Magnetic resonance evaluation of autonomous thyroid nodules treated by percutaneous ethanol injection

Magnetic resonance (MR) imaging was used to evaluate the effect of ultrasound-guided percutaneous ethanol injection (PEI) of autonomous thyroid nodules (ATNs). Nine patients affected with ATN (3.7–32.2 mL volume) underwent PEI (4–19 mL ethanol, subdivided in 3–6 weekly procedures). MR imaging (1.5 T) was performed before each alcoholization and 1 month after the last PEI procedure with the following parameters:T ₁-(550/12) andT ₂-weighted (2200/160) spin-echo images; 4-mm slices, 10% gap; coronal planes. A further seven patients with normal thyroid function, who had undergone PEI 6–18 months before, underwent an MR examination with the same parameters. The signal-to-noise ratio (S/N) of ATN and extranodular gland, as well as ATN volume, were evaluated on theT ₂-weighted images. OnT ₁-weighted images, ATNs appeared mostly hypointense before treatment, with hyperintense areas during treatment, and were lightly hyperintense 6–18 months after treatment. S/N onT ₂-weighted images: extranodular gland 3.5–9.2; ATNs, before treatment 13.2–19.7, before the last procedure 7.7–11.6, 1 month after the last procedure 5.6–10.9; previously treated ATNs, 4.3–8.2. No significant volume reduction was observed with MR 1 month after the last procedure. The MR examination time was about 15 minutes. In conclusion, the effect of PEI on ATNs can be evaluated with an MR examination that is not very time consuming.     

Breath-hold spin-echo MR imaging for evaluation of dynamic enhancement of native and treated hepatocellular carcinoma after intravenous Gd-DTPA administration

Using a simple modification of a standard spin-echo sequence which enables acquisition of three breath-hold images in 15 s, dynamic enhancement of 30 histologically proven hepatocellular carcinomas (17 native tumors, 6 completely necrotic tumors after nonsurgical treatments, and 7 tumors with viable and necrotic portions) after intravenous injection of gadolinium-DTPA was evaluated. Native hepatocellular carcinomas and viable portions in treated nodules showed elective enhancement in images obtained 40 s after contrast injection. Contrast between these lesions and the normal liver decreased thereafter. No contrast uptake was seen in entirely necrotic nodules and necrotic portions of treated nodules. Because of the capability to demonstrate the elective arterial blood supply typical of hepatocellular carcinoma, breath-holdT ₁-weighted spin-echo sequence should replace conventionalT ₁-weighted images for the evaluation of intravenously administered gadolinium-DTPA enhancement of this tumor before and after nonsurgical treatments.     

Quantitative T<Subscript>1</Subscript> and T<Subscript>2</Subscript> MRI signal characteristics in the human brain: different patterns of MR contrasts in normal ageing

Objective

The objective of this study was to examine age-dependent changes in both T₁-weighted and T₂-weighted image contrasts and spin-echo T₂ relaxation time in the human brain during healthy ageing.

Methods

A total of 37 participants between the ages of 49 and 87 years old were scanned with a 3 Tesla system, using T₁-weighted, T₂ weighted and quantitative spin-echo T₂ imaging. Contrast between image intensities and T₂ values was calculated for various regions, including between individual hippocampal subfields.

Results

The T₁ contrast-to-noise (CNR) and gray:white signal intensity ratio (GWR) did not change in the hippocampus, but it declined in the cingulate cortex with age. In contrast, T₂ CNR and GWR declined in both brain regions. T₂ relaxation time was almost constant in gray matter and most (but not all) hippocampal subfields, but increased substantially in white matter, pointing to an age effect on water relaxation in white matter.

Conclusions

Changes in T₁ and T₂ MR characteristics influence the appearance of brain images in later life and should be considered in image analyses of aged subjects. It is speculated that alterations at the cell biology level, with concomitant alterations to the local magnetic environment, reduce dephasing and subsequently prolong spin-echo T₂ through reduced diffusion effects in later life.

