MRI

Japanese: MRI

(3) MRI
a.The device applies radio waves in a static magnetic field to align the energy state of all atomic nuclei, which also aligns their spin state. If the nuclei are then left alone, they will return to a lower energy level (spin state) while emitting radio waves. This phenomenon is called relaxation. The time it takes is the relaxation time. The emitted radio waves are called echoes. These echoes are Fourier transformed and displayed as images to create MR images. MR images show the distribution of hydrogen nuclei. The signal strength of MR images is determined by the density of hydrogen nuclei and the relaxation time of hydrogen nuclei in different molecules.
Signal intensity varies depending on the repetition time (TR), echo time (TE), and pulse delay time (TI). _T1- weighted images are obtained by setting short TR and TE, and T2 _- weighted images are obtained by lengthening both. Fat and subacute hematomas have short _T1 relaxation times and show high signals in _T1- weighted images. Cerebrospinal fluid and edema, which contain more water, have long _T1 and _T2 relaxation times and show low signals in _T1- weighted images and high signals in _T2- weighted images. Gray matter contains 10-15% more water than white matter, and has more contrast. _T2- weighted images are more sensitive than _T1 -weighted images to cerebral edema, cerebral infarction, demyelinating lesions, and chronic hemorrhage (Figure 15-4-19B). Subacute hemorrhage and fat have a significantly shortened _T1 relaxation time, so they show high signals in _T1- weighted images and are more sensitive than T2 _- weighted images. FLAIR images are more effective than spin echo methods for lesions in or near the cerebrospinal fluid. Gradient echo methods are more effective for magnetic field inhomogeneities caused by blood, calcification, air, etc. (Figure 15-4-18B). The images obtained are _T2 * weighted images and are suitable for patients with a history of head trauma or bleeding. Any cross-section, such as coronal or sagittal images, can be easily obtained electrically without changing the patient's position, making it suitable for imaging diagnostics of the brain, which has a complex anatomy.
b. Contraindications and precautions Although MRI is a safe device, there are contraindications and precautions. MRI is contraindicated for people with cardiac pacemakers.
c. MRI contrast agents A chelate preparation of gadolinium (Gd), a paramagnetic metal ion (diethylenetriaminepentaacetic acid: DTPA), is used clinically as a contrast agent. As it is a paramagnetic substance, it shortens the _T1 and _T2 relaxation times, so it shows high signals in _T1 weighted images and low signals in _T2 weighted images. However, the latter requires a sufficient amount locally, and bolus intravenous administration is required. Unlike iodine contrast agents, the presence of hydrogen nuclei is essential. Gd-DTPA is administered at approximately 0.2 mL/kg body weight. Normally, Gd-DTPA does not pass through the BBB, but a contrast effect is observed in lesions where the BBB is disrupted, and normal structures without the BBB (pituitary gland, choroid plexus) are also contrasted. When renal function is impaired, it passes through the BBB, albeit slowly.
Gd-DTPA is a relatively safe contrast agent with few side effects. Side effects were observed in 3.7% of patients with atopy or bronchial asthma. The rate of side effects increased to 6.3% in patients who experienced side effects with iodine contrast agents. It can be used in children as well as adults, but should not be used in children under 6 months of age.
A recently discovered serious side effect of Gd-DTPA is nephrogenic systemic fibrosis (NSF) in patients with renal failure. It develops between 5 and 75 days after administration. In addition to the skin, it can cause fibrosis in organs throughout the body (skeletal muscles, bones, lungs, pleura, pericardium, myocardium, kidneys, testes, and dura mater). Symptoms include burning, itching, swelling, hardening, and tightness of the skin, red or black spots on the skin, yellow spots on the whites of the eyes, and difficulty moving or extending the arms, hands, legs, and feet. Main symptoms include joint stiffness, deep pain in the hip bones or ribs, and muscle weakness. Some cases have been debilitating and even fatal (Dillon, 2012).
When administering GD-DTPA to patients with the following medical histories, it is necessary to obtain their most recent glomerular filtration rate (GFR) (within the past 6 weeks): 1. Renal disease (isolated kidney, kidney transplant, renal tumor), 2. Age 60 or older, 3. History of hypertension, 4. History of diabetes, 5. Severe liver disease, liver transplant, or patients awaiting liver transplant. The incidence of NSF in patients with severe renal failure (GFR: less than 30 mL/min/1.73 ^m2 ) is 0.19-4%. GD-DTPA should not be administered to patients with a GFR of less than 30, and great caution is required when administering to those with a GFR of less than 45.
