In 1997, the Food and Drug Administration (FDA) approved the use of a radiopharmaceutical, technetium-99m sestamibi (Miraluma) for scintimammography, which used a conventional gamma camera to image the breast. Technetium-99m sestamibi, showed a high propensity to accumulate in breast tumors, which allowed for “functional imaging” not affected by anatomic characteristics such as breast density or surgical distortion. There are two mechanisms by which Technetium-99m sestamibi accumulates in breast tumors. The first is that it binds preferentially to the mitochondria in cells; since cancers have a high density of mitochondria (which is a marker for proliferative activity) they accumulate more technetium-99m sestamibi than the adjacent normal tissue.¹ Secondly, breast tumors produce significant neoangiogenesis that enables increased uptake of the radiopharmaceutical.²

Although scintimammography with conventional gamma cameras showed good sensitivity and specificity for larger tumors, its low sensitivity for tumors smaller than 15 mm did not meet prevailing standards for breast imaging.^{3, 4} Improvements in gamma camera technology and optimized image acquisition parameters allowed for higher sensitivity for detecting subcentimeter lesions, and ushered in the upgraded technology to now be referred to as MBI. Two types of gamma cameras are currently available: 1) multicrystal array detectors using sodium iodide, and 2) cadmium zinc telluride (CZT) direct conversion detectors.

The use of cadmium zinc telluride (CZT) direct conversion detectors showcase a technology upgrade in nuclear medicine where photon energy released by a radiopharmaceutical, such as technetium-99m sestamibi, is directly converted into electric signals allowing for accurate identification of event location and energy. With computer and software technology, complex calculations can be performed very quickly to convert the detected radiation into images and information that is useful for radiologists.

LOW BREAST RADIATION RISK COMPARED TO MAMMOGRAPHY

Optimizations in detector design, patient preparation, and radiopharmaceutical delivery have reduced the administered activity from 740–1,100MBq (20–30 mCi) to the current off-label standard of 240–300MBq (6.5–8 mCi).^{5, 6} The average absorbed radiation dose to the breast from 300MBq (8 mCi) of 99mTc-sestamibi is estimated to be 1.1mGy, compared with 3.0–4.5mGy that is associated with mammography and breast tomosynthesis (3D mammography).⁷

Since 99mTc-sestamibi is systemically distributed, tissues outside the breast receive the largest radiation dose. The estimated effective (whole-body) dose for 300MBq (8 mCi) of 99mTc-sestamibi is 2.1–2.6 mSv, which is at, or lower than, annual natural background levels (~3 mSv).⁸ For reference, the effective dose of chest CT can approach 6 mSv.⁹ Tissues with the highest exposures include the colon (7.1mGy), urinary bladder (3.2mGy), and gallbladder (11.5mGy).¹⁰ Various organizations assessing radiation risk and radiation protection (Health Physics Society, American Association of Physicists in Medicine, International Organization for Medical Physics, and United Nations Scientific Committee on the Effects of Atomic Radiation) state that risks from radiation doses of less than 100 mSv are not significantly different from zero.8 Thus, current MBI radiation exposure is deemed to pose negligible risk to the patient, with minimal theoretic risk of inducing cancer in any of these organs.8 The administered activity of 99mTc-sestamibi for MBI continues to decrease with technologic advancement. Tao et al. showed that new image-processing algorithms maintain lesion conspicuity with a simulated half-dose (150MBq [4 mCi]) injection.¹¹ Furthermore, continued advances in CZT module design will improve sensitivity and should allow further dose reduction.

Molecular Breast Imaging

The Dilon Molecular Imaging System is a specialized camera designed to image various organs. This system has been optimized to perform Molecular Breast Imaging (MBI) and is a strong diagnostic tool in early breast cancer detection. Using MBI as an adjunct to mammography results in an almost fourfold increase in invasive cancer detection in women with dense breast tissue. MBI has a higher specificity than MRI and has proven to reduce benign biopsies by 50%. With a negative predictive value of 98% MBI is the beacon in dense breast tissue. The system offers MBI biopsy capability which provides an additional tool to accurately localize the region of interest.

The Dilon MBI procedure is an excellent problem-solving tool when faced with difficult-to-diagnose patients with:

• Dense Breasts
• Inconclusive Mammograms
• Lumps not seen by Mammogram
• Family History of Breast Cancer
• Breast Implants
• Breast Cancer Survivor
• Scar Tissue
• Cannot have an MRI

WHAT CAN MBI DO?

For the breast radiologist:
• Find cancers missed by mammography and ultrasound, especially in difficult and dense breast cases, while leading to fewer benign biopsies than MRI
• Provide a high negative predictive value to eliminate questionable findings on mammography and ultrasound

For the breast surgeon:
• Improve surgical planning by finding additional disease
• Provide a better understanding of the position of the tumor to help in surgical planning
• Provide information useful in monitoring tumor response to chemotherapy

HOW DOES MBI WORK?