     

Measurements of T1 and T2 relaxation times of colon cancer metastases in rat liver at 7 T

The purpose of this study was to investigate the magnetic resonance imaging (MRI) characteristics of colon cancer metastases in rat liver at 7 T. A dedicated RF microstrip coil of novel design was built in order to increase the signal-to-noise ratio and, in combination with respiratory triggering, to minimize motion artifacts. T₁- and T₂-weighted MR imaging was performed to follow tumor growth. T₁-weighted images provided a good anatomical delineation of the liver structure, while the best contrast between metastases and normal liver tissue was achieved with T₂-weighted images.Measurements of T₁ and T₂ relaxation times were performed with inversion recovery FLASH and Carr–Purcell–Meiboom–Gill and inversion recovery FLASH imaging sequences, respectively, for quantitative MR characterization of metastases. Both the T₁ and T₂ of the metastases were significantly higher than those of normal liver tissue. Further, an increase in the T₁ relaxation time of the metastases was observed with tumor growth. These findings suggest that quantitative in vivo MR characterization provides information on tumor development and possibly response to therapy, though additional studies are needed to elucidate the correlation between the changes in relaxation times and tumor microenvironment.     

MR imaging of focal liver lesions: comparison between turbo spin-echo and conventional spin-echo pulse sequences

The aim of this study was to compare conventional spin-echo (CSE)T ₂-weighted (T2W) images with turbo spin-echo (TSE) T2W pulse sequences in their ability to detect focal liver lesions. Seventy-eight consecutive patients with focal liver lesions were entered into this study. All patients were imaged using the gradient-echo (GE) sequence with the breath-hold technique forT ₁-weighted (T1W) images, and CSE and TSE sequences for T2W images. Qualitative evaluation included lesion detection (number of lesions detected) and conspicuity (extent of visualization of lesional borders); quantitative evaluation included the signal-to-noise (S/N) ratio and the contrast-to-noise (C/N) ratio. TSE showed the best performance in terms of lesion detection; however, the difference between TSE and CSE was significant only in the case of benign cysts (p<0.01). Conspicuity was higher with TSE and CSE, and lower with GE. The S/N and C/N ratios of the two T2W sequences were also comparable, and better than those of GE. However, the combined use of GE and TSE resulted in improved lesion detection. The results show that, because the acquisition time is greatly reduced with TSE sequences, these should be considered as first-line approach to magnetic resonance imaging of the liver for the study of focal lesions.     

Dual-contrast single breath-hold 3D abdominal MR imaging

Object: Multiple contrasts are often helpful for a comprehensive diagnosis. In 3D abdominal MRI, breath-hold techniques are preferred for single contrast acquisitions to avoid respiratory artifacts. In this paper, highly accelerated parallel MRI is used to acquire large 3D abdominal volumes with two different contrasts within a single breath-hold. Material and methods: In vivo studies have been performed on six healthy volunteers, combining T ₁- and T ₂-weighted, gradient- or spin-echo based scans, as well as water/fat resolved imaging in a single breath-hold. These 3D scans were acquired with an acceleration factor of six, using a prototype 32-element receive array. Results: The presented approach was tested successfully on all volunteers. The whole liver area was covered by a FOV of 350 × 250 × 200 mm³ for all scans with reasonable spatial resolution. Arbitrary scan protocols generating different contrasts have been shown to be combinable in this single breath-hold approach. Good spatial correspondence with negligible spatial offset was achieved for all different scan combinations acquired in overall breath-hold times between 15 and 25 s. Conclusion: Enabled by highly parallel imaging technology, this study demonstrates the technical feasibility and the promising image quality of single breath-hold dual contrast MRI.     

