Convert CDT into JSON format

Hi 

I am working on version 18.3.

I have a CDT variable called "testCDT" like:

local!testCDT: {
  'type!test'(
   Field_Name: "Air Ticket"
   Field_Value: "321"
  ),
  'type!test'(
   Field_Name: "Bus Ticket"
   Field_Value: "123ab"
  ),
  'type!test'(
   Field_Name: "Plane Ticket"
   Field_Value: ""
  )
}

Now my requirement is to convert it into following JSON format:

{"Air Ticket":"321","Bus Ticket":"123ab","Plane Ticket":""}

If i do a!toJson() on the variable local!testCDT

load(
 local!testCDT: {
  'type!test'(
   Field_Name: "Air Ticket"
   Field_Value: "321"
  ),
  'type!test'(
   Field_Name: "Bus Ticket"
   Field_Value: "123ab"
  ),
  'type!test'(
   Field_Name: "Plane Ticket"
   Field_Value: ""
  )
 },
 a!toJson(local!testCDT)
)

the output I receive is:

"[[{"Field_Name":"Air Ticket","Field_Value":"321"}],[{"Field_Name":"Bus Ticket","Field_Value":"123ab"}],[{"Field_Name":"Plane Ticket","Field_Value":""}]]"

Any suggestions how to achieve my desired JSON format.

Thanks in advance!!