d. Imaging methods for routine brain and spinal cord examinations In Japan, due to the influence of CT, which takes transverse images, many facilities only take transverse images for brain MRI, but routine examinations should take images in three directions: transverse, sagittal, and coronal. When a coronal image is taken, lesions in the pituitary gland, inferior temporal lobe, hippocampus, and parietal regions can be easily seen. Similarly, lesions in the corpus callosum, bulbospinal junction, etc. can be seen on a sagittal image.
e.MRA
Using the gradient echo technique, moving hydrogen nuclei are visualized as high signals, and other stationary tissues as low signals, making it possible to obtain images similar to angiograms (MRA). There are two methods: TOF and PC (phase contrast). The spatial resolution of MRA is lower than that of standard angiograms. Therefore, it is difficult to identify abnormalities in small arteries, and it is not suitable for evaluating vasculitis. It also cannot capture blood flow with a relatively slow flow rate. Furthermore, it is difficult to distinguish between complete and incomplete occlusion. However, it has the advantage of being able to non-invasively capture cerebral aneurysms, arteriovenous malformations, and diseases that cause vasodilation, such as the acute stage of MELAS (Figure 15-4-19C).
f. Echo Planar Method This method can obtain information on the whole brain within 50 to 150 msec, and has paved the way for diffusion-weighted and perfusion imaging. Diffusion-weighted imaging evaluates the minute movement of water molecules, and when that movement is restricted, it is depicted as a high signal. This is the most sensitive method for ischemic lesions within 7 days, and fresh cerebral infarction is depicted as a high signal in diffusion-weighted images. It is used in many facilities not only for the brain but also for the spinal cord, and is useful for depicting spinal cord infarction. It is also sensitive to encephalitis and abscesses (brain abscess and ventriculitis), depicting them as high signal areas (Figure _15-4-20 ). For perfusion imaging, a bolus is administered intravenously, and the difference in magnetic susceptibility between the blood vessels and surrounding tissues due to the paramagnetic substance (contrast agent) entering the blood vessels creates inhomogeneity in the magnetic field, leading to a change in T2*. The perfusion state can be determined from this change in _T2 *.
Diffusion tract imaging (DTI) is a special type of diffusion imaging that can depict the white matter fiber tracts in the brain and their relationship to lesions. It can capture the minute movements of water molecules along the white matter fiber tracts and determine the direction of the white matter fiber tracts. It is useful in elucidating brain growth and white matter lesions. Functional imaging (fMRI) is a method to examine local brain activity after performing a task. It uses the BOLD method (blood oxygen level-dependent contrast). Neuronal activity requires more oxygen-containing blood in that part of the brain, which changes the ratio of oxyhemoglobin to deoxyhemoglobin in the blood. This causes a 2-3% increase in signal intensity in the veins, which is depicted as an image. It is effective in understanding the motor and sensory cortex and auditory center before surgery.
g.susceptibilty-weighted imaging (SWI)
SWI is a new MRI imaging method. Literally translated, it is the "susceptibility weighted" method, but in Japan it is commonly referred to as "SWI." As the name suggests, it is an image that emphasizes changes in magnetic susceptibility. Unlike _T2 * weighted images obtained using gradient echo, it does not image the _T2 * signal attenuation due to the magnetic susceptibility effect, but rather emphasizes image contrast by multiplying the intensity image by a phase image (phase difference due to changes in magnetic susceptibility). The phase difference is greater with higher magnetic field devices, so 3T MRI is more effective. It provides high-resolution depiction of venous blood in intracranial and brain tissue, and can sensitively detect even minute amounts of bleeding.
Intracranial MRI provides contrast between oxygenated (oxyhemoglobinized) brain tissue and deoxyhemoglobinized veins. Arteries are not visualized, and the reduction in signal intensity in veins reflects deoxyhemoglobin concentration, not blood flow. Medullary veins are clearly visible on 3T MRI (Ida et al., 2008).
h.Spectroscopy (MRS)
It is performed for various diseases using ^1H and ^31P as the target nuclides. It can visualize energy metabolism non-invasively. Diseases in which lactate is visualized are relatively limited, and it is effective for MELAS (Figure 15-4-19D). [Akira Yanagishita]
■ References <br /> Aoki, S., Hori, M., et al.: Cases requiring contrast CT: Cerebrospinal region. Japan-German Medical Journal, 56: 80-92, 2011.
Dillon WP: Neuroimging in neurologic diseases. In: Harrison's Principles of Internal Medicine 16th ed (Longo DL, Kasper DL, et al eds), pp3240-3250, McGraw-Hill, New York, 2012.
Masahiro Ida, Shunsuke Sugawara, et al.: MRI _T2 ^* weighted images and neurological disorders, susceptibility-weighted imaging and cerebrovascular disorders. Neurology, 69: 251-260, 2008.