In vivo mouse spinal cord imaging using echo-planar imaging at 11.75 T

Object To evaluate the feasibility of mouse spinal cord MR imaging using echo-planar imaging (EPI). Materials and methods Optimized multi-shot spin-echo-EPI sequences were compared to conventional spin-echo (c-SE) at 11.75 T and used for high-spatially resolved acquisitions and relaxation-time measurements. Results Good quality images were obtained, with clear delineation of gray and white matter. Acquisition-time gain factor was up to 6 (vs. c-SE) and resolution up to 74 × 94 μm² was achieved. T ₁ and T ₂ relaxation times were reliably measured. Conclusion High-temporally and spatially resolved mouse spinal cord EPI imaging is feasible. This technique should greatly benefit to long acquisition-time experiments (diffusion imaging) and imaging of rapidly-evolving pathologies. V. Callot and G. Duhamel equally contributed to this work.     

Dynamic snapshot gradient-echo imaging of head and neck malignancies: time dependency and quality of contrast-to-noise ratio

The purpose of this study was to evaluate the time dependency of the contrast-to-noise ratio (CNR) of head and neck malignancies during contrast-enhanced MR imaging. Then we would compare the CNR of dynamic snapshot gradient-echo (SGE) images with conventional spin-echo (SE) and fast spin-echo (FSE) sequences. Fifteen patients with squamous cell carcinomas were examined with T1W-SE, T2W-FSE, contrast-enhanced Gd-TlW-SE, and T1W-SGE sequences, the latter statically and contrast-enhanced dynamically. The CNR for all sequences and adjacent tissues was computed and the time to reach maximal CNR (T_max) was determined for dynamic studies. The CNR was time dependent with two distinct T_max at 6–18 and 60–160 s which corresponded to two different tumor enhancement patterns. Neither enhancement pattern correlated with distinct histologic findings or tumor grading. The CNR improved for the Gd-TlW-SGE images. The improvement was statistically significant in relation to T1W-SE and Gd-TlW-SE images at the floor of the mouth and at the tongue base. The good CNR of the dynamic Gd-TlW-SGE measurements justifies further investigations of this method in order to improve tumor delineation.     

A novel alternative to classify tissues from <Emphasis Type="Italic">T</Emphasis><Subscript>1</Subscript> and <Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> relaxation times for prostate MRI

Objective

To segment and classify the different attenuation regions from MRI at the pelvis level using the T ₁ and T ₂ relaxation times and anatomical knowledge as a first step towards the creation of PET/MR attenuation maps.

Materials and methods

Relaxation times were calculated by fitting the pixel-wise intensities of acquired T ₁- and T ₂-weighted images from eight men with inversion-recovery and multi-echo multi-slice spin-echo sequences. A decision binary tree based on relaxation times was implemented to segment and classify fat, muscle, prostate, and air (within the body). Connected component analysis and an anatomical knowledge-based procedure were implemented to localize the background and bone.

Results

Relaxation times at 3 T are reported for fat (T ₁ = 385 ms, T ₂ = 121 ms), muscle (T ₁ = 1295 ms, T ₂ = 40 ms), and prostate (T ₁ = 1700 ms, T ₂ = 80 ms). The relaxation times allowed the segmentation–classification of fat, prostate, muscle, and air, and combined with anatomical knowledge, they allowed classification of bone. The good segmentation–classification of prostate [mean Dice similarity score (mDSC) = 0.70] suggests a viable implementation in oncology and that of fat (mDSC = 0.99), muscle (mDSC = 0.99), and bone (mDSCs = 0.78) advocates for its implementation in PET/MR attenuation correction.

Conclusion

Our method allows the segmentation and classification of the attenuation-relevant structures required for the generation of the attenuation map of PET/MR systems in prostate imaging: air, background, bone, fat, muscle, and prostate.

