High-resolution ¹⁸F-fluorodeoxyglucose positron emission tomography and magnetic resonance imaging for pituitary adenoma detection in Cushing disease

Abstract

OBJECT

High-resolution PET (hrPET) performed using a high-resolution research tomograph is reported as having a resolution of 2 mm and could be used to detect corticotroph adenomas through uptake of ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). To determine the sensitivity of this imaging modality, the authors compared ¹⁸F-FDG hrPET and MRI detection of pituitary adenomas in Cushing disease (CD).

METHODS

Consecutive patients with CD who underwent preoperative ¹⁸F-FDG hrPET and MRI (spin echo [SE] and spoiled gradient recalled [SPGR] sequences) were prospectively analyzed. Standardized uptake values (SUVs) were calculated from hrPET and were compared with MRI findings. Imaging findings were correlated to operative and histological findings.

RESULTS

Ten patients (7 females and 3 males) were included (mean age 30.8 ± 19.3 years; range 11–59 years). MRI revealed a pituitary adenoma in 4 patients (40% of patients) on SE and 7 patients (70%) on SPGR sequences. ¹⁸F-FDG hrPET demonstrated increased ¹⁸F-FDG uptake consistent with an adenoma in 4 patients (40%; adenoma size range 3–14 mm). Maximum SUV was significantly higher for ¹⁸F-FDG hrPET–positive tumors (difference = 5.1, 95% CI 2.1–8.1; p = 0.004) than for ¹⁸F-FDG hrPET–negative tumors. ¹⁸F-FDG hrPET positivity was not associated with tumor volume (p = 0.2) or dural invasion (p = 0.5). Midnight and morning ACTH levels were associated with ¹⁸F-FDG hrPET positivity (p = 0.01 and 0.04, respectively) and correlated with the maximum SUV (R = 0.9; p = 0.001) and average SUV (R = 0.8; p = 0.01). All ¹⁸F-FDG hrPET–positive adenomas had a less than a 180% ACTH increase and ¹⁸F-FDG hrPET–negative adenomas had a greater than 180% ACTH increase after CRH stimulation (p = 0.03). Three adenomas were detected on SPGR MRI sequences that were not detected by ¹⁸F-FDG hrPET imaging. Two adenomas not detected on SE (but no adenomas not detected on SPGR) were detected on ¹⁸F-FDG hrPET.

CONCLUSIONS

While ¹⁸F-FDG hrPET imaging can detect small functioning corticotroph adenomas and is more sensitive than SE MRI, SPGR MRI is more sensitive than ¹⁸F-FDG hrPET and SE MRI in the detection of CD-associated pituitary adenomas. Response to CRH stimulation can predict ¹⁸F-FDG hrPET–positive adenomas in CD.

Biochemical remission after transsphenoidal surgery in Cushing disease (CD) is influenced by the ability to visualize a pituitary adenoma on preoperative imaging. Currently, MRI is the gold standard for detecting pituitary microadenomas. Nevertheless, modern pituitary MRI sequences fail to detect a pituitary adenoma in approximately 40% of CD cases due to small tumor size and/or poor MRI contrast-to-noise ratio at the sella-sphenoid interface.^8,11 When adenomas are visible on preoperative imaging, remission rates approaching 90%–100% have been reported.^15,16 When adenomas are not detected on preoperative MRI, remission rates can decrease to 50%–60% in CD.^11,15,17

Standard spin echo (SE) MRI sequences (pre- and postcontrast administration) have been and are currently used to image the pituitary gland for adenomas in CD. Using SE MRI, approximately 50% of adenomas in CD can be detected using preoperative postcontrast MRI studies.¹⁴ Since 2003, the spoiled gradient recalled (SPGR) MRI sequence (pre- and postcontrast administration) has been used to best enhance detection of pituitary adenomas in CD.^2,10,11 While SPGR MRI has increased detection of pituitary adenomas in CD (a 10%–15% increase in sensitivity in CD adenoma detection), a large portion of pituitary adenomas in CD remain undetected on MRI before resection. ^8,11,14

To enhance detection of pituitary adenomas in patients with CD undergoing SE and/or SPGR MRI, investigators have used standard resolution (approximately 6-mm resolution) ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) PET imaging. Data from these MRI and PET studies revealed that ¹⁸F-FDG PET is complementary to SE MRI in detecting pituitary adenomas not detected on preoperative MRI in 25% of CD cases.^1,4 Recently, technological advances have led to development of high-resolution PET (hrPET) using a high-resolution research tomograph scanner.¹⁸ These scanners have a 2-mm resolution.³ These advances suggest that hrPET scanners may be used to detect small biologically active adenomas in CD via ¹⁸F-FDG uptake.^1,4