MRI

magnetic resonance imaging

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

（3）MRI
a.装置
　静磁場中でラジオ波をあて，すべての原子核のエネルギー状態をそろえるとスピンの状態もそろえられる．その後放置すると，原子核はラジオ波を出しながら低いエネルギー準位（スピン状態）に戻ってくる．この現象を緩和とよぶ．そのときの時間が緩和時間である．放出されたラジオ波をエコーとよぶ．そのエコーをフーリエ変換し，画像として表したのがMR画像である．MR画像は水素原子核の分布を示す．MR画像の信号強度は水素原子核の密度と，異なった分子内にある水素原子核の緩和時間によって決まる．
　信号強度は繰り返し時間（TR），エコー時間（TE），パルス遅延時間（TI）によって異なってくる．T₁強調画像はTRおよびTEを短く設定することによって得られ，T₂強調画像は両者を長くすることによって得られる．脂肪および亜急性期の血腫は短いT₁緩和時間をもち，T₁強調像では高信号を示す．水分をより有する脳脊髄液や浮腫は長いT₁およびT₂緩和時間をもち，T₁強調像では低信号，T₂強調像では高信号を示す．灰白質は白質に比べて水分を10～15％よけいに有し，よりコントラストがつく．T₂強調像は，脳浮腫，脳梗塞，脱髄性病変，慢性出血に対してT₁強調像よりも鋭敏である（図15-4-19B）．亜急性期の出血，脂肪はT₁緩和時間の強い短縮をきたすので，T₁強調像で高信号を示し，T₂強調像よりも鋭敏である．FLAIR画像は髄液中あるいはその近くの病変に対して，スピンエコー方法に比べてより有効である．血液，石灰化，空気などによる磁場の不均一に対してはgradient echo法がより有効である（図15-4-18B）．得られる画像はT₂＊強調像であり，頭部外傷あるいは出血の既往のある患者に向いている．　患者の体位を変えることなく電気的に容易に冠状断像，矢状断像などの任意の断面を得られ，複雑な解剖を有する脳の画像診断に適している．
b.禁忌と注意事項
　MRIは安全な装置ではあるが，禁忌および注意事項がある．心臓ペースメーカ装着者は禁忌である．
c.MRIの造影剤
　常磁性体金属イオンであるガドリニウム（Gd）のキレート製剤（ジエチレントリアミン五酢酸：DTPA）が造影剤として臨床に使用されている．常磁性体であるので，T₁およびT₂緩和時間を短縮させるので，T₁強調像では高信号，T₂強調像では低信号を示す．ただし，後者では局所的な十分な量が必要であり，ボーラスでの経静脈性の投与が必要となる．ヨウ素造影剤とは異なり，水素原子核の存在が不可欠である．GD-DTPAは約0.2 mL/kg体重を投与する．正常ではGD-DTPAはBBBを通過しないが，BBBが破綻している病変では造影効果を認め，また，BBBのない正常構造（下垂体，脈絡叢）も造影される．腎機能低下があるときには，ゆっくりではあるが，BBBを通過する．
　GD-DTPAは比較的安全な造影剤であり，副作用は少ない．アトピーおよび気管支喘息のある例では3.7％に副作用を認める．ヨウ素造影剤にて副作用が発生した例では6.3％に上昇する．小児も，成人と同様に使用できるが，6カ月未満は使用すべきではない．
　最近になり判明したGD-DTPAの重大な副作用に，腎不全患者に発生する腎性全身性線維化症（nephrogenic systemic fibrosis：NSF）がある．投与から5～75日の間に発症する．皮膚のほかに，全身臓器（骨格筋，骨，肺，胸膜，心膜，心筋，腎，睾丸，硬膜）の線維化をきたす．皮膚の灼熱，瘙痒，腫脹，硬化，つっぱり，皮膚の赤色もしくは黒色斑，白眼の黄色斑点，腕，手，脚，足を動かすまたはその伸展に困難が伴う．関節の硬直，寛骨もしくは肋骨の深部痛，筋力低下などが主症状である．衰弱から死に至る例もある（Dillon，2012）．
　以下の病歴のある患者にGD-DTPAを投与する際には最新の（過去6週間以内）の腎糸球体濾過率（glomerular filtration rate：GFR）を求めておく必要がある．①腎疾患（孤発腎，腎移植，腎腫瘍）②60歳以上③高血圧の既往④糖尿病の既往⑤重篤な肝疾患，肝移植，肝移植待期患者
　重篤な腎不全患者（GFR：30 mL/min/1.73 m²未満）におけるNSFの発生率は0.19～4％である．GFRが30未満の患者にはGD-DTPAは投与すべきではなく，45未満では投与の際には十分な注意が必要である．
d.脳および脊髄のルーチン検査の撮像方法