     

Post-processing water-fat imaging technique for fat suppression in a low-field MR imaging system,evaluation in patients with rheumatoid arthritis

Purpose: To evaluate the feasibility of the phase difference-based post-processing water-fat imaging method for fat suppression at low-field in imaging of arthritic joints.Materials and methods: Thirty joints (wrist, 10; elbow, 10; knee, 10) in 30 patients with rheumatoid arthritis were imaged using a 0.23TMRI unit. Contrast-enhanced Tl-weighted (Tlw) three-dimensional (3D) gradient-echo (GRE) images with and without fat suppression along with short inversion time inversion-recovery (STIR) images were evaluated by two radiologists. Contrast-enhanced Tlw 3D GRE images and corresponding post-processed fat-suppressed images were scored for conspicuity and delineation of enhancing synovial hypertrophy. The uniformity of fat suppression was evaluated between Tlw 3D GRE fat-suppressed images and STIR images, and general image quality was estimated for all of the three techniques by consensus. For a quantitative analysis, the enhancing synovial hypertrophy-to-fat contrast-to-noise (CNR) values for the T1W 3D GRE images with and without fat suppression were measured. For comparison, synovial bright signal-to-fat CNR values for the STIR images were measured.Results: The post-processing water-fat imaging technique for fat suppression was successfully applied in all examinations. Conspicuity and delineation of enhancing tissue were superior in fat-suppressed Tlw 3D GRE images compared to non-fat-suppressed images (P < 0.0001). As expected, the enhancing synovial hypertrophy tissue-to-fat CNRs were significantly higher in fat-suppressed Tlw 3D GRE images compared to non-fat-suppressed images (P < 0.0001). General image quality was assessed to be best in non-fat-suppressed images, and the difference was significant compared to fat-suppressed images (P < 0.05) and STIR images (P < 0.05).Conclusion: The phase difference-based post-processing water-fat imaging technique for fat suppression can be successfully used at low-field, and it provides high-quality fat suppression images in imaging of arthritic joints.     

NMR microscopy of single neurons at 500 MHZ

Previous NMR microimaging studies at 360 MHz have demonstrated a clear differentiation between the nucleus and cytoplasm in isolated single neurons. In particular, theT ₂ of the cell nucleus is 2.5 times larger than that of the cytoplasm. In order to determine the magnitude of possibleT ₂ ^* influences on these observations, images of single cells have been obtained at 500 MHz using spin-echo and line-narrowing sequences. Comparison of the images acquired by the two sequences, and of the spin-echo images at 360 and 500 MHz, imply that anyT ₂ ^* contributions are relatively small. Consequently, the measuredT ₂ differences in spin-echo imaging represent a true difference in theT ₂ relaxation in the two cellular compartments.     

Gadofluorine 8: initial experience with a new contrast medium for interstitial MR lymphography

Rationale and objectives: Differential diagnosis of malignant and benign lymph nodes is still a problem in lymphographic imaging modalities. Plain magnetic resonance imaging (MRI) and computed tomography (CT) are inadequate for detecting metastases in normal-sized lymph nodes and for differentiating enlarged nodes. Therefore it is important to have a contrast agent that accumulates in healthy lymphatic tissue but does not accumulate in metastatic deposits.Methods: The lymphographic constrast agent Gadofluorine 8 (Schering AG, Berlin, Germany) is a lipophilic but water-soluble gadolinium complex. Lymphographic effects were investigated in guinea pigs, dogs, and tumor-bearing rabbits after interstitial (subcutaneous or intracutaneous) injection. MR imaging was performed using T₁-weighted gradient-echo sequences until 120 min after administration.Results: After interstitial injection Gadofluorine 8 accumulates in regional, lymph nodes, resulting in a pronounced increase in signal intensity in the lymph nodes. Differentiation between normal and metastatic lymph nodes was achieved.Conclusions: Gadofluorine 8 is an innovative contrast agent that can distinguish between normal and tumorous lymph nodes in interstitial MR lymphography.     

