Magnetic Resonance in Medical Sciences Vol. 14(2015) No. 1

We illustrate the fundamental theoretical principles of arterial spin-labeling (ASL) magnetic resonance imaging (MRI) and show a system that employs the second version of quantitative imaging of perfusion using a single subtraction (Q2TIPS) to quantify cerebral blood flow (CBF). We also discuss the effects of the parameters used in Q2TIPS on CBF values as measured with ASL-MRI.

N-acetylaspartate (NAA) appears in a prominent peak in proton magnetic resonance spectroscopy (¹H-MRS) of the brain. Exhibition by NAA of time-dependent attenuation that reflects energy metabolism during the acute stage of cerebral ischemia makes this metabolite a unique biomarker for assessing ischemic stroke. Although magnetic resonance (MR) imaging is a powerful technique for inspecting the pathological changes that occur during ischemic stroke, biomarkers that directly reflect the drastic metabolic changes associated with acute-stage ischemia are strongly warranted for appropriate therapeutic decision-making in daily clinical settings. In this review, we provide a brief overview of NAA metabolism and focus on the use of attenuation in NAA as a means for assessing the pathophysiological changes that occur during the acute stage of ischemic stroke.

Purpose: We propose a new motion tracking method that encodes an object’s position information using tagging magnetic resonance (MR) images as digital codes, and we assess the method’s feasibility in stationary and moving phantoms. The encoding and decoding of tag patterns employ principles of spread spectrum communication.
Methods: We used a segmented fast low angle shot cine sequence (FLASH) with spatial modulation of magnetization (SPAMM) preparation pulses to encode position information as 7-bit code words and used this spread code to decode the position information. To make 7-bit code words using tag images, we adjusted the flip angle and phase of the 4 composite radiofrequency (RF) pulses and the gradient magnetic field strength in the SPAMM pulse to generate 7 types of tag patterns. The proposed method required 7 acquisitions with 7 types of tag patterns. We compared results with images obtained by conventional tagging in stationary and moving phantom experiments.
Results: In a stationary phantom, the proposed method showed the same ability to identify pixel position as conventional tagging method using improved SNR images with the average of 7 acquisitions. In a moving phantom, pixel position was successfully decoded by the proposed method on a pixel-by-pixel basis. In this method, the motion of the phantom was detected by the simple calculation of the correlation coefficient of the code words.
Conclusion: We introduced a spread spectrum communication technique to tagging MR imaging that regards tag patterns as digital codes, and we demonstrated the method’s potential to detect pixel position in sub-pixel resolution.

Purpose: We investigated sensations experienced by a large number of subjects during magnetic resonance (MR) imaging examinations using a 7-tesla scanner and slow table-feed speed.
Methods: After examinations at 7T, 504 of 508 consecutive subjects completed questionnaires using an 11-point scale to rate 14 potential sensations and symptoms during table movement and stationary positioning of the table. We compared scores among the sensations and between table conditions and the mean values of the scores with those reported in previous studies and examined correlations between the scores and subject characteristics.
Results: Vertigo and feelings of curving or leaning in the right or left direction during table movement were experienced frequently and markedly compared to other sensations and sensations experienced when the table was stationary (P < 0.01) and were correlated with subject age and examination time (P < 0.05). However, moderate to severe (scores of 5 to 10) vertigo and a curving/leaning feeling during table movement were noted in only 10.5% (vertigo) and 10.9% (curving/leaning) of subjects, and the mean vertigo score, 1.26, appeared to be substantially lower than that reported in a previous study. Reports of a metallic taste, nausea, and light flashes were significantly rarer and weaker than other sensations (P < 0.05).
Conclusion: Vertigo and feelings of curving during table movement were the most frequent sensations reported during MR imaging examination at 7T. However, the occurrence and severity were low and mild, presumably because of the slow table-feed speed, which suggests that most patients and volunteers found discomfort at 7T acceptable.

Metabolite diffusion is expected to provide more specific microstructural and functional information than water diffusion. However, highly accurate measurement techniques have still not been developed, especially for reducing motion artifacts caused by cardiac pulsation and respiration. We developed a diffusion-weighted line-scan echo-planar spectroscopic imaging (DW-LSEPSI) technique to reduce such motion artifacts in measuring diffusion-weighted images (DWI) of metabolites. Our technique uses line-scan and echo-planar techniques to reduce phase errors induced by such motion during diffusion time. The phase errors are corrected using residual water signals in water suppression for each acquisition and at each spatial pixel specified by combining the line-scan and echo-planar techniques. We apply this technique to a moving phantom and a rat brain in vivo to demonstrate the reduction of motion artifacts in DWI and apparent diffusion coefficient (ADC) maps of metabolites. DW-LSEPSI will be useful for investigating a cellular diffusion environment using metabolites as probes.

