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Biomedical subjects

D G Mitchell

Publications and source records attributed to D G Mitchell.

At least 145 records · Page 8Linked to original sources
PubMed1990

Variable flip angle imaging and fat suppression in combined gradient and spin-echo (GREASE) techniques.

Conventional "proton density" and "T2-weighted" spin-echo images are susceptible to motion induced artifact, which is exacerbated by lipid signals. Gradient moment nulling can reduce motion artifact but lengthens the minimum TE, degrading the "proton density" contrast. We designed a pulse sequence capable of optimizing proton density and T2-weighted contrast while suppressing lipid signals and motion induced artifacts. Proton density weighting was obtained by rapid readout gradient reversal immediately after the excitation RF pulse, within a conventional spin-echo sequence. By analyzing the behavior of the macroscopic magnetization and optimizing excitation flip angle, we suppressed T1 contribution to the image, thereby enhancing proton density and T2-weighted contrast with a two- to four-fold reduction of repetition time. This permitted an increased number of averages to be used, reducing motion induced artifacts. Fat suppression in the presence of motion was investigated in two groups of 8 volunteers each by (i) modified Dixon technique, (ii) selective excitation, and (iii) hybrid of both. Elimination of fat signal by the first technique was relatively uniform across the field of view, but it did not fully suppress the ghosts originating from fat motion. Selective excitation, while sensitive to the main field inhomogeneity, largely eliminated the ghosts (0.21 +/- 0.05 vs. 0.29 +/- 0.06, p less than 0.01). The hybrid of both techniques combined with bandwidth optimization, however, showed the best results (0.17 +/- 0.04, p less than 0.001). Variable flip-angle imaging allows optimization of image contrast which, along with averaging and effective fat suppression, significantly improves gradient- and spin-echo imaging, particularly in the presence of motion.

Adipose Tissue↗
PubMed1990

Neurosonographic features of periventricular echodensities associated with cerebral palsy in preterm infants.

We studied the value of neonatal neurosonograms in preterm infants for predicting the development of cerebral palsy (CP). All infants born at less than 33 weeks of gestation who were admitted to the intensive care nursery of Thomas Jefferson University Hospital from 1982 to 1986 were serially studied with cranial ultrasound methods that reliably detect neonatal periventricular echodensities (PVE) and cysts that are 2 mm in diameter or larger. PVE were graded as mild or as moderate to severe, and cyst size was classified by widest diameter as either large (greater than or equal to 3 mm) or small (less than 3 mm). All 127 surviving infants with PVE or cyst formation or both were followed until spastic forms of CP could be diagnosed or excluded during late infancy. All 26 of the infants in whom spastic CP developed had moderate or severe PVE in the area superior and lateral to the caudothalamic notch as noted on the parasagittal images of the neonatal neurosonograms. All these infants also developed cysts in the periventricular region within the area of the previously noted PVE. Mild or moderate to severe PVE were not associated with the development of spastic CP in 101 infants. Cysts developed in the area of PVE in the neurosonographic studies of 42 of these 101 infants. Mild PVE without cysts and moderate to severe PVE without cysts had negative predictive values for CP of 69% and 76%, respectively. By contrast, the presence of moderate to severe PVEs with large cyst formation had positive and negative predictive values of 90% and 93%, respectively, and was the most sensitive and specific neurosonographic finding for predicting CP, with an efficiency of 92%. The presence or absence of intracranial hemorrhage did not increase the efficiency of the ultrasound test results.

Brain Diseases↗
PubMed1990

Color Doppler imaging in the management of intraocular tumors.

Forty-four intraocular mass lesions were studied using a new, non-invasive ultrasound technique known as color Doppler imaging (CDI). This technique displays color-encoded Doppler flow information throughout a two-dimensional gray scale image thus providing selective analysis of Doppler spectra in small vessels using pulsed Doppler. Abnormal Doppler shifts were demonstrated within 39 neoplastic lesions studied, but Doppler shifts could not be detected in three tumor-simulating lesions. In a group of 12 choroidal melanomas studied after radiation therapy, lower Doppler shifts were seen compared with a group of 28 tumors before therapy. This change in Doppler shift may reflect the decreased vascular supply of the tumor. Color Doppler imaging may be of value as an additional useful tool in the diagnosis and management of intraocular tumors.

Adolescent↗
PubMed1990

Color Doppler imaging of the iliofemoral region.

Color Doppler imaging has been used to evaluate a variety of abnormalities in the iliofemoral region. This pictorial essay demonstrates how use of color Doppler techniques aids in the diagnoses of iliofemoral pseudoaneurysms, fluid collections, aneurysms, arteriovenous fistulas and malformations, venous thrombosis, vascular grafts, and soft-tissue masses including tumors and benign lymphadenopathy.

