AJR 2003; 181:1635-1640
© American Roentgen Ray Society
Interpretation of CT and MRI After Radiofrequency Ablation of Hepatic Malignancies
Piyaporn Limanond1,
Peter Zimmerman,
Steven S. Raman,
Barbara M. Kadell and
David S. K. Lu
1 All authors: Department of Radiology, David Geffen School of Medicine at UCLA,
10833 Le Conte Ave., Los Angeles, CA 90095-1721.
Received December 12, 2002;
accepted after revision April 3, 2003.
Address correspondence to P. Limanond.
Presented at the annual meeting of the American Roentgen Ray Society, San
Diego, CA, May 2003.
Introduction
Radiofrequency ablation is currently a widely used minimally invasive
treatment for primary and secondary hepatic malignancies in patients
ineligible for surgery. Imaging is therefore being performed after ablation
with increasing frequency. Radiologists need to be familiar with the
appearance of lesions on CT and MRI after ablation to evaluate the success of
treatment, detect residual or recurrent tumor, and diagnose new lesions.
Pathophysiology of Areas After Radiofrequency Ablation
Radiofrequency ablation uses alternating electric current to produce ionic
agitation and frictional heat in targeted tissue
[1–3],
resulting in coagulation necrosis and tumor desiccation. A recent ablation
area may have a circumferential rim of peripheral hyperemia from host
inflammatory response to thermal injury, which may subside over time
(Fig. 1). Understanding the
pathophysiology of lesions after radiofrequency ablation is essential for
image interpretation.
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Fig. 1. —Liver of animal model with recent radiofrequency ablation
lesion. Photograph of gross pathologic section shows central white area of
coagulation necrosis and tumor desiccation. Notice circumferential red rim of
peripheral hyperemia from host inflammatory response to thermal ablation.
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Imaging Studies
Multiphasic contrast-enhanced CT and dynamic MRI are accepted as reliable
modalities for evaluating the adequacy of radiofrequency ablation and early
detection of tumor recurrences
[1–4].
Dromain et al. [5] reported a
higher sensitivity in early detection of local recurrence on MRI than on CT
but without significant differences (p = 0.12). A baseline study
should be obtained within the first week after the procedure. Subsequent
follow-up should be performed every 3 months for 1 year, and every 6 months
thereafter. In equivocal cases, follow-up may be more frequently
performed.
Short-Term Follow-Up
CT
Findings of unenhanced CT show the area after ablation as a low-density
lesion occupying the entire volume of original tumor with a variable central
area of high attenuation from tumor desiccation. An additional 5–10 mm
of circumferential margin is preferred, especially in liver metastases, which
do not have clear lesion-to-liver interface and tend to have adjacent
microscopic tumor cells. In a hepatocellular carcinoma with an underlying
cirrhotic liver, radiofrequency heat may be concentrated within a
well-encapsulated tumor and does not usually extend to adjacent high-impedance
cirrhotic tissue [3].
Therefore, a successful radiofrequency ablation area of hepatocellular
carcinoma tends to be the same size as the original tumor.
On enhanced CT, the ablation area is expected to be nonenhancing. However,
a recent ablation area may have an enhancing rim related to hyperemia from
thermal injury
[1–3].
This is more typically present on the arterial dominant phase (Fig.
2A,
2B,
2C,
2D) but may be present on the
portal dominant phase or both phases, depending on technical factors. This
hyperemia may be irregular in contour and may vary in thickness but should
completely surround the ablation area and should subside over time, generally
disappearing after several months. Discrete nodular noncircumferential
enhancement, especially at the ablation margin, is suspicious for residual or
recurrent tumors (Fig. 3A,
3B,
3C). Differentiation of
reactive hyperemia from residual tumors may be difficult. Careful comparison
with imaging before ablation and close follow-up are necessary in equivocal
cases.
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Fig. 2A. —73-year-old woman with successful ablation of hepatocellular
carcinoma seen on 1-day follow-up CT scan. Arterial dominant phase axial CT
scan obtained before ablation shows small hypervascular lesion
(arrow) in posterior segment, right lobe.
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Fig. 2B. —73-year-old woman with successful ablation of hepatocellular
carcinoma seen on 1-day follow-up CT scan. Unenhanced axial CT scan obtained 1
day after ablation shows low-attenuation lesion with central high attenuation
compatible with radiofrequency ablation area. Note that treated lesion is
larger than original, representing desired surgical margin and small amount of
perihepatic fluid that resolved on subsequent study.
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Fig. 2C. —73-year-old woman with successful ablation of hepatocellular
carcinoma seen on 1-day follow-up CT scan. Enhanced CT scan obtained on same
date as B shows nonenhancing radiofrequency ablation area with
circumferential peripheral enhancement on arterial dominant phase.
