A short overview of MRI artefacts

Abstract

Many different artefacts can occur during magnetic resonance imaging (MRI), some affecting the diagnostic quality, while others may be confused with pathology. An artefact is a feature appearing in an image that is not present in the original object. Artefacts can be classified as patientrelated, signal processing-dependent and hardware (machine)-related. This article presents an overview of MRI artefacts and possible rectifying methods. IntroductionArtefacts remain a problematic in magnetic resonance imaging (MRI). Some affect the quality of the examination, while others may be confused with pathology.An artefact is a feature appearing in an image that is not present in the original object. Depending on their origin, artefacts are typically classified as patient-related, signal processingdependent and hardware (machine)-related.It is important to recognise these artefacts and have a basic understanding of their origin, especially those mimicking pathology. In this article emphasis is placed on recognition of artefacts and possible rectifying methods. Patient-related MR artefacts Motion artefactsMotion is one of the most common artefacts in MR imaging, causing either ghost images or diffuse image noise in the phase-encoding direction. The reason for mainly affecting data sampling in the phase-encoding direction is the significant difference in the time of acquisition in the frequency-and phase-encoding directions. Frequency-encoding sampling in all the rows of the matrix (128, 256 or 512) takes place during a single echo (milliseconds). Phase-encoded sampling takes several seconds, or even minutes, owing to the collection of all the k-space lines to enable Fourier analysis. Major physiological movements are of millisecond to seconds duration and thus too slow to affect frequency-encoded sampling, but they have a pronounced effect in the phase-encoding direction.Periodic movements such as cardiac movement and blood vessel or CSF pulsation cause ghost images, while non-periodic movement causes diffuse image noise (Fig. 1). Ghost image intensity increases with amplitude of movement and the signal intensity from the moving tissue.Several methods can be used to reduce motion artefacts, including patient immobilisation, 1 cardiac and respiratory gating, 2 signal suppression of the tissue causing the artefact, 1 choosing the shorter dimension of the matrix as the phase-encoding direction, view-ordering or phase-reordering methods 3 and swapping phaseand frequency-encoding directions 1 to move the artefact out of the field of interest. FlowFlow can manifest as either altered intravascular signal (flow enhancement or flow-related signal loss), or flow-related artefacts (ghost images or spatial misregistration).Flow enhancement, also known as inflow effect, is caused by fully magnetised protons entering the imaged slice while the stationary protons have not fully regained their magnetisation. A short overview of MRI artefacts L J Erasmus MB ChB D Hurter MB ChB M Naudé MB ChB H G Kritzinger MB ChB

Magnetinio Rezonanso Tomografijos Vaizdinimo Artefaktai

“…Fazę koduojantis registravimas trunka kelias sekundes ar net minutes. Pagrindiniai judesiai, trunkantys nuo kelių milisekundžių iki kelių sekundžių, yra per lėti, kad paveiktų dažnio koduojamą registravimą, tačiau turi ryškų efektą fazės koduojama kryptimi [2].…”

“…Ji naudojama teorija ir praktika 2014 -T. 20 (Nr. 2) širdies magnetinio rezonanso tomografijoje, rečiau -smegenų (artefaktams dėl miego arterijų pulsacijos mažinti) bei krūtinės ląstos tyrimuose (stuburo kanalo skysčio pulsacijos artefaktams mažinti). Gradiento momento anuliavimas ir įsotinimo juostos galėtų būti veiksmingesnės ir universalesnės technikos pulsacijos keliamiems artefaktams mažinti [4].…”

“…Šis artefaktas pasireiškia Spin-echo ir Gradient-echo sekose. Poslinkiui ypač jautrūs didelio magnetinio lauko stiprumo MR tomografai [2,8].…”

“…Už vaizdinimo lauko teorija ir praktika 2014 -T. 20 (Nr. 2) ribų esantys audiniai atsiranda kitoje atvaizdo pusėje, fazės koduojama kryptimi.…”

“…Artefaktas atrodo kaip dauginės, reguliariai išsidėsčiusios lygiagrečios tamsios ir šviesios žiedinės struktūros, po truputį išblėstančios periferijos link [2]. Dažnai Gibbso artefaktas matomas T2 režime, tarp hiperintensinio smegenų skysčio ir hipointensinių nugaros smegenų, kur imituoja centrinio nugaros smegenų kanalo dilataciją (hidromieliją).…”