TE Sequences Research Articles

The loss of small objects in variable TE imaging: implications for FSE, RARE, and EPI.

The importance to MR image quality of the order of acquisition of different phase-encoded views with sequences that have variable TR and TE has been recently reported. It has been shown that the effective point spread function (PSF) may be manipulated by varying TE or TR, or both, with each phase-encoding step. This paper explores the behavior of the PSF in a variable TE sequence and its dependence on both imaging and tissue parameters. It is shown that the PSF is different for each tissue type and that its effect on tissue contrast is a function of both the shape and size of the structure. The important problem of signal loss from small objects that arises when the effective PSF is broad and the difficulty in detecting this phenomenon in practical MR images is illustrated. It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin-echo images but that this blurring may be not be obvious in practice because the noise is unaffected by the PSF. It is also shown that the signal from small lesions with short T2 can easily be lost through this blurring mechanism. The importance of signal loss from small objects and its implication for the clinical use of such sequences as fast spin-echo or rapid acquisition relaxation-enhanced and echo planar imaging is stressed.

Atomic arrangement in the mSnTe-nBi2Te3 compounds by electronic structure calculations

mAIVTe-nB2VTe3 systems form well-defined compounds, characterized by a layer structure, where the Te planes are intercalated by the metallic ones. Both ordered and random distributions of A and B atoms in the cation sites have been invoked by the experimentalists. The different possible sequences of filled and unfilled sandwiches of Te planes might induce additional disorder in the unit cell when n>1. The authors' calculations for such a system, specifically for mSnTe-nBi2Te3, based on a tight-binding effective Hamiltonian, give a clear indication in favour of ordered structures. This would explain why these systems do not form solid solutions. The results for the sequence of Te and metallic planes suggest SnBi2Te4 to be the only stable compound in the system, the others quoted in the literature being just combinations of that unit cell with the Bi2Te3 cell.

Valve and great vessel stenosis: assessment with MR jet velocity mapping.

For measurement of poststenotic jet velocities with magnetic resonance (MR) imaging, the authors reduced the echo time (TE) of the field even-echo rephasing (FEER) velocity mapping sequence from 14.0 to 3.6 msec, so minimizing the problem of MR signal loss from turbulent fluid. In vitro use of rotating disk and stenotic flow phantoms confirmed that the 3.6-msec TE sequence enables accurate measurement of jet velocities of up to 6.0 m/sec (r = .996). Peak jet velocity measurements were made with MR imaging in 36 patients with stenosis of native heart valves (n = 9), conduits (n = 19), or Fontan connections (n = 2) or with aortic coarctation (n = 6). Peak velocity measurements made with MR imaging agreed well with measurements made with Doppler ultrasound (US), which were available in 18 cases (standard deviation = 0.2 m/sec). Velocity mapping with fast-echo MR imaging is likely to have considerable importance as a noninvasive means of locating and evaluating stenoses, particularly at sites inaccessible to US, but care must be taken to prevent errors caused by malalignment, signal loss, phase wrap, or partial-volume effects.

Flow artifact reduction in MRI: A review of the roles of gradient moment nulling and spatial presaturation

In the past, flow artifacts and inconsistent depiction of vascular anatomy have represented significant problems in clinical MRI. These difficulties are now generally well addressed by the techniques of gradient moment nulling and spatial presaturation. Gradient moment nulling (GMN) is an effective method for eliminating flow artifacts in gradient echo images, while presaturation is more applicable to the same task in spin echo acquisitions. The GMN technique also has useful applications in spin echo imaging such as combating the effects of tissue and CSF motion in long TE sequences. In contrast to presaturation, however, GMN is not suitable for suppressing artifacts due to pulsatile blood flow in spin echo images.

Serial study of gadolinium‐DTPA MRI enhancement in multiple sclerosis

We performed serial baseline and gadolinium (Gd)-DTPA-enhanced MRI in 4 patients with definite multiple sclerosis. Studies were performed every month for a total of 4 scans. We obtained short TR/short TE sequences at 10 and 60 minutes after Gd-DTPA injection. All patients had multiple hyperintense lesions seen on baseline MRI with long TR/short and long TE. There was Gd-DTPA enhancement in new, enlarging, and preexisting lesions that were unchanged in size. The enhancing lesions were always seen on T2-weighted images. There was no difference in enhancement between the 10- and 60-minute studies. Six of 85 preexisting lesions enhanced whereas all new or enlarging lesions enhanced. Enhancement persisted in only 1/3 of the new or enlarging lesions, suggesting that MR enhancement is a transient phenomenon due to local temporary blood-brain barrier breakdown. Our data indicate that Gd-DTPA enhancement monitoring is more sensitive than unenhanced MRI for detecting disease activity in MS.