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Fig. 3A. —45-year-old man with hepatocellular carcinoma and residual
tumor seen on short-term follow-up CT scan after ablation. Arterial dominant
phase axial CT scan obtained immediately after radiofrequency ablation shows
noncircumferential thick enhancing area at perivascular margin.
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Fig. 3B. —45-year-old man with hepatocellular carcinoma and residual
tumor seen on short-term follow-up CT scan after ablation. Because of findings
on A, close follow-up CT scan was obtained at 1 month and shows
enlarging area of enhancement.
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Fig. 3C. —45-year-old man with hepatocellular carcinoma and residual
tumor seen on short-term follow-up CT scan after ablation. CT scan obtained
after second session of ablation 1 month after B shows successful
ablation with extensive ablated margin up to inferior vena cava and right
hepatic vein. Notice right pleural effusion, a common finding after ablation
of lesion adjacent to diaphragm.
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A linear ablation tract may be seen along the passage of the radiofrequency
probe. The tract should be a nonenhancing (except for peripheral hyperemia)
lesion and should not be misinterpreted as a new lesion. Another potential
finding is a peripheral-based wedge-shaped area of enhancement on the arterial
dominant phase adjacent to the ablation area (Fig.
4A,
4B). This finding is likely a
perfusion alteration due to small arteriovenous shunts from needle injury
[1]. The characteristics of a
peripheral-based wedge-shaped pattern are helpful to differentiate this entity
from residual tumor.
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Fig. 4A. —61-year-old man with hepatocellular carcinoma and perfusion
alteration from radiofrequency ablation on short-term follow-up CT. Arterial
dominant phase axial CT scan shows non-enhancing radiofrequency ablation area
(arrow).
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Fig. 4B. —61-year-old man with hepatocellular carcinoma and perfusion
alteration from radiofrequency ablation on short-term follow-up CT. Arterial
dominant phase CT scan obtained at more cranial level shows adjacent
peripheral wedge-shaped enhancing area (arrow). This is compatible
with transient hepatic attenuation defect from radiofrequency ablation. This
perfusion alteration resolved on follow-up CT (not shown).
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MRI
The characteristic MRI signals of coagulation necrosis after radiofrequency
ablation are intermediate to high signal-to-liver parenchyma on T1-weighted
and low signal on T2-weighted images. A T2 hyperintense rim around the
ablation area is a possible finding, likely related to edema from thermal
injury. Any discrete areas of T1 hypointense and T2 hyperintense signal should
raise the possibility of residual or recurrent tumor. However, a recent
ablation area may have heterogeneous signal on both T1- and T2-weighted images
because of nonuniform evolution of inflammation and necrosis
[2], resulting in difficulty in
the interpretation of unenhanced MRI. Gadolinium-enhanced MRI is therefore
routinely used to maximize the accuracy of the study. As with CT, the ablation
area is nonenhancing except for circumferential peripheral hyperemia (Fig.
5A,
5B,
5C,
5D), and residual or recurrent
tumor is manifest as a noncircumferential nodular area of enhancement.
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Fig. 5A. —74-year-old woman with hepatocellular carcinoma and
successful ablation seen on 1-day follow-up MRI. Axial T1-weighted fast
low-angle shot image shows T1 hyperintense signal lesion with signal
characteristics representative of tumor desiccation.
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Fig. 5B. —74-year-old woman with hepatocellular carcinoma and
successful ablation seen on 1-day follow-up MRI. Axial T2-weighted HASTE image
shows T2 hypointense lesion with T2 hyperintense rim (white
arrow).
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Fig. 5C. —74-year-old woman with hepatocellular carcinoma and
successful ablation seen on 1-day follow-up MRI. Gadolinium-enhanced MRI shows
circumferential enhancement (arrow) around lesion but no enhancement
of lesion itself.
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Fig. 5D. —74-year-old woman with hepatocellular carcinoma and
successful ablation seen on 1-day follow-up MRI. Gadolinium-enhanced MRI of
contiguous image shows circumferential enhancement (arrow) around
ablation tract.
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Special Considerations
Lesions adjacent to major vessels have a higher risk of incomplete ablation
because of a "heat sink" effect (i.e., the dissipation of heat by
flow in contiguous vessels)
[6]. Because radiofrequency
heat cannot easily traverse vessels, the ablation extent is usually limited by
major vessels (Fig. 6A,
6B) and may not provide the
desired margin.
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Fig. 6A. —61-year-old man with hepatocellular carcinoma after right
lobectomy. This case shows limitation of perivascular tumor ablation. Enhanced
CT scan obtained before radiofrequency ablation shows perivascular lesion in
medial segment, left lobe.
