A review of substitute CT generation for MRI-only radiation therapy

Abstract

Radiotherapy based on magnetic resonance imaging as the sole modality (MRI-only RT) is an area of growing scientific interest due to the increasing use of MRI for both target and normal tissue delineation and the development of MR based delivery systems. One major issue in MRI-only RT is the assignment of electron densities (ED) to MRI scans for dose calculation and a similar need for attenuation correction can be found for hybrid PET/MR systems. The ED assigned MRI scan is here named a substitute CT (sCT). In this review, we report on a collection of typical performance values for a number of main approaches encountered in the literature for sCT generation as compared to CT. A literature search in the Scopus database resulted in 254 papers which were included in this investigation. A final number of 50 contributions which fulfilled all inclusion criteria were categorized according to applied method, MRI sequence/contrast involved, number of subjects included and anatomical site investigated. The latter included brain, torso, prostate and phantoms. The contributions geometric and/or dosimetric performance metrics were also noted. The majority of studies are carried out on the brain for 5–10 patients with PET/MR applications in mind using a voxel based method. T1 weighted images are most commonly applied. The overall dosimetric agreement is in the order of 0.3–2.5%. A strict gamma criterion of 1% and 1mm has a range of passing rates from 68 to 94% while less strict criteria show pass rates > 98%. The mean absolute error (MAE) is between 80 and 200 HU for the brain and around 40 HU for the prostate. The Dice score for bone is between 0.5 and 0.95. The specificity and sensitivity is reported in the upper 80s% for both quantities and correctly classified voxels average around 84%. The review shows that a variety of promising approaches exist that seem clinical acceptable even with standard clinical MRI sequences. A consistent reference frame for method benchmarking is probably necessary to move the field further towards a widespread clinical implementation.

Registration free automatic identification of gold fiducial markers in MRI target delineation images for prostate radiotherapy

“…Multiple methods for creating sCT for prostate RTP has been presented. [6][7][8][9][10] The dosimetric accuracy in high-quality sCTs for prostate cancer RTP is sufficient. 6,7,[11][12][13] The sCTs can also be used as reference images for image-guided RT.…”

Wykorzystanie obrazowania rezonansem magnetycznym w radioterapii (planowanie leczenia i procedury IGRT) oraz związana z tym rola elektroradiologa

“…Z tego powodu nie jest możliwa kalkulacja rozkładu dawki promieniowania bezpośrednio na obrazach MRI. Dodatkowo tkanka kostna obrazowana w konwencjonalnych sekwencjach MR (tj.T1 i T2) daje bardzo słaby sygnał, podobnie jak powietrze, mimo, że wykazują skrajne wręcz gęstości elektronowe i zdolności pochłaniania promieniowania jonizującego [11,12].…”

“…synthetic-bądź pseudo-CT (ang. sCT), czyli syntetyczne CT, z obrazów MRI [12]. Dwie główne strategie pozyskiwania sCT to:…”

“…Progowy sposób wykorzystuje stałą wartość relaksacji danej tkanki, aby zróżnicować i podzielić je na kategorie. Metoda probabilistyczna odnosi się do metod statystycznych takich jak statystyka baeysowska, losowe pola markowa, sieci neuronowe, liniowa analiza dyskryminacyjna, analiza głównych składowych, bądź innych, które pozwalają na określenie prawdopodobieństwa, że voxel należy do danej klasy tkanek [12,13].…”

“…metoda atlasu i metoda oparta o kalkulacje voxela, bywają stosowane jako hybrydy [12,15,16]. Różnice w rozkładach dawek kalkulowanych na podstawie CT i sCT są akceptowalne dla dalszego procesu leczenia -sięgają ok. 2% .…”