Magnetic resonance imaging features in melanoma

T 1 and T 2 relaxation time shortening secondary to paramagnetic compounds has been described in melanoma. The purpose of this paper is to evaluate the signal behavior of melanoma involved in various body areas using short TR, TE and long TR, TE sequences. Twenty-seven sites of melanoma were evaluated with MR using T 1-weighted and T 2-weighted techniques. Using fat and muscle signal intensities as references tissues, lesions were graded into high, low or intermediate intensity categories for each of the sequences. Four signal patterns emerged. The typical pattern characterized by high signal on T 1-weighted images and low singal on T 2-weighted images reflected T 1 and T 2 shortening. The other pattern categories comprised of lesions demonstrating low signal T 1-weighted images and high signal on T 2-weighted images, high singal on both T 1- and T 2-weighted images and lesions showing intermediate signal on either T 1- or T 2-weighted images. We observed a tendency away from the typical signal pattern in extraocular melanoma cases with only one of 14 demonstrating this pattern. Moreover, only seven of thirteen ocular melanomas exhibited such behavior. Possible explanations for this finding as well as the existence of a variety of MR appearances to melanoma are offered. We conclude that while signal patterns showing T 1 and T 2 shortening are typical of melanoma, the absence of these does not exclude the diagnosis.

Prostate cancer: Comparison of pre-operative 0.35 T MRI with whole-mount histopathology

Magnetic resonance imaging of the prostate was performed in eight patients prior to radical prostatectomy. The results of the imaging studies were then directly compared to histopathologic findings from whole-mount histologic sections. Magnetic resonance imaging identified 82% of cancers greater than 5 mm in minimal diameter. Cancers were identified as areas of decreased signal intensity compared to the high signal intensity peripheral zone on long TR TE sequences. Cancers were best detected when they involved the middle level of the gland and the posterior half of the prostate. Of the individual tumors identified by imaging, the amount of tumor involvement was underestimated by 37% and overestimated by 22% by MRI. We conclude that magnetic resonance imaging can identify prostate cancer, but has limitations as a screening modality and in accurately assessing the amount of involvement of the prostate gland by cancer.

Magnetic resonance imaging of arachnoid cysts

A retrospective review of magnetic resonance imaging (MRI)—computed tomography (CT) correlation was performed in 29 patients with arachnoid cysts. Short TR, short TE spin echo (SE) pulse sequences provided the best anatomic definition whereas multiple echo long TR, TE sequences allowed comparison of the signal intensity of the cyst with that of cerebrospinal fluid (CSF). Simple arachnoid cysts were isointense while neoplastic, hemorrhagic or inflammatory cysts were hyperintense relative to CSF. The CT differential diagnosis of an arachnoid cyst (depending upon its location) may include other cystic collections such as craniopharyngioma, epidermoid, astrocytoma, and chronic subdural hematoma. However, on MRI the combination of extraaxial location, morphological features, and signal intensity matching that of CSF allows one to make the diagnosis of an uncomplicated arachnoid cyst with confidence.

On approximation spectrum of bounded self-adjoint operators

In this formula i t is agreed that whenever the transformation (PnTPn-XI)IHn does not have an inverse we write H((PnTPn-XI)IHn)'III = +~ . Denote by ~a(T) the approximation spectrum of T and by o(T) the spectrum of T . I t is easily seen that Oa(T) is compact. In fact i t follows from the definit ion that X belongs to the complement of Oa(T) i f f there exists an in f in i te sequence of integers n I < n 2 < . . such that Hnk) II I supll((P. TP,~ X I ) I < ~o k "k '~ I f T is self-adjoint i t is clear that ~(T) C~a(T) . In this case we wil l use the following property of ~a(T) , ~ is in ~a(T) i f and }, ~ E O(PnTPnlH n) for each only i f there is a sequence { n}n=i such that 1n n and lim ~'n = X . In this paper we consider the following problem. When does the equality Oa(T) = ~(T) holds, T being a self-adjoint operator with a f in i te number of nonzero diagonals. We say that T is a 2n + 1 diagonal operator i f t i j = 0 for

Magnetic resonance imaging of the spinal cord and canal: Potentials and limitations

Preliminary experience with magnetic resonance imaging (MRI) of the spinal cord and canal in 17 patients indicates considerable promise in the diagnosis of neoplastic, degenerative, and congenital lesions. The ability to image the cord directly rather than indirectly as in myelography, the absence of bone artifact as in computed tomography, and the multiplanar capabilities indicate that MRI will be the procedure of choice in the examination of the spinal cord. Current limitations include partial-volume effects due to slice thickness and the inability to perform contiguous sections when using multiplanar techniques. The relative increase in signal from cerebrospinal fluid with long TR and TE sequences in spin-echo imaging may result in less sensitivity than in the brain for detection of cord edema and/or infarction.

Magnetic resonance imaging of the spinal cord and canal: potentials and limitations.

Preliminary experience with magnetic resonance imaging (MRI) of the spinal cord and canal in 17 patients indicates considerable promise in the diagnosis of neoplastic, degenerative, and congenital lesions. The ability to image the cord directly rather than indirectly as in myelography, the absence of bone artifact as in computed tomography, and the multiplanar capabilities indicate that MRI will be the procedure of choice in the examination of the spinal cord. Current limitations include partial-volume effects due to slice thickness and the inability to perform contiguous sections when using multiplanar techniques. The relative increase in signal from cerebrospinal fluid with long TR and TE sequences in spin-echo imaging may result in less sensitivity than in the brain for detection of cord edema and/or infarction.