MBI uses a pharmaceutical tracer that is absorbed by the breast tissue. The cancerous cells in the breast absorb a greater amount of the tracer than normal, healthy cells. As a result, the tracer “lights up” the cancerous areas inside the breast and the malignant cells appear as “dark spots” on the MBI image. Patients find the exam to be very comfortable, unlike mammography, as there is minimal compression and they are seated throughout the process.

Its similarity to mammographic positioning makes the MBI an easy modality for any trained mammographer to learn and use, and allows easy comparison to the original mammography views by the radiologist. The breast cancer scanning procedure may begin five minutes after the patient has been injected with a small amount of radioactive tracer (Tc-99m Sestamibi). The entire breast cancer screening study takes approximately 40-45 minutes and images are immediately available for a physician’s interpretation.

MBI CLINICAL ASPECTS

The Dilon Molecular Breast Imaging (MBI) Procedure can detect cancers missed by mammography and ultrasound, while leading to fewer benign biopsies than MRI. According to clinical studies, MBI has very high sensitivity for identifying earlier stage cancers as small as 1 mm. Clinical indications of MBI include:
• Diagnosing breast cancer patients with an unresolved diagnostic dilemma
• Equivocal exams
• Dense-breast with indeterminate breast abnormalities
• Breast implants
• Discordant findings
• As an alternate to MRI in patients who can not have an MRI study
• Monitor for recurrence
• Evaluate the extent of disease (initial staging)
• Detect multi-centric, multi-focal, or bilateral disease
• Assess response to neo-adjuvant chemotherapy
• As an adjunct to a limited mammogram
• Surgical planning for residual disease

Dilon 6800: Technical Specifications

Digital Detector

Processing Mode

Design

Crystal Material

Overall Size

 

FOV and UFOV

Pixel Size

Crystal Thickness

Number of Crystals

Photomultiplier Tubes

Shielding

Dead space

Axes of Rotation

Detector Rotation

Detector Tilt

PSPMT & Pixilated Crystals

NaI[Tl]

10.9 x 25.1 x 29.2 cm

[4.3 x 9.9 x 11.5 in]

15.2 x 20.3 cm [6.0 x 8.0 in]

2.96 mm x 2.96 mm

6 mm

3072

48

5 mm Pb

10 mm [0.4 in]

2

±130º

360º

Databases

Image Import/Export

Processing Functions

 

 

Annotation

Report Output

Image File Output

Local and remote

Interfile or DICOM

Threshold,

saturation, rotation,

smoothing, zoom

Text, box, arrows

BMP file, printer

BMP, JPEG, PNG

Imaging

System dimensions

Intrinsic Spatial

Resolution

Energy Resolution

Energy Range

Uniformity

Image Event Rate

3.3 mm

 

13.5%

70 – 200 KeV

± 10% across full FOV

10k counts/sec (max)

System Total Weight

SmartShield Weight

System Dimensions

(with retracted arm

for storage and/or

transportation)

Arm Extension

Maximum Arm Height

Arm Vertical Travel

249 Kg [550 lbs]

5.5 kg [12 lbs]

132 (h) x 71 (w) x 127 (d)

cm [52 x 28 x 50 in]

 

 

86 cm [33.75 in]

103 cm [40.5 in]

51 cm [20.0 in]

Workstation

Operational Requirements

Acquisition Workstation

 

Operating System

Monitor

Monitor Resolution

Archival Medium

Cables

PC, monitor, keyboard with

touchpad

Windows XP Pro

17 inch color flat panel

1280 x 1024 pixels

USB/CD-RW

Power and Ethernet

Operating Temperature

Storage Temperature

Operating Humidity

Storage Humidity

Atmospheric Pressure

Power or Voltage

Frequency

Current

Power

UPS Backup

18-30 ºC [65-85 ºF]

0-50 ºC [32-122 ºF]

30-80% (Non-condensing)

5-80% (Non-condensing)

70-105 kPa [10-15 lb/in2]

120 ~ 230 ~

60 Hz 50 Hz or 60 Hz

6 A 3 A

720 VA (max)

20 minutes (max

Acquisition

Communications

Imaging Protocol

Protocol Elements

 

Display Data

 

Persistence Screen

 

 

Peaking

User Definable

Organ, view, isotope, time/

counts, detector rotation

Elapsed time, acquired

counts, remaining time

Integral to workstation,

adjustable zoom and

refresh

Manual

Hardware

Protocol

Data Format

10/100 Base T

Ethernet

DICOM 3.0 Interfile V3.3

DICOM Services

Query/Retrieve, Print,

Store

Worklist Management