Based on improvements in both MRI and PET as defined above, the combination of these 2 modalities may provide an opportunity to enhance detection of CD-associated pituitary adenomas. To assess the sensitivity of these imaging techniques for detection of CD-associated pituitary adenomas (alone and in a complementary fashion), we prospectively compared high-resolution MRI (SE and SPGR sequences) with ¹⁸F-FDG hrPET for detection of corticotroph adenomas in CD. Furthermore, to determine if biological features of adenomas in CD could be used to predict ¹⁸F-FDG uptake, we analyzed biological factors in patients with CD undergoing ¹⁸F-FDG hrPET.

Methods

Patients

Patients with CD were enrolled in a clinical trial (registration no. NCT01459237) (clinicaltrials.gov) conducted at the National Institute of Neurological Diseases and Stroke (NINDS) (12-N-0007) to prospectively evaluate the use of ¹⁸F-FDG hrPET for CD. All patients or guardians (in the case of minors) signed informed consent.

Diagnosis of CD

To distinguish between adrenocorticotropic hormone (ACTH)–dependent causes (either ectopic or pituitary) of hypercortisolism, patients underwent biochemical evaluation including high-dose dexamethasone testing, corticotropin-releasing hormone (CRH) stimulation testing, and MRI of the pituitary. Patients with high-dose dexamethasone suppression of cortisol, CRH-induced elevation of ACTH and cortisol, as well as MRI evidence of a pituitary adenoma were given a diagnosis of CD. Patients with negative findings on MRI or indefinite endocrinological evaluation underwent inferior petrosal sinus sampling (IPSS) to confirm or exclude a pituitary source of ACTH secretion.¹³

MRI of the Pituitary Gland

All patients underwent MRI of the pituitary gland before resection. MRI sequences included 3-mm SE sequences in coronal and sagittal orientation, as well as 1-mm 3D gradient echo (spoiled gradient or SPGR) sequence, as described previously.¹⁴ Coronal T1-weighted SE scans were obtained using the following parameters: TR 400 msec, TE 9 msec, and 12-cm FOV. Contiguous 3-mm-thick slices were obtained. SPGR scans were performed using the following parameters: TR 9.6 msec, TE 2.3 msec, and 12-cm FOV. Contiguous 1-mm–thick coronal slices were obtained for SPGR sequences. MRI findings were assessed by a neuroradiologist before surgery for the presence of an adenoma. Adenoma volume was assessed by a formula calculating ellipsoid volume of brain tumors (volume = [maximal mediolateral dimension × maximal anteroposterior dimension × maximal vertical dimension]/2).¹⁹

¹⁸F-FDG hrPET Protocol

PET imaging was performed using a high-resolution research tomography scanner (Siemens AG). Subjects were asked to fast (except water) for at least 6 hours prior to the imaging session. Commercially manufactured ¹⁸F-FDG was injected intravenously over a period of approximately 1 minute. The ¹⁸F-FDG dose was standardized at 10 mCi in patients who were 18 years and older. For patients younger than 18 years, the dose was calculated at 0.08 mCi/kg. The subjects rested quietly in a dark room for approximately 30 minutes after ¹⁸F-FDG administration. During the scan, a cap with small light reflectors was placed on the subject’s head to monitor head position with a Polaris Vicra head tracking system (Northern Digital Inc.). Information about head movement was used in the image reconstruction process to reduce the blurring effect of head movement on the PET images. After the patient was positioned in the scanner, a transmission scan was obtained with a ¹³⁷Cesium rotating pin source to correct emission images for attenuation. Then a PET emission scan of the brain was obtained in 3D mode starting about 50 minutes after ¹⁸F-FDG injection for up to 40 minutes. Standardized uptake values (SUVs) for ¹⁸F-FDG were calculated after normalization of focal ¹⁸F-FDG uptake for body weight and injected dose of ¹⁸F-FDG.¹²

Surgical Procedure

All patients underwent a standard sublabial transsphenoidal approach for pituitary adenoma resection, as described previously.⁹ Briefly, wide bony removal was performed at the sellar face and sellar floor until the cavernous sinuses were visualized. Once dura overlying the pituitary gland was opened, the exposed pituitary gland was inspected for the presence of tumor. The pituitary capsule was incised adjacent to the expected adenoma location. The pituitary pseudocapsule was developed to complete selective adenectomy. When cavernous sinus invasion was suspected, the involved medial wall of the cavernous sinus was removed.

