Imaging the brain with MRI

原文

https://www.ivis.org/system/files/google_drive/node/64586/field_chpt_content/eyJzdWJkaXIiOiJcL25vZGVcLzY0NTg2XC9maWVsZF9jaHB0X2NvbnRlbnQifQ--pOLptwzcuUIL_wzQJwxyVkwnpyKbVy3rks59eruP4Oo.pdf

The increasing use of MRI in veterinary patients over the last 25 years has meant a dramatic increase in the ability to diagnose and therefore treat many diseases, especially those affecting the central nervous system. Initially, veterinary patients were scanned at medical or research facilities (unusually at unsocial hours), then a few veterinary institutions installed mostly mid- and high-field scanners and more recently low field systems designed for the veterinary market have become popular. Now most referral facilities and many first opinion practices either have their own scanner or ready access to one via mobile MR scanners in large lorries. The rapid expansion of teleradiology services has also meant that images acquired can be interpreted by experts even when the user is an MRI novice.

在過去的25年里，越來越多的獸醫(yī)病患使用核磁共振成像，這意味著診斷和治療許多疾病的能力大大提高，特別是那些影響中樞神經系統(tǒng)的疾病。最初，獸醫(yī)病患在醫(yī)療或研究機構進行掃描(通常在非社交時間)，然后一些獸醫(yī)機構大多安裝了中高場掃描儀，最近為獸醫(yī)市場設計的低場系統(tǒng)已經流行起來?，F(xiàn)在，大多數(shù)轉診機構和許多第一意見診所要么有自己的掃描儀，要么通過大型卡車上的移動磁共振掃描儀，隨時可以使用。遠程放射學服務的迅速擴展也意味著，即使用戶是核磁共振成像的新手，獲得的圖像也可以由專家進行判讀解釋。

This lecture will address MRI of the brain in veterinary patients, considering the technique and approach to interpretation. It is based on 24 years of experience with on-site MRI at the AHT, since 2000 with a 1.5 Tesla magnet.

本講座將討論獸醫(yī)病患的腦部核磁共振成像，并考慮其判讀的技術和方法。它是基于AHT 24年的現(xiàn)場核磁共振經驗，自2000年以來，使用的是1.5特斯拉的磁共振設備。

High field versus low field magnets
高場強 vs 低場強磁共振

High field, superconducting magnets (1.5T and above: 3T are being installed in some veterinary institutions) have a number of advantages including shorter scan times, better image quality and the ability to perform certain types of scan such as spectral fat suppression and diffusion weighted imaging. Disadvantages include their much higher cost, major housing requirements and closed bore. Low field, permanent magnets were originally designed for use in claustrophobic human patients and for interventional MRI but have become popular in veterinary imaging as they are cheaper to purchase and can be installed in a smaller area. However, imaging times are much longer, image quality is often poorer (especially of T2-weighted scans) and types of study can be limited. Nevertheless, the manufacturers are constantly developing new sequences to try to overcome some of these limitations.

高場超導磁體（1.5T及以上：一些獸醫(yī)機構正在安裝3T的）具有許多優(yōu)點，包括更短的掃描時間，更好的圖像質量以及執(zhí)行某些類型掃描的能力，如質譜脂肪抑制和彌散加權成像。缺點包括成本高得多，主要的外殼要求和封閉孔。低場永磁體最初是為幽閉恐懼癥病患和介入性核磁共振設計的，但由于它們購買更便宜，可以安裝在更小的區(qū)域，因此在獸醫(yī)成像中很受歡迎。然而，成像時間要長得多，圖像質量通常較差(特別是T2加權掃描)，并且研究類型可能受到限制。盡管如此，制造商仍在不斷開發(fā)新的序列，試圖克服這些限制。

Patient restraint and positioning?

病患保定和擺位

The relatively long scanning time required means that general anaesthesia is usually essential. Due to the powerful magnetic field around high field scanners the equipment used must be non-ferrous and MRI compatible. Suitable anaesthetic and monitoring equipment is available in the medical market, but is expensive. With low field systems it is usually possible to use conventional equipment but with long circuits so that any non-MRI-compatible equipment is safely outside the stray magnetic field. Shorter scans (such as ‘mini-scans’ for Chiari/syringomyelia screening) are sometimes performed under heavy sedation and on occasion it may be possible to scan comatose patients with no further chemical restraint. Image quality may be poorer but in such cases large lesions are likely to be present.