　わが国では横断像で撮像されるCTの影響を受け，脳のMRIも横断像のみ撮像している施設が多いが，ルーチン検査として横断像，矢状断像，冠状断像の3方向の撮像をすべきである．冠状断像を撮像すれば，下垂体，側頭葉下部，海馬，頭頂部の病変が容易に認められる．同様に，矢状断像にて，脳梁，延髄脊髄移行部などの病変を認めることができる．
e.MRA
　gradient echo法を利用して，動いている水素原子核のみを高信号として描出し，その他の静止している組織を低信号としてみせることによって血管造影に似た画像を得ることができる（MRA）．2つの方法があり，TOF法とPC（phase contrast）法である．MRAの空間分解能は通常の血管造影に比べて落ちる．それゆえに，小動脈の異常を指摘することは困難であり，血管炎などの評価には向いていない．また，比較的流速の遅い血流もとらえることができない．さらに，完全閉塞か不完全閉塞かの区別もしにくい．しかし，脳動脈瘤や動静脈奇形，さらに血管拡張をきたす疾患，たとえばMELASの急性期（図15-4-19C）に対して，無侵襲的にとらえることができる利点はある．
f.エコープランナー法
　全脳の情報を50～150 msecの間に得ることができ，拡散強調像および灌流画像への道を開いた方法である．　拡散強調像は水分子の微小運動を評価し，その運動を制限する状況では高信号として描出される．7日以内の虚血性病変に対して最も鋭敏な方法であり，新鮮な脳梗塞が拡散強調像にて高信号として描出される．脳のみではなく，脊髄にも多くの施設で使用され，脊髄梗塞の描出に役に立っている．脳炎および膿瘍（脳膿瘍および脳室炎）に対しても鋭敏で高信号領域として描出される（図15-4-20）．　灌流画像は静脈にボーラス投与され，血管に入った常磁性物質（造影剤）のために血管と周囲組織との磁化率の違いが磁場の不均一性を生じ，T₂＊の変化をまねく．このT₂＊の変化から灌流状態を求めることができる．
　拡散線維路画像（diffusion tarct imaging：DTI）は脳の白質線維路を描出し，それと病変との関係を描出することができる特殊な拡散画像である．白質線維路に沿った水分子の微小運動をとらえ，白質線維路の方向性をとらえることができる．脳の成長と白質病変の解明に役に立つ．　機能画像（fMRI）はある課題を行った後の，脳の局所の活動性を調べる方法である．BOLD法（blood oxygen level-dependent contrast）を使用する．ニューロンの活動は脳のその部位に酸素を含んだ血液をより必要とする．それによって血液内のオキシヘモグロビンとデオキシヘモグロビンの割合を変化させる．それが静脈内に2～3％の信号強度上昇を起こし，それを画像として描出する．術前の運動感覚皮質や聴覚中枢の把握に有効である．
g.susceptibilty-weighted imaging（SWI）
　SWIは新しいMRI撮像法である．直訳すると「磁化率強調」法であるが，わが国では「SWI」で浸透している．名前のとおり，磁化率変化を強調した画像である．gradient echo法によるT₂＊強調像とは異なり，磁化率効果によるT₂＊信号減衰を画像化したものではなく，強度画像に位相画像（磁化率変化による位相差）を乗じて画像コントラストを強調している．位相差は高磁場装置ほど大きいので，3TのMRIがより有効である．頭蓋内，脳組織においては静脈血を高精細に描出し，微量の出血も鋭敏に検出する．
　頭蓋内では酸素（オキシヘモグロビン）化された脳実質組織とデオキシヘモグロビン化された静脈とのコントラストが得られる．動脈は描出されず，静脈内の信号低下も血流ではなく，デオキシヘモグロビン濃度を反映する．3TのMRIでは髄質静脈が明瞭に認められる（井田ら，2008）．
h.スペクトロスコピー（MRS）
　¹Hや³¹Pを対象核種として種々の疾患に対して施行されている．エネルギー代謝を非侵襲的に描出することができる．乳酸が描出される疾患は比較的限られており，MELASなどには有効である（図15-4-19D）．［柳下　章］
■文献
青木茂樹，堀　正明，他：造影CTが必要とされる症例2．脳脊髄領域．日獨医報，56: 80-92, 2011.
Dillon WP: Neuroimging in neurologic diseases. In: Harrison’s Principles of Internal Medicine 16th ed (Longo DL, Kasper DL, et al eds), pp3240-3250, McGraw-Hill, New York, 2012.
井田正博，菅原俊介，他：MRI T₂^＊強調画像と神経疾患 susceptibility-weighted imagingと脳血管障害．神経内科，69: 251-260, 2008.