Histopathology

Routine H&E staining was performed to evaluate the surgical specimens, as well as reticulin staining. ACTH immunostaining was performed to confirm the diagnosis of corticotroph adenoma.

Statistical Analysis

An unpaired t-test was used to detect a difference between the means. Fisher’s exact test was used for analysis of contingency tables. Pearson correlation coefficients were calculated when generating correlation matrices for continuous variables. The McNemar test was used to compare sensitivities of diagnostic tests. Statistical significance was set at p ≤ 0.05.

Results

Patients

Ten consecutive patients (7 females and 3 males) were included (mean age 30.8 ± 19.3 years [SD]; range 11–59 years) (Table 1). All patients were diagnosed with CD on the basis of clinical and biochemical criteria. While 1 patient presented with recurrent CD, 9 patients were diagnosed with new-onset CD (Table 1). All 10 patients underwent MRI of the pituitary gland and ¹⁸F-FDG hrPET imaging before surgery.

TABLE 1.

Summary statistics for the patients in this study^*

Case No.	Age (yrs), Sex	CD Diagnosis	Max Dia (mm)	Vol (mm3)	Invasion	MRI SPGR	MRI SE	hrPET
1	15, F	New	5	30	No	No	No	No
2	37, F	New	14	336	Yes	Yes	Yes	Yes
3	13, M	New	3	13.5	No	No	No	No
4	15, M	New	4	16	No	Yes	Yes	No
5	59, F	New	10	320	Yes	Yes	Yes	Yes
6	37, F	Recurrent	6	108	Yes	Yes	No	No
7	58, F	New	3	13.5	No	Yes	No	Yes
8	15, F	New	5	40	No	Yes	No	Yes
9	48, F	New	4	12	No	Yes	Yes	No
10	11, M	New	10	180	No	No	No	No

^*Max Dia indicates maximum diameter of adenoma at surgery; Vol, volume of adenoma calculated at surgery; Invasion, presence of invasion noted at surgery; MRI SPGR, presence of hypoenhancing tumor noted on SPGR sequence preoperatively; MRI SE, presence of hypoenhancing tumor noted on SE sequence preoperatively; hrPET, presence of adenoma detected by increased FDG uptake preoperatively.

Imaging Findings

MRI

A pituitary adenoma was visible in 4 patients using postcontrast SE MRI, and an adenoma was visible in 7 patients using SPGR MRI sequences. All pituitary adenomas that were detected by SE MRI were also detected using SPGR MRI. Adenoma location on MRI corresponded with the surgical location of the adenoma in every case. There was no association between MRI adenoma detection and biochemical findings.

MRI Compared With ¹⁸F-FDG hrPET Imaging

Three adenomas were detected on SPGR MRI but not on ¹⁸F-FDG hrPET. ¹⁸F-FDG hrPET did not detect any adenomas that SPGR MRI did not detect. All adenomas detected on SE MRI were detected by ¹⁸F-FDG hrPET. Furthermore, ¹⁸F-FDG hrPET detected 2 adenomas that were not detected by SE MRI (Table 1). These 2 adenomas were 3 and 5 mm in diameter (Fig. 1). Adenoma location on ¹⁸F-FDG hrPET–positive imaging corresponded with the surgical location of the adenoma.

Fig. 1.

Images obtained in a patient in whom a hypoenhancing adenoma (arrowheads) was detected on SPGR sequences (A and B), but not on SE sequences (C and D). Corresponding ¹⁸F-FDG hrPET images (E and F) revealed a focus of increased ¹⁸F-FDG uptake (arrowhead).

¹⁸F-FDG hrPET Imaging

Increased SUVs corresponding to an adenoma were detected in 4 patients. The maximum SUV (SUV_Max) in ¹⁸F-FDG hrPET–positive adenoma ranged from 6.0 to 12.8 (mean 9.5 ± 1.6) (Table 2). The average SUV (SUV_Avg) in the sella ranged from 2.3 to 4.0 (mean 3.1 ± 0.4) in ¹⁸F-FDG hrPET–positive cases. SUV_Max in ¹⁸F-FDG hrPET–negative cases ranged from 3.7 to 5.0 (mean 4.4 ± 0.2). SUV_Avg for sella ranged from 2.0 to 2.8 (mean 2.5 ± 0.1) in ¹⁸F-FDG hrPET–negative cases. While SUV_Max was significantly higher for ¹⁸F-FDG hrPET–positive adenomas (p = 0.004) compared with ¹⁸F-FDG hrPET–negative adenomas, SUV_Avg was not (p = 0.08).