掃描所需的時間相對較長，這意味著通常需要全身麻醉。由于高場掃描儀周圍的強大磁場，所使用的設備必須是非鐵金屬和MRI兼容的。醫(yī)療市場上有合適的麻醉和監(jiān)測設備，但價格昂貴。對于低磁場系統(tǒng)，通常可以使用傳統(tǒng)的設備，但要使用長線路，這樣任何不兼容核磁共振成像的設備都可以安全地置于雜散磁場之外。較短的掃描(如篩查脊髓空洞的“迷你掃描”)有時可以在重度鎮(zhèn)靜下進行，有時可能在沒有進一步化學保定的情況下掃描昏迷的病患。圖像質量可能較差，但在這種情況下，可能存在較大的病變。

The recumbency of the patient depends on the shape of the RF coil and in some cases on the animal’s conformation or certain medical issues. Many patients are scanned in dorsal recumbency, which allows for easy positioning, but ventral recumbency can also be used and is advised in patients where there may be a risk of regurgitation. Positioning in lateral recumbency is often more difficult but can be helpful for very narrow dogs which are hard to restrain supine or prone, and for comatose patients. Positioning aids and restraint bands are helpful.

病患的躺姿取決于射頻線圈的形狀，在某些情況下取決于動物的構造或某些醫(yī)療疾病問題。許多病患采用背臥位掃描，這便于定位，但也可以采用腹臥位，并建議有食道反流風險的病患采用腹臥位。側臥位通常更困難，但對于很難仰臥保定或俯臥的非常扁的狗狗和昏迷病患來說是有幫助的。擺位輔助工具和保定帶是有用的。

Choice of radiofrequency (RF) coil?

射頻線圈的選擇

Medical MRI scanners come with a range of shapes and sizes of RF coils which are designed for use in people but which are equally suitable for small animals. Images are optimum when the area to be scanned fills the coil fairly well: images will be poor if the head is much smaller than the RF coil. The usual RF coil for examination of the head is a human extremity coil but for large and giant dogs a human head coil is required. It is also possible to obtain excellent brain scans using a surface spine coil with the dog in dorsal recumbency, as the brain is close to the coil surface. Veterinary MR scanners have their own purpose-designed RF coils.

醫(yī)學核磁共振掃描儀有一系列形狀和大小的射頻線圈，這些線圈是為人類設計的，但同樣適用于小動物。當要掃描的區(qū)域相當好地填充線圈時，圖像是最佳的；如果頭部比射頻線圈小得多，圖像就會很差。通常用于檢查頭部的射頻線圈是人的肢體線圈，但對于大型和巨型犬需要人的頭部線圈。使用表面脊柱線圈，犬背部平躺，也可以獲得出色的腦部掃描，因為大腦靠近線圈表面。獸醫(yī)磁共振掃描儀有自己專門設計的射頻線圈。

Image planes and pulse sequences

成像平面和脈沖序列?

Standard image planes for the brain are dorsal, transverse and sagittal. Each is important for different parts of the brain and all three planes should usually be used in each patient. Oblique planes are helpful for the optic nerve and orbit.

腦的標準圖像平面為背側面、橫斷面和矢狀面。每個平面對大腦的不同部位都很重要，通常每個病患都應該使用這三個平面。斜面對視神經和眼眶有幫助。

A number of different pulse sequences should be used in order to obtain the maximum information about the nature of a brain lesion. The exceptions may be when major pathology such as severe hydrocephalus or a large mass is evident and no treatment is proposed: these will be evident on the three plane localiser. Standard sequences include T1-weighting (T1W) and T2W: intermediate proton density weighting (PDW) can produce anatomically pleasing images but does not add further information. T1W is performed before and after intravenous injection of an MRI contrast medium, and most veterinary brain scans are not complete without a post-contrast T1W study. Following administration of contrast medium subtraction images can also be created, and for structures close to bone marrow, fat suppressed post-contrast T1W images are helpful. Fluid-suppressed inversion recovery (FLAIR) is a standard sequence for the brain as high signal from normal CSF is suppressed and pathology close to the ventricles or subarachnoid space will be better seen. A T2* gradient echo sequence is the most sensitive way to detect haemorrhage with high field magnets but is less sensitive in low field systems. A variety of 3D gradient echo sequences can also be used. With high field systems diffusion weighted imaging (DWI), perfusion imaging, diffusion tensor imaging and spectroscopy may be possible although clinical applications are currently limited.?

