A retrospective study comprised 45 patients, with 51 HCC lesions, who underwent microwave ablation and were followed up with MRI. Data were collected from our PACS in the period between January 2017 and December 2018. All patients signed a written informed consent before the MRI examination. The first follow-up MRI examination was done within 4 to 6 weeks after the ablation procedure as a baseline study and for early detection of any recurrence or residue. Further follow-up MRI was done after 3 to 4 months in nonviable cases.
HCC patients underwent MWA procedure as the only therapeutic intervention.
MRI contraindications include claustrophobia and contraindication to contrast media. Also, patients with other liver tumors were excluded and patients who underwent other therapeutic procedures or systemic therapy.
The MRI examination was performed on 1.5 Tesla Philips MRI scanner (Achieva, Netherlands). Pre-contrast sequences include axial T1 with acquisition parameters (TR 250 ms, TE 30 ms, flip angle 15°, FOV 300–350, slice thickness 7), axial T2 with acquisition parameters (TR 1000 ms, TE 80 ms, flip angle 90°, FOV 300–350, slice thickness 7), SPAIR, and coronal T2-weighted images. The diffusion imaging were then acquired using respiratory-triggered single-shot spin echo echoplanar sequence technique with the following acquisition parameters: TR 1700 ms, TE 76 ms, slice thickness 8 mm, b values 0, 500, 1000 mm2/s). 3D T1 fat-suppressed gradient sequence (THRIVE) images were used for dynamic images after manual Gd-DTPA injection (0.1 mmol/kg body weight) flushed with 20 ml saline. Acquisition parameters were (TR 4.4 ms, TE 2.2 ms, flip angle 10°, FOV 300–350, slice thickness 2–3 mm). The dynamic sequence consisted of a pre-contrast series followed by four post-contrast series with 19–20-s intervals (17 s for image acquisition and 3–4 s for rebreathing). All series were acquired at end expiration to reduce the incidence of misregistration artifact. The delayed phase was then acquired using the same parameters after 5 min of contrast injection.
Post processing and image analysis were performed by three experienced radiologists with 9, 11, and 12 years of experience in abdominal imaging. Each reader interpreted the images independently using the available workstation (Phillips Extended MR workspace 18.104.22.168 Netherlands 2011). The morphological features of the ablation zone (size, site, and signal pattern at different sequences) were assessed followed by assessment of the enhancement pattern of the ablation zone and areas suspicious for malignancy.
Interpretation of the dynamic study
The LI-RADS TR algorithm was used in the assessment of treatment response and patient’s categorization.
Arterial phase hyperenhancement (APHE)
This is the enhancement of the entire or a part of a lesion relative to the liver parenchyma during the early or late arterial phase . It is then classified into rim or non-rim enhancement. Rim enhancement is evident along the observation periphery , and this type of enhancement is usually seen in malignancies other than HCC (categorized as LR-M) . Non-rim enhancement is considered as a major feature of HCC in LI-RADS . To confirm the presence or absence of enhancement, we used the subtraction technique to remove the pre-contrast high-T1 signal, and thus confirming that the hyperintensity is due to true enhancement.
Delayed washout is a visually assessed reduction in enhancement (the entire or a part of an observation), relative to the liver . It is classified into non-peripheral or peripheral washout. Non-peripheral washout is one of the major features of HCC at LI-RADS, while peripheral washout is considered a feature of other malignancies and categorized LR-M .
Delayed enhancing capsule
This is a major feature of HCC.
Interpretation of the diffusion images
For qualitative assessment, visual assessment of areas of interest at diffusion images was done, then correlated with the ADC map to confirm whether it represents true diffusion restriction or T2 shine through.
For quantitative assessment and ADC measurement, the ADC maps were generated on the workstation from the three b values used. The region of interest (ROI) was drawn over the area to be measured. We measured the ADC values of the entire ablation zones, the surrounding parenchyma (in case of parenchymal enhancement), and any area suspicious for malignancy.
The diffusion observation was categorized into the following:
Definitely restricted: if the observation exhibited high signal in diffusion images and low signal at the ADC map
Facilitated diffusion: when the observation exhibited low signal (hypo or isointense to liver parenchyma) in diffusion images and high signal at the ADC map
Uncertain: when the observation exhibited hyperintensity in diffusion images yet with no corresponding hypointensity on ADC. In this case, the tiebreaking rules of LI-RADS TR algorithm were applied stating that, if the reader was unsure between two categories, choose the one that reflect the lower certainty. Hence, the uncertainty in diffusion analysis was considered negative for malignancy.
Interpretation of perilesional enhancement
Early after ablation, the hepatic parenchyma close to the ablation zone may show inflammatory changes and hyperemia. This is an expected benign imaging finding and appears as ill-defined arterial enhancement of the hepatic parenchyma that usually persists in the delayed phases of the study .
A well-defined, delayed enhancing rim is also considered as treatment-related expected enhancement and caused by scar tissue .
Well-defined nodular, mass-like, or thick irregular tissue enhancement at the margin of the ablation zone may suggest tumor recurrence.
The size of the enhancing component or areas of delayed washout was measured at its single longest diameter not crossing the non-enhancing area.
The patients’ categories
The patients’ categories following the LI-RADS TR algorithm are listed below.
LR-TR non evaluable: Poor image quality or degraded images hindering proper assessment
LR-TR nonviable “probably or definitely nonviable”: Is considered in cases with no pathological enhancement or if there is treatment-specific expected enhancement. Also, if complete disappearance of the ablation zone is noted, it is considered equivalent to non-enhancement.
LR-TR equivocal “equivocally viable”: If the enhancement was atypical for either treatment-specific enhancement or tumoral viability
LR-TR viable “probably or definitely viable”: Mass-like, nodular, or thick irregular tissue showing APHE or delayed washout usually at the margins or less commonly within the treatment zone. In case with enhancement similar to the pretreatment pattern, it is also considered as LR-TR viable.
Tiebreaking rules: In case of being unsure between two categories, the reader chooses the category that reflects the lower certainty (if lower certainty of viability or nonviability, choose LR-TR equivocal).
Ancillary features favoring malignancy like T2 hyperintensity and restricted diffusion are still not included in the Treatment Response Algorithm.
The reference standard was the dynamic MRI features according to LI-RADS treatment response v2018. Follow-up was considered in nonviable and equivocal categories, while tissue biopsy and pathological confirmation were not done due to its technical difficulty and guided by clinical practice and the general condition of the patients.
The data were analyzed using the statistical package SPSS. Data were expressed as means and standard deviations. For comparing categorical data, unpaired student t test to calculate the P values between the mean ADC of the different study groups. P value ≤ 0.05 was considered as significant.
ROC Curves were done for diffusion and ADC, which is commonly used to characterize the sensitivity/specificity tradeoffs for a binary classifier. The area under the curve is viewed as a measure of a forecast’s accuracy that was donated by AUC.