluation by independent observers.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1988

Acquisition order and motional artifact reduction in spin warp images.

Multiple averaging can be a powerful tool against motional artifacts if significant motion occurs between the redundant acquisitions taken at a given gradient strength. However, if the time delay between these redundant measurements is too short, data or images depicting the patient is exactly the same position will be combined. Pooling such identical data has no effect on motional artifacts. This problem can be solved by increasing TR, increasing the number of redundant acquisitions, or changing the order in which acquisitions are taken. Usually all acquisitions at a particular value of the warp gradient are taken before proceeding to the next gradient value. This order minimizes motion between redundant acquisitions and so maximizes artifacts. The effect of other acquisition orders on both periodic and nonrepetitive motion is discussed. Human images for breathing and phantom results for single-occurrence motions are presented.

Humans↗
PubMed1988

Normal SPECT thallium-201 bull's-eye display: gender differences.

The bull's-eye technique synthesizes three-dimensional information from single photon emission computed tomographic 201TI images into two dimensions so that a patient's data can be compared quantitatively against a normal file. To characterize the normal database and to clarify differences between males and females, clinical data and exercise electrocardiography were used to identify 50 males and 50 females with less than 5% probability of coronary artery disease. Results show inhomogeneity of the 201TI distributions at stress and delay: septal to lateral wall count ratios are less than 1.0 in both females and males; anterior to inferior wall count ratios are greater than 1.0 in males but are approximately equal to 1.0 in females. Washout rate is faster in females than males at the same peak exercise heart rate and systolic blood pressure, despite lower exercise time. These important differences suggest that quantitative analysis of single photon emission computed tomographic 201TI images requires gender-matched normal files.

Adult↗
PubMed1987

Leptomeningeal metastasis: MR imaging.

Seven patients with central nervous system neoplasia and leptomeningeal metastases, proved either at initial diagnosis or on follow-up with contrast material-enhanced computed tomography (CT), were evaluated with magnetic resonance (MR) imaging. In two patients, diffuse sulcal enhancement on CT scans was inapparent on T1- or T2-weighted MR images. Likewise, in four patients diffuse cisternal enhancement on CT scans was not identifiable with MR. Nodular or focal cisternal masses were identified with both CT and MR imaging in three patients; in two, however, MR imaging provided less information. Ependymal and subependymal metastases identified with CT (two patients) were indistinguishable on MR images from periventricular abnormalities of radiation therapy and/or hydrocephalus. These findings suggest that leptomeningeal metastasis may be so subtle or inapparent as to be overlooked with MR imaging alone. Thus, CT and MR imaging should be considered complementary techniques for initial diagnosis and follow-up of tumors with a propensity for leptomeningeal metastasis.

Adult↗
PubMed1986

SPECT liver imaging using an iterative attenuation correction algorithm and an external flood source.

The results obtained from the inclusion of a new intrinsic attenuation correction algorithm into a protocol for SPECT liver imaging are presented in this study. A total of six patients were evaluated with this protocol. The new algorithm uses a transmission tomographic acquisition that is obtained before a standard emission tomograph, and requires the use of an external flood source. The transmission tomograph results in an attenuation image, or map, of the patient. The attenuation map then serves as input into the final intrinsic correction algorithm, that also uses data from a standard emission acquisition. The results of the six patients studied show that the algorithm can correct for attenuation effects without degrading image quality. In all the cases studied, the attenuation corrected images made the cases easier to interpret than did the images obtained without attenuation correction.

Aged↗
PubMed1985

An attenuated projector-backprojector for iterative SPECT reconstruction.

A new ray-driven projector-backprojector which can easily be adapted for hardware implementation is described and simulated in software. The projector-backprojector discretely models the attenuated Radon transform of a source distributed within an attenuating medium as line integrals of discrete pixels, obtained using the standard sampling technique of averaging the emission source or attenuation distribution over small square regions. Attenuation factors are calculated for each pixel during the projection and backprojection operations instead of using precalculated values. The calculation of the factors requires a specification of the attenuation distribution, estimated either from an assumed constant distribution and an approximate body outline or from transmission measurements. The distribution of attenuation coefficients is stored in memory for efficient access during the projection and backprojection operations. The reconstruction of the source distribution is obtained by using a conjugate gradient or SIRT type iterative algorithm which requires one projection and one backprojection operation for each iteration.

Heart↗
PubMed1985

Jugular venous thrombosis: MR imaging.

Magnetic resonance (MR) imaging of jugular venous thrombosis was investigated in three patients who had symptoms suggestive of this condition; the diagnosis was later confirmed by computed tomography, by venography, and clinically. Bright intraluminal signal intensity was seen throughout the course of the affected jugular vein on MR images in all three patients, in sharp contrast to the lack of signal from the corresponding site in the uninvolved venous system. Temporal changes in signal intensity from the acute to subacute stage of thrombosis were evaluated for one patient. A relative increase in signal intensity for the subacute phase was believed to be related to a decrease in the T1 relaxation time. MR may be the imaging modality of choice in the investigation of venous thrombosis.