Prediction of pathological tumor volume in clinically localized prostate cancer: value of endorectal coil magnetic resonance imaging

The purpose of this study was to determine whether endorectal coil magnetic resonance imaging (MRI) enables accurate assessment of pathologic tumor volume in patients with clinically localized prostate carcinoma. Twenty-four patients with biopsy-proved prostate carcinoma underwent MRI at 0.5 T before radical prostatectomy. Tumor volumes were determined independently on axial fast-spin-echo (SE) T₂-weighted MR images and whole-mount pathology slides of the surgical specimens. At pathology, tumor volumes ranged from 0.17 to 9.42 cm³ (mean±SD, 3.11±2.99 cm³). A strong correlation (r=.944) was found between measurements of tumor volume based on MR images and pathological specimens. The error was less than 0.5 cm³ in 14 cases, in the range of 0.5–1 cm³ in 7 cases, and more than 1 cm³ in 3 cases. By using an MR tumor volume of 2 cm³ as cutoff value, extracapsular tumor spread could be predicted with a sensitivity of 81.2%, a specificity of 100%, and an accuracy of 87.5%. Tumor volume determinations based on MR images seem to be accurate enough to be helpful in clinical decision-making.     

High-resolution MR imaging appearance of colonic tissue in rabbits using an endoluminal coil

Purpose: This study assessed the value of high-resolution magnetic resonance imaging (MRI) of the distal colon by means of a dedicated endoluminal magnetic resonance receiver coil on a 1.5-T clinical scanner. Materials and Methods: To this end, single-loop, receive-only radio-frequency coils, housed in 18 F sheaths, were built. A 1.5-T clinical imager was used. A 18 French diameter internal MRI receiver coil was inserted into the distal colon in 15 New Zealand rabbits to obtain high-resolution magnetic resonance images by using T1-weighted Flash sequences with and without Fat Saturation (FS), T2-weighted True-Fisp, turbo spin-echo, and T1-weighted Flash FS after contrast media injection. Images were compared to histological sections. Results: An adequate image quality was obtained in all specimens without significant artefacts. Based on histological reports, a five-layer structure of the wall was considered normal. On different MR sequences, only two layers were identified on the images of all rabbits specimens. The nearest layer to the mucosal surface was usually seen as a hyper intense layer and likely corresponds to the mucosa. The highest difference of signal value between internal and external layers was performed on 2D Fat saturation T1 weighted gradient echo. Comparison of mean signal value between the internal and external layers was statistically different in for each sequence used in our protocol (P< 0.05). Conclusion: Dedicated endoluminal RF coil provides good spatial resolution at the region of interest. On this prospective study of in vivo rabbit, evaluation of colon walls allowed to provide detailed information.     

Magnetic resonance imaging of bovine ovaries in vitro

A sample of 20 bovine ovaries were imaged in vitro using nuclear magnetic resonance (NMR) techniques to determine the visibility of various physiologic structures. In particular, the possibility of using NMR imaging to differentiate atretic follicles from physiologically selected and ovulatory follicles was examined. Five of the 20 ovaries were preserved in formalin, whereas the remaining 15 were preserved in a saline solution and imaged within 18 hours of death. Images weighted by T₁ and T₂ proton spin relaxation rates were obtained along with some three-dimensional (3-D) data sets acquired via a fast imaging with steady-state precession technique. Physiologically different structures were easily identified in the images from their morphology, especially in the 3-D images. Weighting by T₁ and T₂ was able to separate structures in the fresh ovaries in the following manner. Atretic and cohort follicles appear dark in T₁-weighted images and bright in T₂-weighted images. Ovulatory follicles appear bright in both T₁-and T₂-weighted images, whereas prephysiologic selection follicles present an intermediate brightness in T₁-weighted images and appear dark in T₂-weighted images. The corpus luteum appears bright in T₁-weighted images and dark in T₂-weighted images, whereas cysts in the corpus luteum appear dark in T₁-weighted images and bright in T₂-weighted images. The varying brightness of the follicles at different stages of development is hypothesized to be related to different hormone and protein concentrations in the follicular fluid. For example, it is known that physiologically selected preovulatory follicles contain high concentrations of estrogens in a viscous follicular fluid. The increased viscosity may occur only when the follicle fluid contains high concentrations of estrogen and contributes to bright T₁-weighted images. The possibility of using nuclear relaxation-weighted NMR imaging for the study of follicular dynamics and other ovarian biology therefore shows great promise.     