為了獲得關于腦損傷性質的最大信息，應該使用許多不同的脈沖序列。例外情況可能是當嚴重腦積水或大腫塊等主要病理明顯且未建議治療時：這些在三平面定位儀上很明顯。標準序列包括T1加權(T1W)和T2W：中間質子密度加權(PDW)可以產生解剖學上令人愉悅的圖像，但不能增加進一步的信息。T1W是在靜脈注射MRI增強劑之前和之后進行的，大多數(shù)獸醫(yī)腦部掃描如果沒有增強劑后的T1W研究是不完整的。在使用增強劑后，還可以創(chuàng)建減影圖像，對于靠近骨髓的結構，脂肪抑制的增強后T1W圖像是有幫助的。由于來自正常腦脊液的高信號被抑制，靠近腦室或蛛網膜下腔的病變將被更好地看到，因此液體抑制反轉恢復(FLAIR)是腦的標準序列。T2*梯度回波序列是高場磁體檢測出血最敏感的方法，但在低場系統(tǒng)中不太敏感。還可以使用各種3D梯度回聲序列。盡管目前臨床應用有限，但高場強系統(tǒng)的彌散加權成像(DWI)、灌注成像、彌散張量成像和質譜正逐漸成為可能。

Principles of interpretation?

判讀原則

A sound knowledge of MRI neuroanatomy and the ability to correlate visible abnormalities with clinical signs is necessary. A number of atlases and electronic facilities are available for normal brain MRI. There is a wide variation in the appearance of the brain with breed and head conformation, even within specific breeds and it is essential not to over-diagnose an incidental variant as pathology. Comparison of the right and left sides of the brain on dorsal and transverse images is very important, although it must be appreciated that slight asymmetry can be a normal finding, especially of lateral ventricle size. A number of important but incidental variants will be demonstrated in the lecture. If lesions are detected they should be described using general principles as below, and examples will be shown:

具備MRI神經解剖學的良好知識和將可見異常與臨床癥狀相關聯(lián)的能力是必要的。正常的腦MRI有一些圖譜和電子設備軟件。隨著品種和頭部形態(tài)的不同，大腦的外觀也有很大的變化，即使是在特定的品種中，也不能把偶然的變異過度診斷為病理。雖然必須認識到輕微的不對稱可能是正常的發(fā)現(xiàn)，特別是側腦室的大小，但在背側面和橫斷面上比較左右腦是非常重要的。一些重要但偶然的變異將在講座中展示。如果檢測到病變，應使用以下一般原則進行描述，并將展示示例:

Number: solitary, multiple, diffuse?

Distribution when multiple lesions are seen: e.g. unilateral or bilateral, symmetrical or asymmetrical?

Location: extra-axial, intra-axial, intra-ventricular, pituitary: white matter, grey matter or both: then a more precise description of the location e.g. cerebral, cerebellar, brainstem?

Size: mm size in three orthogonal planes?

Shape and margination: e.g. rounded, irregular etc.; well-defined, ill-defined, semidefined.?

Associated oedema: absent, minimal, mild, marked, severe etc.?

Mass effect: midline shift, ventricular distortion, herniation (sub-falcial, subtentorial, through foramen magnum, though craniectomy), secondary cervical cord changes?

Signal characteristics on difference RF pulse sequences: hyperintense, hypointense, isointense (and iso to what), signal void: suppressing?

Contrast enhancement degree and pattern: e.g. mild/moderate, marked; homogeneous, heterogeneous, ring/rim enhancement?

Other features: e.g. haemorrhage, cyst formation, cavitation, extra-cranial changes?

Change over time, if sequential studies are performed.?

Following a full description of the lesion a list of differential diagnoses is possible which permits planning for further tests if indicated.

數(shù)量：單個，多個，彌散

多發(fā)病變時分布：如單側或雙側，對稱或不對稱

位置：軸外、軸內、腦室內、垂體：白質、灰質或兩者兼而有之：然后是更精確的位置描述，如大腦、小腦、腦干

大?。喝齻€正交平面的毫米大小

形狀及邊緣：如圓形、不規(guī)則等；邊界清晰的，不清晰的，半清晰的。

相關水腫：無水腫、輕微水腫、輕度水腫、明顯水腫、嚴重水腫等。

腫物效應：中線移位，腦室變形，疝出(鐮下，幕下，通過枕骨大孔，經顱骨切除術)，繼發(fā)頸髓改變

不同射頻脈沖序列的信號特征：高強度、低強度、等強度(和相對什么等強度)，信號空洞：抑制

對比增強程度和模式：如輕度/中度，明顯；均質，非均質，環(huán)形/邊緣增強

其他特征：如出血、囊腫形成、空腔、顱外改變

如果進行了系列研究，病變隨時間的變化。

在病變的完整描述之后，可以列出鑒別診斷列表，以便在需要時計劃進一步的檢查。

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