Purpose: We measured T₁ and T₂ values of cerebral postmortem magnetic resonance (PMMR) imaging and compared the data of cadavers with that of living human subjects.
Materials and Methods: We performed PMMR imaging of the brains of 30 adults (22 men, 8 women; mean age, 58.2 years) whose deaths were for reasons other than brain injury or disease at a mean of 29.4 hours after death. Before imaging, the bodies were kept in cold storage at 4°C (mean rectal temperature, 15.6°C). We measured T₁ and T₂ values in the brain bilaterally at 5 sites (bilateral caudate nucleus, putamen, thalamus and gray matter and white matter of the frontal lobe) and compared the data of PMMR imaging with that from MR imaging of the corresponding sites in 24 healthy volunteers (9 men, 15 women; mean age, 51.8 years). We also investigated the influence of body temperature on T₁ and T₂ values.
Results: Compared with MR imaging findings in the living subjects, PMMR imaging showed significantly shorter T₁ values in the caudate nucleus, putamen, thalamus and gray matter and white matter of the frontal lobe and significantly longer T₂ values in the gray matter and white matter of the frontal lobe; T₂ values in the caudate nucleus, putamen, and thalamus showed no such differences. T₁ values correlated significantly with body temperature in all 5 brain sites measured, but T₂ values did not.
Conclusion: Compared with findings of cerebral MR imaging in living adult subjects, those of PMMR imaging tended to demonstrate shorter T₁ values and longer T₂ values. We attribute this to increased water content of tissue, reduced pH, and reduced body temperature after death.

Purpose: Magnetic resonance (MR) imaging offers the highest sensitivity for detecting bone necrosis. We evaluated osteonecrosis in rabbit models by calculating the percentage of fat to (fat + water) [F/(F+W)] on MR spectroscopy (MRS) and compared MR spectroscopy and imaging findings with corresponding histological results.
Methods: To model the natural course of articular osteonecrosis, we removed the fourth tarsal bone in 45 rabbits, froze it for 5 min in liquid nitrogen to produce complete cellular necrosis, and then replaced the bone into the knee joint. We performed Carr-Purcell-Meiboom-Gill proton spectroscopic imaging to assess necrotic bone at 3 days and one, 2, 3, 4, 8, 12, 16, and 20 weeks after osteonecrosis and calculated the percentage of F/(F+W) of each bone. We also performed conventional T₁- and T₂-weighted imaging and compared all data to histological findings to analyze the natural course of necrosis.
Results: T₁-weighted MR imaging demonstrated obvious low signal intensity at 2 to 8 weeks and recovery at 12 to 20 weeks, whereas T₂-weighted imaging demonstrated inconsistent intensities throughout the period. The postoperative percentage of F/(F+W) measured using line scan MRS decreased to 8.88% at 3 weeks, 6.22% at 8 weeks, and 34.40% at 20 weeks results that were mostly consistent with MR imaging findings. Histological findings demonstrated complete absence of osteocyte nuclei and loss of osteoid-osteogenesis at 3 to 8 weeks. Recovery of bone marrow was identified as an increase in the area of fat after 12 weeks.
Conclusion: Osteonecrosis delineated by T₁-weighted MR imaging demonstrated fat content in the bone marrow that correlated with histology. The present MRS modality can be used to calculate the percentage of F/(F+W) of osteonecrosis to enable objective assessment of recovery and quantification of osteonecrosis to provide a numerical value for osteonecrosis.

Objective: We used magnetic resonance (MR) imaging and hormonal levels to evaluate the influence of chemotherapy for cervical cancer on female pelvic reproductive organs.
Materials and Methods: We retrospectively evaluated 16 pre- and 11 postmenopausal patients with cervical cancer who underwent neoadjuvant chemotherapy (NACT) and radical surgery. We evaluated morphological changes in the uterus and ovaries by MR imaging both quantitatively and qualitatively, measuring the volume of the uterine body and bilateral ovaries, endometrial thickness, and signal intensity of the myometrium and bilateral ovaries and assessing visibility of the junctional zone and bilateral ovarian follicles. We compared both quantitative and qualitative factors between pre- and post-NACT. Pre- and post-NACT hormonal values of estradiol, progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) of 8 patients in the premenopausal group were obtained and analyzed statistically.
Results: In the premenopausal group, we observed a statistically significant decrease in all quantitative parameters as well as in the visibility of the left ovarian follicle. In the postmenopausal group, only endometrial thickness changed significantly. Premenopausal patients showed a statistically significant decrease in levels of progesterone, FSH, and LH after chemotherapy.
Conclusions: MR demonstrated changes in the uterus and ovaries in premenopausal subjects who underwent chemotherapy that resembled those changes classically reported in physiological postmenopausal subjects. These changes are likely due to ovarian toxicity and secondary hormonal changes. MR imaging might be a valuable tool for obtaining information regarding chemotherapy-induced infertility.

Decreased absolute tumor blood flow (TBF) measured by arterial spin labeling perfusion imaging (ASL-PI) on 3-tesla magnetic resonance imaging demonstrated the reduced size and growth hormone (GH) secretion of a large GH-producing pituitary adenoma in a 32-year-old man in response to octreotide therapy. The study shows the usefulness of ASL-PI in providing a biomarker of the antiangiogenic effect of octreotide.

We measured the size of the endolymph using a newly proposed 10-min magnetic resonance (MR) imaging protocol and compared values with those of the previously reported 31- and 17-min protocols in 15 patients. Values for the 3 protocols did not differ significantly in either the cochlea or vestibule. Correlation among the 3 protocols was strong or relatively strong. Evaluation of endolymphatic hydrops is feasible using the newly proposed 10-min protocol.