TABLE 2.

Summary statistics for differences in tumors that were positive or negative on ¹⁸F-FDG hrPET

Variable	18F-FDG hrPET*		p Value
	Positive	Negative
Age in yrs	42.25 ± 10.40	23.17 ± 6.306	0.10
Sex (female/male)	4/0	3/3	0.20
Dural invasion (present/absent)	2/2	5/1	0.50
Vol in mm3	177.4 ± 87.19	59.92 ± 28.29	0.17
SUVMax	9.475 ± 1.621	4.367 ± 0.1820	0.004†
SUVAvg	3.100 ± 0.3536	2.467 ± 0.1202	0.08
ACTH MN in pg/ml	52.30 ± 12.56	17.63 ± 2.89	0.01†
ACTH AM in pg/ml	55.05 ± 15.80	20.32 ± 5.27	0.04†
Cortisol MN in mg/dl	20.90 ± 4.16	15.09 ± 2.82	0.26
Cortisol AM in mg/dl	23.04 ± 5.69	19.35 ± 4.43	0.62
Diurnal variation in ACTH in pg/ml	−11.01 ± 7.40	−0.70 ± 7.65	0.38
Diurnal variation in cortisol	2.13 ± 3.15	1.03 ± 4.47	0.86
Basal IPSS ACTH in pg/ml	534.1 ± 439.0	70.6 ± 25.9	0.16
Maximum IPSS ACTH in pg/ml	11826 ± 10811	1036 ± 427.7	0.18
Maximum IPSS gradient in pg/ml	46.72 ± 16.33	60.66 ± 33.18	0.80
ACTH % rise w/CRH stimulation	112.2 ± 31.76	237.9 ± 32.79	0.03†
Cortisol % rise w/CRH stimulation	362.8 ± 153.9	321.5 ± 78.91	0.81
ACTH-to-cortisol ratio	3.134 ± 0.68	2.693 ± 0.81	0.69

AM = morning; MN = midnight.

^*All values are the means ± SD except for sex and dural invasion variables. These values are the number of patients.

^†Statistically significant (p < 0.05).

Factors Associated With ¹⁸F-FDG Uptake

¹⁸F-FDG hrPET positivity was not associated with age (p = 0.1), sex (p = 0.2), presence of dural invasion (p = 0.5), or adenoma volume (p = 0.2). ¹⁸F-FDG hrPET positivity was associated with midnight and morning ACTH levels (p = 0.01 and p = 0.04, respectively). Both midnight and morning ACTH values correlated with SUV_Max (R = 0.9, p = 0.001) and SUV_Avg (R = 0.8, p = 0.01) (Fig. 2). There was no association between the IPSS central-to-peripheral ratio and ¹⁸F-FDG hrPET positivity (p = 0.79) or maximum central ACTH values (p = 0.2). Basal IPSS ACTH values were higher in ¹⁸F-FDG hrPET–positive tumors (mean 534 ± 439 pg/ml) than in ¹⁸F-FDG hrPET–negative tumors (mean 70.63 ± 25.95 pg/ml), but the difference was not statistically significant (p = 0.2).

Fig. 2.

A and B: ¹⁸F-FDG hrPET findings were associated with biochemical data. Tumors detected on ¹⁸F-FDG hrPET had higher midnight (A) and morning (B) serum ACTH levels. C and D: Correlation between serum ACTH (black, midnight [MN]; red, morning [AM]) with both maximum SUV (SUV Max; C) and average SUV (SUV Avg; D) for the sella.

CRH stimulation demonstrated that change in ACTH level after stimulation (mean of 15 and 30 minutes) was significantly less in ¹⁸F-FDG hrPET–positive tumors (mean change 112.2 ± 31.8%) than in ¹⁸F-FDG hrPET–negative tumors (mean change 237.9 ± 32.8%; p = 0.03). All ¹⁸F-FDG hrPET–positive tumors were associated with an increase in ACTH after stimulation less than 180%, and all ¹⁸F-FDG hrPET–negative tumors had an increase in ACTH after stimulation greater than 180% (Fig. 3). No association was found with cortisol response to CRH stimulation (p = 0.8) with ¹⁸F-FDG hrPET–positive tumors.