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Fig. 6B. —61-year-old man with hepatocellular carcinoma after right
lobectomy. This case shows limitation of perivascular tumor ablation. CT scan
obtained after ablation shows ablation margin limited by hepatic vessels
(arrows).
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Long-Term Follow-Up
Evaluation of long-term follow-up imaging (> 4–6 months) is
generally easier than that in the immediate study performed after ablation
because of the resolution of inflammation. On CT, the radiofrequency ablation
areas and tracts become better marginated and decrease in size, while having
no enhancement (Fig. 7A,
7B,
7C,
7D). Signs of tumor recurrence
include development of noncircumferential nodular enhancement and increase in
lesion size (Fig. 8A,
8B).
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Fig. 7A. —44-year-old woman with colon carcinoma metastasis after
intraarterial infusion catheter placement with successful ablation seen on
long-term follow-up CT scan. Enhanced CT scan obtained 1 week after ablation
shows nonenhancing radiofrequency ablation area with peripheral enhancement
(arrow) on arterial dominant phase. Notice circumferential
enhancement pattern around ablation area that is likely inflammatory response
after radiofrequency ablation (compare with noncircumferential enhancement
pattern of residual tumor in Fig.
3A).
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Fig. 7B. —44-year-old woman with colon carcinoma metastasis after
intraarterial infusion catheter placement with successful ablation seen on
long-term follow-up CT scan. Portal dominant phase axial CT scan does not show
peripheral enhancement.
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Fig. 7C. —44-year-old woman with colon carcinoma metastasis after
intraarterial infusion catheter placement with successful ablation seen on
long-term follow-up CT scan. Five-month follow-up CT scan obtained in arterial
dominant phase shows reduction in size of lesion and resolution of peripheral
enhancement.
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Fig. 7D. —44-year-old woman with colon carcinoma metastasis after
intraarterial infusion catheter placement with successful ablation seen on
long-term follow-up CT scan. Portal dominant phase axial CT scan shows same
findings as in C.
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Fig. 8A. —61-year-old woman with colon cancer metastases and recurrent
tumor seen on CT scan obtained 6 months after ablation. CT scan obtained 2
weeks after ablation shows nonenhancing radiofrequency ablation area. Note
ablation tract (arrows) anterior to ablation area.
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Fig. 8B. —61-year-old woman with colon cancer metastases and recurrent
tumor seen on CT scan obtained 6 months after ablation. Six-month follow-up CT
scan shows tumor recurrence at perivascular margin (open arrows).
However, ablation tract (arrow) has decreased in size.
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On MRI, the ablation area shows more homogeneous T1 hyperintense and T2
hypointense signal (Fig. 9A,
9B). Signs of recurrence
include new enhancement, lesion size increase, and development of T1
hypointense and T2 hyperintense signal areas (Fig.
10A,
10B,
10C,
10D).
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Fig. 9A. —66-year-old man with hepatocellular carcinoma and reduction
of ablation size seen on long-term follow-up MRI. Axial T2-weighted image
obtained 2 weeks after ablation shows T2 hypointense lesion with T2
hyperintense rim.
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Fig. 9B. —66-year-old man with hepatocellular carcinoma and reduction
of ablation size seen on long-term follow-up MRI. Seven-month follow-up MRI
shows shrinkage of radiofrequency ablation area (white arrow) with
resolution of T2 hyperintense rim, indicating resolution of inflammatory
response.
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Fig. 10A. —44-year-old man with colon cancer metastases and recurrent
tumor seen on long-term follow-up MRI after ablation. Enhanced CT scan
obtained 1 month after ablation shows nonenhancing ablation area in medial
segment, left lobe.
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Fig. 10B. —44-year-old man with colon cancer metastases and recurrent
tumor seen on long-term follow-up MRI after ablation. Enhanced CT scan
obtained 3 years after ablation shows shrinkage of ablation area
(arrow).
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Fig. 10C. —44-year-old man with colon cancer metastases and recurrent
tumor seen on long-term follow-up MRI after ablation. Follow-up MRI obtained 3
months after B shows recurrent tumor as enlarged T1 hypointense signal
area.
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Fig. 10D. —44-year-old man with colon cancer metastases and recurrent
tumor seen on long-term follow-up MRI after ablation. Axial
gadolinium-enhanced T1-weighted image obtained on same date as C shows
noncircumferential nodular enhancement of recurrent tumor.
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Conclusion
Accurate assessment of lesions after radiofrequency ablation is essential
to evaluate the adequacy of treatment and guide further management.
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Introduction
Pathophysiology of Areas After...