Adult↗
PubMed1985

A count-based algorithm for attenuation-corrected volume determination using data from an external flood source.

We discuss the basic features of a count-based algorithm for attenuation-corrected volume determination, for use with planar gamma camera images. The attenuation correction is arrived at by combining the results of two 180 degrees opposed images with a transmission image obtained with an external flood source. A sample of the imaged radioactive volume is used to convert the attenuation-corrected count to an absolute volume. The algorithm is best suited to the measurement of small to medium-sized volumes of uniform activity, such as are encountered in cardiac blood-pool imaging.

Heart↗
PubMedDate not supplied

Direct hepatic tumor injection in rats: can it be used for analysis of MR imaging contrast agent?

With a recently described rat model technique for direct hepatic injection of tumor cells for imaging research, there were concerns that the injection itself might produce lesions detectable with magnetic resonance (MR) imaging, thereby producing false-positive results. To examine this possibility, the authors prospectively studied 14 Sprague-Dawley rats after direct hepatic injection of cells from a rat hepatoma cell line. The rats were imaged with a variety of pulse sequences before and after intravenous injection of the contrast agent manganese dipyridoxal diphosphate at a dose of 8 mumol/kg. No intrahepatic lesions could be detected with MR imaging during the first 6 days after direct hepatic injection of the tumor cells. Therefore, the direct injection technique should be accurate for evaluating various MR imaging sequences and contrast agents for early hepatic tumor detection.

Animals↗
PubMedDate not supplied

Are hepatic and muscle T2 values different at 0.5 and 1.5 Tesla?

In an effort to determine whether T2 values of liver and muscle change with increasing field strength, 144 abdominal MR examinations were retrospectively evaluated. These patients were evaluated with a dual echo T2-weighted spin-echo sequence. Eighty-two of the examinations were performed at 0.5 Tesla and 72 at 1.5 Tesla (T). Eleven of the patients were evaluated with both MR systems with the same sequences. T2 values were also obtained from a Fe NH4(SO4)2 12H2O phantom. The T2 values of liver decreased from 57.8 +/- 11.3 at 0.5 T to 43.7 +/- 8.3 at 1.5 T. The T2 values of muscle decreased from 44.2 +/- 9 at 0.5 T to 35.4 +/- 7.2 at 1.5 T. Patients who were examined on both systems also demonstrated a decrease in both liver and muscle T2 values. For concentrations in the range of hepatic T2's, the phantom demonstrated a decrease in T2 values from 0.5 to 1.5 T ranging from 20.3 to 23.4%. All the T2 changes were statistically significant (p less than .05). The findings suggest that T2 values may depend on field strength, or may vary due to other hardware-related differences.

Evaluation Studies as Topic↗
PubMedDate not supplied

Controlled eddy currents: applications to MR imaging.

We describe how the artifact caused by eddy currents generated in free standing copper coils may be controlled and used to advantage in magnetic resonance (MR) imaging. The eddy currents distort the excitation field of the MR imager in the vicinity of the coil; the coils may then be used to reduce the signal intensity of tissues adjacent to them. Two examples of how this signal reduction may be used to advantage are given. In one example a coil was used to eliminate an aliasing artifact by removing the signal from an unwanted object. In another example a coil was placed on the anterior abdominal wall of a subject, thereby reducing the high-intensity signal from subcutaneous fat and the resulting ghosting from respiratory motion.

Abdominal Muscles↗
PubMedDate not supplied

Flow-sensitive MR imaging of ventriculoperitoneal shunts: in vitro findings, clinical applications, and pitfalls.

To determine ventriculoperitoneal shunt patency during routine MR imaging of the head, 23 patients were studied with T1-weighted fast-field-echo scans. Without knowledge of the results of previous MR/CT studies or of the patients' clinical history, we reviewed the fast-field-echo studies and divided them according to those judged to have shunt flow (18) and those judged not to have flow (five). Fast-field-echo sequences showed high signal intensity, consistent with CSF flow and shunt patency in 17 medium-pressure systems and one high-pressure system. No signal was seen in five patients with high-pressure valve shunts. Combined clinical evaluation and MR/CT studies showed that three patients had probable shunt malfunction. One patient had true shunt malfunction; and although malfunction was thought to be present in two symptomatic patients, surgical revision showed the shunts to be patent. The possibility of temporary shunt obstruction is postulated to explain the clinical and MR findings in those two cases. The remaining two cases (9% of the patients) had no clinical evidence of shunt malfunction, and the MR findings probably reflected periodic CSF flow. One patient had an intracranial segment that was not connected and showed no flow on MR. No false-positive results (apparent flow in a nonfunctioning shunt) occurred. Using a standard medium-pressure shunt system, we constructed and imaged a phantom, which confirmed our clinical observations. T1-weighted fast-field-echo sequences may be useful in assessing patency of medium-pressure CSF shunt systems.

Cerebrospinal Fluid Shunts↗