Fig. 3.

¹⁸F-FDG hrPET–positive tumors had an attenuated response to CRH stimulation. All ¹⁸F-FDG hrPET–positive tumors had an attenuated response, specifically, less than 180% change in ACTH level during CRH stimulation test. The red line demonstrates cutoff at 180% change that separates the ¹⁸F-FDG hrPET–positive tumors from negative tumors.

Surgical and Clinical Findings

Surgical Findings

A pituitary adenoma was found in all cases at surgery. Adenoma location on MRI corresponded with the surgical location of the tumor. Similarly, adenomas seen on ¹⁸F-FDG hrPET corresponded with surgical findings. Maximum adenoma diameter ranged from 3 to 14 mm (mean 6.2 ± 3.5 mm). Adenoma volume ranged from 12.0 to 336.0 mm³ (mean 106.9 ± 128.4 mm³). Invasion of the adjacent dura was identified in 3 cases (anterior sella dura in 1 case and medial cavernous sinus wall in 2 cases).

Clinical Results

All patients achieved biochemical remission after surgery. Briefly, biochemical remission was defined by postoperative morning serum cortisol and 24-hour urinary free cortisol. Patients achieved either a reduction of serum cortisol below 5 μg/dl and urinary free cortisol below the reference range (hypocortisolemia) or within the reference range (eucortisolemia).¹¹ Histology confirmed an ACTH-positive adenoma in all cases.

Discussion

MRI in CD

Adoption of SPGR MRI sequences led to an increased sensitivity (80%) compared with SE MRI sequences (49%).¹⁴ The SPGR MRI technique utilizes faster acquisition times leading to minimal movement and pulsation artifacts. Although a lower signal-to-noise ratio is reported for SPGR sequences than for SE sequences, a higher contrast-to-noise ratio is achievable. Moreover, SPGR MRI allows for 1-mm–thick sections, which can potentially permit detection of smaller adenomas.¹⁴ Subsequently, SPGR MRI is superior for detecting smaller tumors that have a contrast difference relative to the normal pituitary gland. For these reasons, we have found that SPGR MRI provides optimal detection of pituitary adenomas in CD patients.^2,11,14

Biological Rationale for ¹⁸F-FDG PET in CD

Large series of whole-body ¹⁸F-FDG PET imaging have examined the features that underlie incidental ¹⁸F-FDG uptake in the sella. They found that ¹⁸F-FDG uptake in the pituitary gland is found in less than 0.1% of cases.^5,7 More than 95% of these patients had an underlying pituitary adenoma detected on MRI as a source of increased ¹⁸F-FDG uptake.⁷ These findings demonstrated that ¹⁸F-FDG PET imaging of the pituitary gland/sella has high specificity for detection of pituitary adenomas⁵ and could potentially be used for detection of pituitary adenomas in this region. Specifically, these prior data suggested that ¹⁸F-FDG PET imaging could be exploited to detect biologically (high metabolic) active CD-associated adenomas that suppress the metabolic activity of the surrounding pituitary gland.

Based on the pituitary findings from whole-body ¹⁸F-FDG PET imaging and the biological features of CD-associated adenomas, groups have used standard resolution ¹⁸F-FDG PET imaging in CD patients for adenoma detection. These studies found that in 25% of cases, ¹⁸F-FDG PET imaging played a complementary role to SE MRI, detecting ACTH-secreting adenomas that were not visible on SE MRI.^1,4 Furthermore, no false-positive cases were reported with the use of ¹⁸F-FDG PET imaging in CD in these series.^1,4 Nevertheless, some lesions not detected using ¹⁸F-FDG PET imaging were visualized on SE MRI, but no biological explanation for these results was established that would improve understanding of the application of the ¹⁸F-FDG PET imaging modality.

Here, we analyzed the effectiveness of ¹⁸F-FDG hrPET imaging (2-mm resolution) and biological factors associated with ¹⁸F-FDG hrPET imaging positivity in patients with CD.