MR T2-weighted image of subacute cerebral infarct can be isointense to the surrounding brain: MR fogging in cerebral infarct

Cerebral infarcts initially showing as markedly hyperintense on magnetic resonance (MR)T ₂-weighted images decreased in intensity and became nearly isointense to normal brain tissue in subsequent MR studies. This MR fogging was observed in 7 (23%) out of 31 cases of cortical infarct and 4 (20%) out of 20 cases of perforator infarct in the second to sixth weeks of the disease. In all fogging cases, significant contrast enhancement (CE) was seen in the fogging area after intravenous administration of MR contrast agent. The CE study is recommended in MR of cerebral infarct during the subacute and early chronic stage.     

T2-weighted breathold imaging of the liver: a quantitative and qualitative comparison of fast spin echo and half Fourier single shot fast spin echo imaging

The imaging characteristics of two EPI-hybrid breath-hold sequences, T2-weighted fast spin-echo [FSE, effective echo time (TE_eff) 138ms] and half Fourier single shot turbo spin-echo (HASTE, TE_eff 60 ms), were compared in hepatic imaging. A total of 111 patients with suspected hepatic disease were studied at 1.5 Tesla using a body phase-array coil. The signal-to-noise (S/N) and contrast-to-noise (C/N) ratios for organs and lesions were calculated and quantitatively compared. Organ delineation, visualization of anatomical structures and pathological lesions, artifacts, and total image quality were qualitatively assessed and statistically compared. The final diagnoses were metastases from colorectal, breast, and pancreatic cancer in 23/111, hepatocellular carcinoma in 15/111, cysts in 19/111, hemangiomas in 9/111, several other lesions in 7/111, and no lesions in 38/111 of the cases. A total of 139 lesion in 73% of the patients were seen while 85% of the lesions were at least 1.5 cm in size. Regarding S/Ns HASTE was significantly (P<0.03) superior to FSE with only minor (P>0.05) differences in C/Ns between the two sequences for anatomical and pathological structures. HASTE demonstrated in almost all (97.3%) of the cases no artifacts, while on fast SE imaging moderate to minor artifacts were present in 23.5–51.7% of the cases. The overall image quality and diagnostic confidence was rated significantly higher (good 43.2%, excellent 53.2%) for HASTE than for fast SE imaging (good 44.8%, excellent 17.6%). Providing comparable C/Ns for anatomical and pathological structures, breatheld HASTE imaging proved to be superior to fast SE in T2-weighted imaging of the upper abdomen regarding general image quality, and, with adequate technical prerequisites, may be a suitable substitute of fast T2-imaging techniques.     

The effect of temperature on MR relaxation times and signal intensities for human tissues

T ₁ andT ₂ relaxation times were measured on human tissue samples of adipose tissue, muscle, bone marrow and osteolytic skeletal metastases at temperatures ranging from +37°C to –10°C. Relative signal intensities forT ₁, proton density andT ₂-weighted imaging sequences were also calculated.T ₁ andT ₂ of adipose tissue decreased almost linearly with decreasing temperature while for muscle, bone marrow and metastasesT ₁ andT ₂ decreased slightly to moderately, with temperature reduction to about –5 °C at which temperature a sudden marked decrease occurred. Calculated signal intensities showed a decrease in image contrast with temperature reduction and reversal of contrast between adipose tissue and the other tested tissues with all imaging sequences at temperatures around 0°C.     

Temperature dependence of the magnetic volume susceptibility of human breast fat tissue: an NMR study

Object  

Proton resonance frequency shift (PRFS)-based MR thermometry (MRT) is hampered by heat-induced susceptibility changes when applied in tissues containing fat, e.g., the human breast. In order to assess the impact of fat susceptibility changes on PRFS-based MRT during thermal therapy in the human breast, reliable knowledge of the temperature dependence of the magnetic volume susceptibility of fat, dχ_fat/dT, is a prerequisite. In this work we have measured dχ_fat/dT of human breast fat tissue, using a double-reference method to ensure invariance to temperature-induced changes in the proton electron screening constant.