Color↗
PubMed1990

Zones of the uterus: discrepancy between US and MR images.

The location of the boundaries among the endometrium, inner myometrium (dark), and outer myometrium (intermediate or bright) were compared on T2-weighted magnetic resonance (MR) and intravaginal ultrasound (US) images obtained within 24 hours of each other. Twelve women with normal reproductive cycles underwent a total of 21 pairs of examinations. The endometrium, inner myometrium (junctional zone), and outer myometrium were measured by three independent observers. The endometrial thickness was thinner with MR imaging (6.5 mm with MR imaging vs 7.9 mm with US early in the cycle [P = .001]; 9.9 mm with MR imaging vs 11.3 mm, respectively, late in the cycle [P = .045]). The junctional zone was thicker with MR imaging (4.4 vs 2.3 mm early [P less than .001]; 3.9 vs 2.2 mm late [P = .003]). The combined thickness of the endometrium and junctional zone was thicker with MR imaging (15.5 vs 12.7 mm early [P = .002]; 17.3 vs 15.8 mm late [P = .064]), confirming that the boundary between the junctional zone and outer myometrium is different with MR imaging versus US.

Adult↗
PubMed1990

Right carotid artery ligation in neonates: classification of collateral flow with color Doppler imaging.

Extracorporeal membrane oxygenation (ECMO) is a lifesaving procedure for neonates that involves permanent ligation of the right common carotid artery. To determine the collateral flow patterns that develop after ligation, 58 infants underwent a total of 115 color Doppler imaging studies during (n = 54), within 1 month after (n = 36), or more than 1 month after (n = 25) ECMO. On the basis of the direction of flow in the right internal carotid artery (ICA) proximal and distal to the right posterior communicating artery (PCoA), 85 of the 115 studies were placed in one of three categories: (a) Anterior communicating artery (ACoA) dominance (n = 22), with retrograde flow in the entire right ICA, was most common during ECMO and was never seen more than 1 month after ECMO. (b) PCoA dominance (n = 47), with antegrade right ICA flow distal to the right PCoA but retrograde proximally, was common during all time periods. (c) External carotid artery (ECA) dominance (n = 16), with antegrade flow throughout the right ICA, was noted in 53% of studies more than 1 month after ECMO but in only 9% of earlier studies. Patterns changed from ACoA to PCoA to ECA dominance in 17 infants, but no change in the opposite direction was noted.

Carotid Arteries↗
PubMed1990

Usefulness of a short femur in the in utero detection of skeletal dysplasias.

In 28 fetuses studied during a 4 1/2-year period, the initial femur was below 2 standard deviations (SDs) of the mean when compared with the biparietal diameter. These fetuses were considered at risk for skeletal dysplasias and were followed up. Studies were performed at a mean gestational age of 26.7 weeks (range, 15.3-41.0 weeks). Group 1 had a femur length 1-4 mm below the 2-SDs line (range, -2.0 to -4.0 SDs); no other abnormalities were detected. Interval examination of 12 femurs showed that 10 either remained shortened to the same degree or had a growth spurt. At birth, all subjects were healthy except one with mild growth retardation and one with a chromosomal abnormality. Of the two subjects that failed to continue normal growth, one was healthy and the other was a heterozygous achondroplastic dwarf. Group 2 had greater femoral shortness; all measurements were more than 5 mm below the 2-SD line (range, -4.3 to -31.0 SDs). All had fetal abnormalities and significant skeletal dysplasias. The authors conclude that the number of millimeters below the 2-SDs line is an accurate, easy criterion for evaluation of femoral length.

Bone Diseases, Developmental↗
PubMed1990

Color Doppler artifact in anechoic regions.

For color Doppler imaging, several types of signal processing are employed in order to produce acceptable images of blood flow in blood vessels while suppressing color in moving solid tissue. The processing can produce an artifact in which color may arise from noise or from tissue motion and fill anechoic regions preferentially. This artifact may complicate the differentiation of areas with blood flow from anechoic regions without flow. By using four different color Doppler ultrasound units to image a tissue-equivalent phantom containing anechoic cylinders, artifactual color resulted when gain was raised sufficiently. This color was concentrated in anechoic regions of a gray-scale image that did not contain flow. In two instruments, this artifact was only observed when the transducer was vibrated, simulating tissue motion. In these instruments, the identification of low-frequency, high-amplitude Doppler signals is used to locate moving solid tissue and so suppress color in these regions. In the other two instruments, the presence of echoes within the image suppressed the assignment of color. With both types of processing, color may appear artifactually in echo-free regions without flow, such as fluid collections. Presence or absence of flow should be confirmed by Doppler spectral analysis. An understanding of the origin and appearance of artifactual color can prevent its occurrence from detracting from the usefulness of color Doppler imaging.