Current Study

Sensitivity of Imaging Modalities

Consistent with previous data,¹⁴ the findings in the current study showed an improvement in adenoma detection sensitivity from 40% to 70% with SPGR over standard SE MRI sequences. The sensitivity of ¹⁸F-FDG hrPET was 40% for the detection of CD adenomas in the current series, which was less than the sensitivity (70%) of SPGR MRI techniques for detection of pituitary adenomas in this series of patients with CD. Similarly, previous studies reported an advantage of ¹⁸F-FDG PET over SE MRI sequences, as we found in the current series.^1,4 Nevertheless, we found no advantage of ¹⁸F-FDG hrPET over high-resolution SPGR MRI. These data underscore the effectiveness of SPGR MRI for detecting ACTH-secreting adenomas in CD.

Regardless of the imaging modality, when an adenoma was identified it corresponded with the location (side of gland) and size of the tumor. These findings indicate that methods to improve the sensitivity of either MRI or PET will not only assist in diagnosis but will have direct surgical implications. Specifically, any of these modalities, including ¹⁸F-FDG hrPET, can guide surgical exploration (and perhaps partial hypophysectomy) to the correct location within the pituitary gland and enhance effectiveness of resection. This is particularly important for very small (2 mm in diameter or less) imaging-invisible CD-associated adenomas that have not formed a pseudocapsule⁶ and may be difficult to distinguish from normal pituitary gland.

Biological Implications

Previous studies in CD have not detected an association with biochemical markers for ACTH-secreting adenomas and ¹⁸F-FDG uptake using standard-resolution PET imaging.^1,4 Similar to De Souza colleagues⁴ (who used standard-resolution PET imaging), we did not find an association between IPSS findings and ¹⁸F-FDG uptake. Likewise, we did not find an association between IPSS findings and ¹⁸F-FDG positivity. While Alzahrani and colleagues¹ did not find an association between ¹⁸F-FDG uptake using standard PET imaging, we found an association between ¹⁸F-FDG hrPET positivity and quantitative ¹⁸F-FDG uptake with midnight and morning ACTH levels. The differences between our findings and those of Alzahrani and colleagues¹ could be due to resolution of the PET imaging used and/or the timing of ACTH sampling (timing is not reported in the study by Alzahrani and colleagues).

These correlations (ACTH level association with ¹⁸F-FDG uptake) reveal intrinsic variability among CD-associated adenomas. Specifically, adenomas that comparatively and constitutively oversecrete ACTH, as demonstrated by higher midnight ACTH, may respond less to diurnal hypothalamic commands and have a higher metabolic signature leading to increased ¹⁸F-FDG uptake. These findings are underscored by the reduced sensitivity of ¹⁸F-FDG hrPET–positive adenomas to CRH stimulation. ¹⁸F-FDG hrPET–positive adenomas had a significantly attenuated response to CRH stimulation compared with ¹⁸F-FDG hrPET–negative adenomas (Fig. 3). Adenomas that have increased constitutive overexpression of ACTH and increased glucose uptake may have an attenuated response to CRH stimulation (i.e., less responsive to CRH stimulation).

Clinical Implications

The findings of the current study demonstrate that the biochemical phenotype of CD-associated adenomas plays the largest role in ¹⁸F-FDG PET positivity. Specifically, biological aspects have a larger role than adenoma size or dural invasion in determining ¹⁸F-FDG uptake. These findings are supported by preoperative biochemical findings and the detection of very small adenomas (3 mm in diameter) by ¹⁸F-FDG hrPET imaging but lack of detection of larger and more CRH stimulation responsive adenomas. Consequently, it is possible that biological features can be exploited to apply ¹⁸F-FDG hrPET imaging in cases with comparatively blunted responses to CRH stimulation (less than 180% increase of ACTH; Fig. 3) and high midnight ACTH levels that do not have evidence of an adenoma on SPGR MRI. In such circumstances, it is possible that ¹⁸F-FDG hrPET imaging could play a noninvasive diagnostic role (versus IPSS).

Conclusions

¹⁸F-FDG hrPET can detect functioning corticotroph adenomas as small as 3 mm in size. While ¹⁸F-FDG hrPET allowed for detection of small adenomas in CD, it offers no advantages over SPGR MRI sequences. High midnight ACTH level and an attenuated response to CRH stimulation can predict ¹⁸F-FDG hrPET–positive adenomas in CD.

Abbreviations

ACTH

adrenocorticotropic hormone

CD

Cushing disease

CRH

corticotropin-releasing hormone

hrPET

high-resolution PET

IPSS

inferior petrosal sinus sampling

SE

spin echo

SPGR

spoiled gradient recalled

SUV

standardized uptake value

SUV_Avg

average SUV

SUV_Max

maximum SUV