Aorta, Abdominal↗
PubMed1990

Magnetic resonance imaging of bone marrow: diagnostic value in diffuse hematologic disorders.

Magnetic resonance imaging (MRI) has value in characterizing normal and abnormal bone marrow because of its ability to distinguish fat from other tissues. Due to this advantage, hematologic disorders resulting in alterations of the normal cellular and fatty marrow distribution can be appreciated. In this article, the role of MRI in diffuse hematologic disorders is emphasized. At birth, almost all marrow is cellular, but by age 25, cellular marrow is restricted to the axial skeleton and proximal femoral and humeral metaphysis. The remainder is fatty, consisting of 80% fat, 15% water, and 5% protein. With increased need for hematopoiesis, reconversion from fatty to cellular marrow occurs in many diffuse disease states. Diffuse diseases that affect bone marrow production are divided into four categories representing conditions that affect the pluripotent hematopoietic stem cell. These include stem cell failure resulting in aplastic anemia, uncontrolled stem cell proliferation as exemplified by polycythemia vera, stem cell dysplasia such as sickle cell anemia, and malignant transformations or replacement. The MRI appearance of these disorders is discussed in this article. The use of spin-echo (SE) sequences is the most common approach to bone marrow imaging. With T1-weighted SE images, fatty marrow will appear bright and cellular marrow, with lower fat content, will exhibit a lower density signal. With T2-weighted SE pulse sequences, contrast between fatty marrow and cellular marrow decreases. Contrast between fatty and cellular marrow is enhanced with chemical shift imaging, including Dixon out-of-phase imaging, as emphasized in this article. MRI presents a more global view of the bone marrow than biopsy material and should provide a better understanding of diffuse hematologic disease progression and resolution.

Bone Marrow↗
PubMed1990

[Imaging of intraocular and orbital vessels using angiodynography].

Color Doppler imaging (CDI) is a recent advance in ultrasonography. It allows simultaneous two-dimensional structural imaging and evaluation of blood-flow characteristics. The velocity information of the scan is extracted and added to the images as color information. A total of 20 normal eyes and 30 patients with intraocular tumors were examined with this technique. The central retinal artery, the posterior cliliary arteries, the opthalmic artery and the central retinal vein could be located in all patients. In the intraocular tumors blood flow was detected reliably. Spectrum analysis of the displayed vessels allows semiquantitative assessment of blood flow in these vessels. This new technique of color Doppler imaging offers a new modality to study ocular and orbital blood flow.

Blood Flow Velocity↗
PubMed1990

Recent advances in magnetic resonance imaging of the knee.

The examination of the knee has rapidly become the most important non-neurologic application of MR imaging. The widespread availability of high signal-to-noise knee coils has made routine imaging with T2-weighted sequences in both coronal and sagittal planes possible in 30 minutes. The spin-echo sequence remains the most important imaging technique, although many newer sequences have also been applied to the knee, with varying degrees of success. Important pitfalls in diagnosis, such as high signal intensity in the posterior horn of the medial meniscus and the transverse meniscal ligament, displaced buckethandle meniscal tears, and discoid menisci, can be recognized with experience. Common clinical problems that are encountered include meniscal cysts, osteonecrosis, and bone bruises. Detection of subtle injuries of the medial collateral ligament, patellar tendon, and anterior cruciate ligament requires careful observation. Intra-articular loose bodies can be reliably detected with MR imaging, and it should be recognized that localized pigmented villonodular synovitis can sometimes simulate the appearance of a loose body. MR imaging is a valuable noninvasive procedure that is complementary to arthroscopy in the evaluation of diseases of the knee.

Humans↗
PubMed1989

Iatrogenic femoral arteriovenous fistula: diagnosis with color Doppler imaging.

Color Doppler imaging was used to identify and localize arteriovenous fistulas in six patients after cardiac catheterization. In all six, there was a focal region of persistent intraarterial flow throughout the cardiac cycle at the site of the fistula, in contrast to normal arterial flow, which is forward only in systole. In all cases, turbulent pulsatile venous flow was present at the site of the fistula and nonturbulent pulsatile venous flow was present proximal to the fistula. In five cases a focal area of apparent extravascular tissue vibration was identified. In four cases the communication between the artery and vein was identified. Familiarity with these color Doppler imaging findings should allow for accurate noninvasive diagnosis of arteriovenous fistulas, at the patient's beside if necessary.

Arteriovenous Fistula↗